Java 치트시트

// Main.java
public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        System.out.printf("Name: %s, Age: %d%n", "Alice", 30);
    }
}

// Compile: javac Main.java  -> Main.class
// Run:     java Main
// Package: java -cp . com.example.Main

// Every Java program needs:
// 1. A class (public class, file name must match)
// 2. A main method: public static void main(String[] args)

// Primitive types (8 total)
int age = 30;              // 32-bit integer
long bigNum = 100_000L;   // 64-bit integer
double price = 19.99;     // 64-bit float (default for decimals)
float pi = 3.14f;         // 32-bit float
boolean active = true;    // true/false
char grade = 'A';         // 16-bit Unicode character
byte b = 127;             // 8-bit signed
short s = 32767;          // 16-bit signed

// Reference types
String name = "Alice";    // Object (not primitive)
int[] nums = {1, 2, 3};  // Array object

// Constants
final double PI = 3.14159; // can't be reassigned

// var (Java 10+, local type inference)
var count = 42;    // inferred as int
var list = new ArrayList<String>(); // inferred as ArrayList<String>

// Wrapper classes (Object versions of primitives)
Integer wrapped = Integer.valueOf(42);  // explicit
Integer auto = 42;                       // autoboxing
int unboxed = auto;                      // unboxing

// Useful methods
int max = Integer.MAX_VALUE;     // 2147483647
String bin = Integer.toBinaryString(42);
int parsed = Integer.parseInt("42");
String s = String.valueOf(42);

// Other wrappers: Double, Boolean, Character, Long, Float
Double d = 3.14;
Boolean b = Boolean.TRUE;
Character c = 'A';

// Be careful with == on wrappers
Integer a = 127, b2 = 127;  // a == b2: true (cached)
Integer x = 128, y = 128;   // x == y: false (not cached)
// Always use .equals() for Integer comparison

// Package declaration (must be first line)
package com.example.app;

// Import specific class
import java.util.List;
import java.util.ArrayList;

// Import all classes in a package
import java.util.*;

// Static import (for static members)
import static java.lang.Math.PI;
import static java.lang.Math.sqrt;

// Usage
double area = PI * 5 * 5;
double root = sqrt(16);

// Fully qualified name (no import needed)
java.time.LocalDate today = java.time.LocalDate.now();

// Package naming convention: reverse domain
// com.google.gson, org.apache.commons, io.netty.channel

import java.util.Scanner;

// Console output
System.out.println("Hello");      // with newline
System.out.print("No newline");   // without newline
System.out.printf("Pi: %.2f%n", 3.14159); // formatted

// Console input with Scanner
Scanner scanner = new Scanner(System.in);
System.out.print("Enter name: ");
String name = scanner.nextLine();

System.out.print("Enter age: ");
int age = scanner.nextInt();

System.out.printf("Hi %s, age %d%n", name, age);
scanner.close(); // always close

// Command-line arguments
// java Main arg1 arg2
// args[0] = "arg1", args[1] = "arg2"

String s = "Hello, World";

// Length and access
int len = s.length();        // 12
char c = s.charAt(0);        // 'H'

// Comparison
s.equals("Hello, World");    // true (content comparison)
s.equalsIgnoreCase("hello, world"); // true
s.compareTo("Apple");        // positive (s > "Apple")
"abc".compareTo("abd");      // negative

// Search
s.indexOf("World");          // 7 (-1 if not found)
s.lastIndexOf("l");          // 10
s.contains("World");         // true
s.startsWith("Hello");       // true
s.endsWith("World");         // true

// Extract
s.substring(7);              // "World"
s.substring(0, 5);           // "Hello"

// Transform
s.toUpperCase();             // "HELLO, WORLD"
s.toLowerCase();             // "hello, world"
s.replace("o", "0");         // "Hell0, W0rld"
s.trim();                    // remove whitespace
s.split(", ");               // ["Hello", "World"]

// String concatenation (creates new String each time)
String s = "Hello" + ", " + "World";
String formatted = String.format("%s is %d", "Alice", 30);

// StringBuilder (mutable, efficient for many concatenations)
StringBuilder sb = new StringBuilder();
sb.append("Hello");
sb.append(", ");
sb.append("World");
sb.insert(5, " there");
sb.delete(5, 11);
sb.reverse();
String result = sb.toString();

// StringBuffer (thread-safe, slower than StringBuilder)
StringBuffer sbf = new StringBuffer("thread-safe");

// Join strings
String joined = String.join(", ", "a", "b", "c"); // "a, b, c"

// Repeat (Java 11+)
String repeated = "ab".repeat(3); // "ababab"

// printf / format specifiers
System.out.printf("Int: %d%n", 42);
System.out.printf("Float: %.2f%n", 3.14159);  // 3.14
System.out.printf("String: %s%n", "hello");
System.out.printf("Char: %c%n", 'A');
System.out.printf("Bool: %b%n", true);
System.out.printf("Hex: %x%n", 255);           // ff
System.out.printf("Octal: %o%n", 8);           // 10

// Width and padding
System.out.printf("[%5d]%n", 42);      // [   42]
System.out.printf("[%-5d]%n", 42);     // [42   ]
System.out.printf("[%05d]%n", 42);     // [00042]
System.out.printf("[%8.2f]%n", 3.14);  // [    3.14]

// String.format returns a String
String s = String.format("Name: %s, Age: %d", "Alice", 30);

// Text blocks (Java 15+)
String json = """
    {
        "name": "Alice",
        "age": 30
    }
    """;

import java.util.regex.*;

// String methods
"hello123".matches("[a-z]+\d+"); // true
"a,b,c".split(",");               // ["a", "b", "c"]
"hello".replaceAll("l", "L");     // "heLLo"

// Pattern and Matcher
Pattern p = Pattern.compile("\\d{4}-\\d{2}-\\d{2}");
Matcher m = p.matcher("Date: 2024-01-15");

if (m.find()) {
    System.out.println(m.group()); // "2024-01-15"
}

// Find all matches
while (m.find()) {
    System.out.println(m.group());
}

// Replace with regex
String result = "2024-01-15".replaceAll(
    "(\\d{4})-(\\d{2})-(\\d{2})",
    "$3/$2/$1"); // "15/01/2024"

// Common patterns
String email = "^[\\w.]+@[\\w.]+\\.\\w+$";
String phone = "^\\d{3}-\\d{4}$";

// Math class
double sqrt = Math.sqrt(16);     // 4.0
double pow = Math.pow(2, 10);    // 1024.0
int abs = Math.abs(-5);          // 5
int max = Math.max(3, 7);        // 7
int min = Math.min(3, 7);        // 3
double rounded = Math.round(3.7); // 4
double ceil = Math.ceil(3.1);    // 4.0
double floor = Math.floor(3.9);  // 3.0
double random = Math.random();   // 0.0 to 1.0

// Constants
double pi = Math.PI;             // 3.14159...
double e = Math.E;               // 2.71828...

// Integer/Long methods
int sum = Integer.sum(3, 4);     // 7
int max2 = Integer.max(3, 7);    // 7

// Rounding modes
double r = Math.round(3.5);      // 4 (round half up)
double r2 = Math.floor(3.5 + 0.5); // alternative

// Random (java.util.Random)
import java.util.Random;
Random rand = new Random();
int n = rand.nextInt(100);  // 0-99
double d = rand.nextDouble(); // 0.0-1.0
boolean b = rand.nextBoolean();

int score = 85;

if (score >= 90) {
    System.out.println("A");
} else if (score >= 80) {
    System.out.println("B");
} else if (score >= 70) {
    System.out.println("C");
} else {
    System.out.println("F");
}

// Ternary operator
String grade = score >= 60 ? "Pass" : "Fail";

// Nested if
if (score >= 60) {
    if (score >= 90) {
        System.out.println("Excellent");
    }
}

// Note: conditions must be boolean (no truthy/falsy)
// if (score) { } // Error: int is not boolean

// Traditional switch (fall-through)
int day = 3;
switch (day) {
    case 1:
        System.out.println("Mon");
        break;
    case 2:
    case 3:
    case 4:
        System.out.println("Midweek");
        break;
    case 6:
    case 7:
        System.out.println("Weekend");
        break;
    default:
        System.out.println("Invalid");
}

// Switch expression (Java 14+, no fall-through)
String type = switch (day) {
    case 1, 2, 3, 4, 5 -> "Weekday";
    case 6, 7 -> "Weekend";
    default -> "Invalid";
};

// Switch with yield (for complex blocks)
int result = switch (day) {
    case 1, 2, 3, 4, 5 -> {
        int hours = 8;
        yield hours * 5;
    }
    case 6, 7 -> 0;
    default -> -1;
};

// For loop
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

// Enhanced for (for-each)
int[] nums = {1, 2, 3, 4, 5};
for (int n : nums) {
    System.out.println(n);
}

List<String> names = List.of("Alice", "Bob");
for (String name : names) {
    System.out.println(name);
}

// While loop
int count = 0;
while (count < 3) {
    System.out.println(count);
    count++;
}

// Do-while (runs at least once)
int i = 0;
do {
    System.out.println(i);
    i++;
} while (i < 3);

// Break and continue
for (int j = 0; j < 10; j++) {
    if (j == 5) break;      // exit loop
    if (j % 2 == 0) continue; // skip iteration
    System.out.println(j);
}

// Labels for breaking out of nested loops
outer:
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        if (i == 1 && j == 1) {
            break outer; // exits both loops
        }
        System.out.println(i + "," + j);
    }
}

// Labeled continue
outer:
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        if (j == 1) {
            continue outer; // skip to next i
        }
        System.out.println(i + "," + j);
    }
}

// Alternative: extract to method and use return
void findPair(int[][] matrix, int target) {
    for (int i = 0; i < matrix.length; i++) {
        for (int j = 0; j < matrix[i].length; j++) {
            if (matrix[i][j] == target) return; // exit method
        }
    }
}

// Declare and initialize
int[] nums = {1, 2, 3, 4, 5};
int[] empty = new int[5]; // [0, 0, 0, 0, 0]
String[] names = new String[3]; // [null, null, null]

// Access and modify
nums[0] = 10;
int first = nums[0]; // 10
int length = nums.length; // 5

// Multidimensional
int[][] matrix = {{1, 2}, {3, 4}};
int val = matrix[0][1]; // 2

// Arrays utility class
import java.util.Arrays;
int[] sorted = {3, 1, 2};
Arrays.sort(sorted); // [1, 2, 3]
int[] copy = Arrays.copyOf(nums, 3);
String str = Arrays.toString(nums); // "[10, 2, 3, 4, 5]"
boolean eq = Arrays.equals(nums, copy);

// Fill
int[] filled = new int[5];
Arrays.fill(filled, 42); // [42, 42, 42, 42, 42]

// Binary search (sorted array only)
int idx = Arrays.binarySearch(sorted, 2); // index of 2

public class Calculator {
    // Method with return type
    public static int add(int a, int b) {
        return a + b;
    }

    // Void method (no return)
    public static void printResult(int result) {
        System.out.println("Result: " + result);
    }

    // Method with default (no overloading needed)
    public static String greet(String name, String greeting) {
        return greeting + ", " + name + "!";
    }

    public static void main(String[] args) {
        int sum = add(3, 4);
        printResult(sum);

        String msg = greet("Alice", "Hello");
        System.out.println(msg);
    }
}

public class MathUtils {
    // Overloaded methods (same name, different params)
    public static int add(int a, int b) {
        return a + b;
    }

    public static double add(double a, double b) {
        return a + b;
    }

    public static int add(int a, int b, int c) {
        return a + b + c;
    }

    public static String add(String a, String b) {
        return a + b;
    }
}

// Java picks the most specific match
MathUtils.add(1, 2);        // int version -> 3
MathUtils.add(1.5, 2.5);    // double version -> 4.0
MathUtils.add(1, 2, 3);     // 3-param version -> 6
MathUtils.add("Hello", "!"); // String version -> "Hello!"

// Varargs: variable number of arguments
public static int sum(int... nums) {
    int total = 0;
    for (int n : nums) {
        total += n;
    }
    return total;
}

sum(1, 2, 3);           // 6
sum(1, 2, 3, 4, 5);     // 15
sum();                  // 0 (empty array)
int[] arr = {1, 2, 3};
sum(arr);               // 6 (pass array to varargs)

// Java is ALWAYS pass-by-value
public static void modify(int x) {
    x = 100; // doesn't affect the caller's variable
}

int n = 5;
modify(n);
System.out.println(n); // still 5

// For objects, the reference is passed by value
public static void addItem(List<String> list) {
    list.add("new"); // modifies the same list object
}

// Factorial
public static int factorial(int n) {
    if (n <= 1) return 1;        // base case
    return n * factorial(n - 1); // recursive case
}
// factorial(5) = 5 * 4 * 3 * 2 * 1 = 120

// Fibonacci
public static int fib(int n) {
    if (n < 2) return n;
    return fib(n - 1) + fib(n - 2);
}

// Tail recursion (Java doesn't optimize this)
public static int factorialTail(int n, int acc) {
    if (n <= 1) return acc;
    return factorialTail(n - 1, n * acc);
}
// Call: factorialTail(5, 1)

// Be careful: deep recursion causes StackOverflowError
// For deep recursion, use iteration or a loop instead

import java.util.function.*;

// Lambda syntax: (params) -> expression
Function<Integer, Integer> square = x -> x * x;
Function<String, Integer> length = s -> s.length();
BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;

// With type annotations
BinaryOperator<Integer> multiply = (Integer a, Integer b) -> a * b;

// Multi-line lambda
Function<String, String> process = s -> {
    String upper = s.toUpperCase();
    return upper.substring(0, 3);
};

// Predicate (boolean test)
Predicate<Integer> isEven = n -> n % 2 == 0;
Predicate<String> isEmpty = String::isEmpty; // method reference

// Consumer (no return)
Consumer<String> printer = s -> System.out.println(s);
Consumer<String> printer2 = System.out::println; // method reference

// Supplier (no input, produces value)
Supplier<Double> random = () -> Math.random();

// Usage
int result = square.apply(5); // 25
boolean even = isEven.test(4); // true
printer.accept("Hello"); // prints "Hello"

public class Person {
    // Fields (instance variables)
    private String name;
    private int age;

    // Constructor
    public Person(String name, int age) {
        this.name = name;  // 'this' refers to the current instance
        this.age = age;
    }

    // Methods
    public String getName() { return name; }
    public int getAge() { return age; }

    public void setAge(int age) {
        if (age >= 0) this.age = age;
    }

    public String greet() {
        return "Hi, I'm " + name;
    }

    @Override
    public String toString() {
        return "Person{name='" + name + "', age=" + age + "}";
    }
}

// Usage
Person p = new Person("Alice", 30);
System.out.println(p.getName());  // "Alice"
System.out.println(p);            // uses toString()

public class BankAccount {
    // Access modifiers:
    public String owner;      // accessible everywhere
    private double balance;   // class only
    protected String type;    // class + subclasses + same package
    String id;                // package-private (default)

    // Static field (shared by all instances)
    private static int accountCount = 0;

    // Static constant
    public static final double MIN_BALANCE = 100.0;

    // Static method (call without instance)
    public static int getAccountCount() {
        return accountCount;
    }

    // Static initializer (runs once when class loads)
    static {
        System.out.println("BankAccount class loaded");
    }

    public BankAccount(String owner) {
        this.owner = owner;
        this.balance = MIN_BALANCE;
        accountCount++; // increment shared counter
    }
}

int count = BankAccount.getAccountCount(); // static method call

// Parent class
class Animal {
    protected String name;

    public Animal(String name) {
        this.name = name;
        System.out.println("Animal constructor");
    }

    public void eat() {
        System.out.println(name + " is eating");
    }
}

// Child class (extends)
class Dog extends Animal {
    private String breed;

    public Dog(String name, String breed) {
        super(name); // must be first line — call parent constructor
        this.breed = breed;
    }

    // Override parent method
    @Override
    public void eat() {
        super.eat(); // call parent's eat()
        System.out.println(name + " the " + breed + " eats dog food");
    }

    public void bark() {
        System.out.println("Woof!");
    }
}

Dog dog = new Dog("Rex", "Labrador");
dog.eat();   // calls Dog's eat()
dog.bark();  // Dog-specific method

// Abstract class (can't be instantiated)
abstract class Shape {
    protected String color;

    public Shape(String color) {
        this.color = color;
    }

    // Abstract method (must be implemented by subclasses)
    public abstract double area();

    // Concrete method (inherited)
    public String describe() {
        return color + " " + this.getClass().getSimpleName();
    }
}

// Interface (pure contract, Java 8+ can have default methods)
interface Drawable {
    void draw(); // abstract by default

    // Default method (Java 8+)
    default void drawTwice() {
        draw();
        draw();
    }

    // Static method in interface
    static Drawable empty() {
        return () -> System.out.println("nothing");
    }
}

// A class can extend one class and implement multiple interfaces
class Circle extends Shape implements Drawable {
    private double radius;

    public Circle(String color, double radius) {
        super(color);
        this.radius = radius;
    }

    @Override
    public double area() {
        return Math.PI * radius * radius;
    }

    @Override
    public void draw() {
        System.out.println("Drawing " + describe());
    }
}

// Polymorphism: one interface, many forms
Shape s1 = new Circle("red", 5);
Shape s2 = new Square("blue", 3);

// Calls the overridden method (runtime dispatch)
System.out.println(s1.area()); // Circle's area
System.out.println(s2.area()); // Square's area

// instanceof check
if (s1 instanceof Circle) {
    Circle c = (Circle) s1; // downcast
    System.out.println("Radius: " + c.radius);
}

// Pattern matching (Java 16+)
if (s1 instanceof Circle c) {
    System.out.println("Radius: " + c.radius); // c is already cast
}

// Upcasting (automatic)
Circle circle = new Circle("green", 2);
Shape shape = circle; // upcast (no explicit cast needed)

// Object class methods (all classes inherit from Object)
circle.equals(circle);   // reference equality by default
circle.hashCode();       // hash code
circle.getClass();       // Class<Circle>
circle.toString();       // string representation

// Record (Java 16+): concise data class
public record Point(int x, int y) {}

// Equivalent to a class with:
// - final fields x, y
// - constructor
// - getters x(), y()
// - equals, hashCode, toString

Point p = new Point(3, 4);
System.out.println(p.x());      // 3
System.out.println(p.y());      // 4
System.out.println(p);          // Point[x=3, y=4]

// Compact constructor (validation)
public record Age(int value) {
    public Age {
        if (value < 0 || value > 150) {
            throw new IllegalArgumentException("Invalid age");
        }
    }
}

// Enum (named constants)
public enum Direction {
    UP, DOWN, LEFT, RIGHT;

    public Direction opposite() {
        return switch (this) {
            case UP -> DOWN;
            case DOWN -> UP;
            case LEFT -> RIGHT;
            case RIGHT -> LEFT;
        };
    }
}

Direction d = Direction.UP;
Direction opp = d.opposite(); // DOWN

import java.util.*;

// ArrayList (fast random access, slow insert/delete in middle)
List<String> list = new ArrayList<>();
list.add("Alice");
list.add("Bob");
list.add(0, "Carol");     // insert at index
list.set(1, "Dave");      // replace at index
String name = list.get(0); // "Carol"
list.remove(0);            // remove by index
list.remove("Dave");       // remove by value
int size = list.size();    // 1
boolean has = list.contains("Bob");

// LinkedList (fast insert/delete at ends)
LinkedList<Integer> linked = new LinkedList<>();
linked.addFirst(1);
linked.addLast(2);
linked.removeFirst();
linked.peek(); // see first element

// Immutable list (Java 9+)
List<String> immutable = List.of("a", "b", "c");
// immutable.add("d"); // UnsupportedOperationException

// Iterate
for (String s : list) {
    System.out.println(s);
}
list.forEach(System.out::println); // method reference

import java.util.*;

// HashSet (fast, unordered)
Set<String> set = new HashSet<>();
set.add("apple");
set.add("banana");
set.add("apple"); // duplicate ignored
System.out.println(set.size()); // 2
System.out.println(set.contains("apple")); // true
set.remove("banana");

// TreeSet (sorted, slower)
Set<Integer> sorted = new TreeSet<>();
sorted.add(3);
sorted.add(1);
sorted.add(2);
System.out.println(sorted); // [1, 2, 3]

// LinkedHashSet (maintains insertion order)
Set<String> ordered = new LinkedHashSet<>();
ordered.add("c");
ordered.add("a");
ordered.add("b");
System.out.println(ordered); // [c, a, b]

// Set operations
Set<Integer> a = new HashSet<>(Set.of(1, 2, 3));
Set<Integer> b = new HashSet<>(Set.of(2, 3, 4));
a.addAll(b);    // union: [1, 2, 3, 4]
a.retainAll(b); // intersection: [2, 3]
a.removeAll(b); // difference: [1]

// Immutable set
Set<String> immutable = Set.of("x", "y", "z");

import java.util.*;

// HashMap (fast, unordered)
Map<String, Integer> ages = new HashMap<>();
ages.put("Alice", 30);
ages.put("Bob", 25);
ages.put("Alice", 31); // overwrite

// Access
int age = ages.get("Alice"); // 31
int defaultAge = ages.getOrDefault("Eve", 0); // 0

// Check
boolean has = ages.containsKey("Alice");
boolean hasVal = ages.containsValue(25);

// Remove
ages.remove("Bob");

// Iterate
for (Map.Entry<String, Integer> entry : ages.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

ages.forEach((key, val) -> System.out.println(key + "=" + val));

// Compute (Java 8+)
ages.compute("Alice", (k, v) -> v + 1); // increment
ages.putIfAbsent("Carol", 28);
ages.merge("Alice", 1, Integer::sum); // add 1

// TreeMap (sorted by keys)
Map<String, Integer> sorted = new TreeMap<>();
// LinkedHashMap (maintains insertion order)
Map<String, Integer> ordered = new LinkedHashMap<>();

import java.util.*;

// Queue (FIFO)
Queue<String> queue = new LinkedList<>();
queue.add("first");   // throws if full (capacity-restricted)
queue.offer("second"); // returns false if full
String head = queue.peek(); // see head (null if empty)
String removed = queue.poll(); // remove and return head

// Deque (double-ended)
Deque<Integer> deque = new ArrayDeque<>();
deque.addFirst(1);
deque.addLast(2);
deque.peekFirst(); // 1
deque.peekLast();  // 2
deque.pollFirst(); // 1
deque.pollLast();  // 2

// PriorityQueue (min-heap by default)
PriorityQueue<Integer> pq = new PriorityQueue<>();
pq.add(3);
pq.add(1);
pq.add(2);
System.out.println(pq.poll()); // 1 (smallest first)

// Max-heap (reverse order)
PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Collections.reverseOrder());
maxHeap.add(1);
maxHeap.add(3);
System.out.println(maxHeap.poll()); // 3 (largest first)

// Stack (legacy, prefer Deque)
Deque<Integer> stack = new ArrayDeque<>();
stack.push(1); // add to front
stack.push(2);
stack.pop();   // 2 (remove from front)

// Generic class
public class Box<T> {
    private T value;

    public void set(T value) { this.value = value; }
    public T get() { return value; }
}

Box<String> stringBox = new Box<>();
stringBox.set("hello");
String s = stringBox.get();

Box<Integer> intBox = new Box<>();
intBox.set(42);

// Generic method
public static <T> T firstOf(List<T> list) {
    return list.get(0);
}

String first = firstOf(List.of("a", "b"));

// Bounded type parameter
public static <T extends Comparable<T>> T max(List<T> list) {
    T result = list.get(0);
    for (T item : list) {
        if (item.compareTo(result) > 0) {
            result = item;
        }
    }
    return result;
}

// Wildcards
void process(List<?> list) { }          // any type
void processNums(List<? extends Number> list) { } // Number or subclass
void addNums(List<? super Integer> list) { }      // Integer or superclass

import java.util.*;

// Iterator
List<String> list = List.of("a", "b", "c");
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    String s = it.next();
    System.out.println(s);
    // it.remove(); // safe removal during iteration
}

// ListIterator (bidirectional)
ListIterator<String> lit = list.listIterator();
while (lit.hasNext()) {
    lit.set(lit.next().toUpperCase()); // replace
}

// Comparable (natural ordering)
public class Person implements Comparable<Person> {
    String name;
    int age;

    @Override
    public int compareTo(Person other) {
        return Integer.compare(this.age, other.age);
    }
}

// Comparator (custom ordering)
Comparator<Person> byName = Comparator.comparing(p -> p.name);
Comparator<Person> byAgeDesc = Comparator.comparingInt((Person p) -> p.age).reversed();

List<Person> people = new ArrayList<>();
people.sort(byName);
people.sort(byAgeDesc);
Collections.sort(people); // uses Comparable

import java.util.*;
import java.util.stream.*;

List<Integer> nums = List.of(1, 2, 3, 4, 5, 6);

// Filter and collect
List<Integer> evens = nums.stream()
    .filter(n -> n % 2 == 0)
    .collect(Collectors.toList()); // [2, 4, 6]

// Map (transform)
List<String> doubled = nums.stream()
    .map(n -> "num" + n)
    .collect(Collectors.toList());

// Reduce
int sum = nums.stream().reduce(0, Integer::sum); // 21
int product = nums.stream().reduce(1, (a, b) -> a * b);

// Count
long count = nums.stream().filter(n -> n > 3).count(); // 3

// Find
Optional<Integer> first = nums.stream().filter(n -> n > 3).findFirst();
boolean anyMatch = nums.stream().anyMatch(n -> n > 5);
boolean allMatch = nums.stream().allMatch(n -> n > 0);

// ForEach
nums.stream().forEach(System.out::println);

// toList() shortcut (Java 16+)
List<Integer> result = nums.stream().filter(n -> n > 3).toList();

List<String> names = List.of("Alice", "Bob", "Charlie", "David");

// Sorted
List<String> sorted = names.stream()
    .sorted()
    .toList();

// Sorted by length
List<String> byLength = names.stream()
    .sorted(Comparator.comparing(String::length))
    .toList();

// Distinct
List<Integer> distinct = List.of(1, 2, 2, 3, 3, 3).stream()
    .distinct()
    .toList(); // [1, 2, 3]

// Limit and Skip
List<Integer> limited = nums.stream()
    .skip(2)   // skip first 2
    .limit(3)  // take next 3
    .toList();

// FlatMap (flatten nested structures)
List<List<Integer>> nested = List.of(List.of(1, 2), List.of(3, 4));
List<Integer> flat = nested.stream()
    .flatMap(List::stream)
    .toList(); // [1, 2, 3, 4]

// Peek (debug, side-effect)
nums.stream()
    .peek(n -> System.out.println("before: " + n))
    .filter(n -> n > 2)
    .peek(n -> System.out.println("after: " + n))
    .toList();

// Grouping
Map<Integer, List<String>> byLength = names.stream()
    .collect(Collectors.groupingBy(String::length));

import java.util.stream.*;

List<Person> people = List.of(
    new Person("Alice", 30),
    new Person("Bob", 25),
    new Person("Charlie", 35)
);

// Join strings
String joined = people.stream()
    .map(Person::getName)
    .collect(Collectors.joining(", ")); // "Alice, Bob, Charlie"

// Group by
Map<Integer, List<Person>> byAge = people.stream()
    .collect(Collectors.groupingBy(Person::getAge));

// Partition (boolean)
Map<Boolean, List<Person>> partition = people.stream()
    .collect(Collectors.partitioningBy(p -> p.getAge() > 28));

// Count by group
Map<Integer, Long> countByAge = people.stream()
    .collect(Collectors.groupingBy(Person::getAge, Collectors.counting()));

// Summarizing
IntSummaryStatistics stats = people.stream()
    .collect(Collectors.summarizingInt(Person::getAge));
System.out.println(stats.getAverage()); // 30.0
System.out.println(stats.getMax());     // 35

// To map
Map<String, Integer> nameToAge = people.stream()
    .collect(Collectors.toMap(Person::getName, Person::getAge));

// Reducing
int totalAge = people.stream()
    .collect(Collectors.reducing(0, Person::getAge, Integer::sum));

import java.util.Optional;

// Creating Optional
Optional<String> present = Optional.of("hello");
Optional<String> empty = Optional.empty();
Optional<String> nullable = Optional.ofNullable(null); // empty if null

// Checking
present.isPresent(); // true
empty.isEmpty();     // true (Java 11+)

// Getting values
String val = present.get(); // throws if empty (avoid!)
String safe = present.orElse("default");
String computed = present.orElseGet(() -> computeDefault());
String orThrow = present.orElseThrow(() -> new RuntimeException("missing"));

// Transform (map/flatMap)
Optional<Integer> length = present.map(String::length); // Optional[5]
Optional<String> upper = present.map(s -> s.toUpperCase());

// Filter
Optional<String> filtered = present.filter(s -> s.length() > 3);

// ifPresent
present.ifPresent(s -> System.out.println(s));
present.ifPresentOrElse(
    s -> System.out.println("Got: " + s),
    () -> System.out.println("Empty")
);

// Chaining (avoid null checks)
String result = getUser(1)
    .map(User::getProfile)
    .map(Profile::getEmail)
    .orElse("no email");

import java.util.function.*;

// Built-in functional interfaces
Function<String, Integer> strToInt = Integer::parseInt;
BiFunction<String, String, String> concat = String::concat;

Predicate<String> isEmpty = String::isEmpty;
BiPredicate<String, String> contains = String::contains;

Consumer<String> printer = System.out::println;
BiConsumer<String, Integer> printPair = (s, i) -> System.out.println(s + ":" + i);

Supplier<List<String>> listFactory = ArrayList::new;

// Primitive specializations
IntFunction<String> intToStr = String::valueOf;
ToIntFunction<String> length = String::length;
IntPredicate isPositive = n -> n > 0;
IntConsumer intPrinter = System.out::println;
IntSupplier randomInt = () -> (int)(Math.random() * 100);

// Binary operators
BinaryOperator<Integer> max = Integer::max;
IntBinaryOperator sum = Integer::sum;

// Unary operators
UnaryOperator<String> trim = String::trim;
IntUnaryOperator negate = n -> -n;

// Custom functional interface
@FunctionalInterface
interface StringProcessor {
    String process(String input);

    // Can have default methods
    default StringProcessor andThen(StringProcessor after) {
        return input -> after.process(process(input));
    }
}

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Math error: " + e.getMessage());
} catch (Exception e) {
    System.out.println("General error: " + e);
} finally {
    // Always runs (even if return/throw in try/catch)
    System.out.println("Cleanup");
}

// Multi-catch (Java 7+)
try {
    riskyOperation();
} catch (IOException | SQLException e) {
    // Handle both exceptions the same way
    log.error(e);
}

// Try-with-resources (auto-close, Java 7+)
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"));
     PrintWriter pw = new PrintWriter("output.txt")) {
    String line = br.readLine();
    pw.println(line);
} catch (IOException e) {
    e.printStackTrace();
}
// br and pw are auto-closed (in reverse order)

// Checked exceptions (must be declared or caught)
public void readFile(String path) throws IOException {
    BufferedReader br = new BufferedReader(new FileReader(path));
    // IOException is checked — compiler enforces handling
}

// Unchecked exceptions (RuntimeException, no need to declare)
public int divide(int a, int b) {
    if (b == 0) {
        throw new IllegalArgumentException("Divisor cannot be zero");
        // RuntimeException — no 'throws' needed
    }
    return a / b;
}

// Common checked exceptions:
// IOException, SQLException, ClassNotFoundException

// Common unchecked exceptions:
// NullPointerException, IllegalArgumentException,
// IndexOutOfBoundsException, ArithmeticException,
// ClassCastException, IllegalStateException

// Custom checked exception
class DataException extends Exception {
    public DataException(String msg) { super(msg); }
}

// Custom unchecked exception
class ValidationException extends RuntimeException {
    public ValidationException(String msg) { super(msg); }
}

import java.nio.file.*;
import java.io.*;

// Read entire file (small files)
List<String> lines = Files.readAllLines(Path.of("input.txt"));
String content = Files.readString(Path.of("config.json")); // Java 11+
byte[] bytes = Files.readAllBytes(Path.of("image.png"));

// Write file
Files.writeString(Path.of("output.txt"), "Hello, World!");
Files.write(Path.of("data.bin"), bytes);

// Append
Files.writeString(Path.of("log.txt"), "entry\n",
    StandardOpenOption.APPEND, StandardOpenOption.CREATE);

// Stream lines (large files, lazy)
try (Stream<String> lineStream = Files.lines(Path.of("large.txt"))) {
    lineStream.filter(l -> l.contains("ERROR"))
              .forEach(System.out::println);
}

// Copy, move, delete
Files.copy(Path.of("src.txt"), Path.of("dest.txt"));
Files.move(Path.of("old.txt"), Path.of("new.txt"));
Files.delete(Path.of("temp.txt"));

// Check existence
boolean exists = Files.exists(Path.of("file.txt"));

// Create directories
Files.createDirectories(Path.of("a/b/c"));

import java.io.*;
import java.nio.file.*;

// BufferedReader (efficient text reading)
try (BufferedReader br = Files.newBufferedReader(Path.of("input.txt"))) {
    String line;
    while ((line = br.readLine()) != null) {
        System.out.println(line);
    }
}

// BufferedWriter (efficient text writing)
try (BufferedWriter bw = Files.newBufferedWriter(Path.of("output.txt"))) {
    bw.write("First line");
    bw.newLine();
    bw.write("Second line");
}

// PrintWriter (convenient formatting)
try (PrintWriter pw = new PrintWriter("output.txt")) {
    pw.println("Hello");
    pw.printf("Name: %s, Age: %d%n", "Alice", 30);
}

// Scanner (parsing input)
try (Scanner sc = new Scanner(Path.of("data.txt"))) {
    while (sc.hasNextLine()) {
        String line = sc.nextLine();
        // Parse tokens
        Scanner lineSc = new Scanner(line);
        if (lineSc.hasNextInt()) {
            int n = lineSc.nextInt();
        }
    }
}

// InputStreamReader (bytes to chars, e.g., from InputStream)
Reader reader = new InputStreamReader(System.in);

import java.time.*;
import java.time.format.*;
import java.time.temporal.*;

// Current date/time
LocalDate today = LocalDate.now();      // 2024-01-15
LocalTime now = LocalTime.now();        // 14:30:45.123
LocalDateTime dt = LocalDateTime.now(); // both
ZonedDateTime zdt = ZonedDateTime.now(); // with timezone

// Create specific
LocalDate date = LocalDate.of(2024, 1, 15);
LocalTime time = LocalTime.of(14, 30, 0);
LocalDateTime specific = LocalDateTime.of(2024, 1, 15, 14, 30);

// Parsing and formatting
LocalDate parsed = LocalDate.parse("2024-01-15");
String formatted = date.format(DateTimeFormatter.ofPattern("dd/MM/yyyy"));
// "15/01/2024"

// Manipulation (immutable, returns new)
LocalDate tomorrow = today.plusDays(1);
LocalDate lastMonth = today.minusMonths(1);
LocalDate nextYear = today.plusYears(1);

// Period (date-based)
Period age = Period.between(LocalDate.of(1990, 1, 1), today);
System.out.println(age.getYears()); // 34

// Duration (time-based)
Duration dur = Duration.between(time, LocalTime.now());
System.out.println(dur.toMinutes());

// Instant (machine time, UTC)
Instant instant = Instant.now();
Instant epoch = Instant.ofEpochSecond(0);

import java.util.concurrent.*;

// Create a thread
Thread thread = new Thread(() -> {
    System.out.println("Running in: " + Thread.currentThread().getName());
});
thread.start();
thread.join(); // wait for completion

// ExecutorService (thread pool)
ExecutorService executor = Executors.newFixedThreadPool(4);
Future<Integer> future = executor.submit(() -> {
    Thread.sleep(1000);
    return 42;
});
Integer result = future.get(); // blocks until done
executor.shutdown();

// CompletableFuture (async, Java 8+)
CompletableFuture<String> cf = CompletableFuture
    .supplyAsync(() -> "Hello")
    .thenApply(s -> s + " World")
    .thenApply(String::toUpperCase);
String asyncResult = cf.join(); // "HELLO WORLD"

// Parallel stream
List<Integer> nums = List.of(1, 2, 3, 4, 5);
int sum = nums.parallelStream().mapToInt(Integer::intValue).sum();

// Synchronized
synchronized (this) {
    // only one thread at a time
}

// Atomic variables
AtomicInteger counter = new AtomicInteger(0);
counter.incrementAndGet();
counter.compareAndSet(0, 1);

// Anonymous class (verbose, pre-Java 8)
Runnable r1 = new Runnable() {
    public void run() {
        System.out.println("Old way");
    }
};

// Lambda expression (Java 8+)
Runnable r2 = () -> System.out.println("Lambda");

// With parameters and body
Comparator<Integer> cmp = (a, b) -> {
    int diff = a - b;
    return diff;
};

// Type inference (omit types)
Comparator<Integer> cmp2 = (a, b) -> a - b;

// Single param, no parentheses needed
Consumer<String> printer = s -> System.out.println(s);

// Zero params need empty parens
Runnable noop = () -> {};

// Functional interface: exactly one abstract method
@FunctionalInterface
interface MathOperation {
    int operate(int a, int b);
}

// Implement with lambda
MathOperation add = (a, b) -> a + b;
MathOperation mul = (a, b) -> a * b;
MathOperation max = (a, b) -> Math.max(a, b);

int result = add.operate(3, 4);  // 7
int m = mul.operate(3, 4);       // 12

// @FunctionalInterface is optional but recommended
// It prevents accidentally adding a second abstract method
// Default and static methods don't count toward the limit
interface StringProcessor {
    String process(String s);
    default StringProcessor andThen(StringProcessor next) {
        return s -> next.process(this.process(s));
    }
}

import java.util.function.*;

// Function<T,R>: input -> output
Function<String, Integer> len = String::length;
int n = len.apply("hello");  // 5

// Predicate<T>: input -> boolean (for filtering)
Predicate<String> isEmpty = String::isEmpty;
boolean e = isEmpty.test("");  // true

// Consumer<T>: input -> void (side effects)
Consumer<String> print = System.out::println;
print.accept("hi");

// Supplier<T>: no input -> output (factories, lazy)
Supplier<Double> random = Math::random;
double r = random.get();

// BiFunction<T,U,R>: two inputs -> output
BiFunction<String, Integer, String> repeat =
    (s, i) -> s.repeat(i);

// Primitives variants avoid boxing
IntFunction<String> f = i -> "n=" + i;
IntPredicate positive = i -> i > 0;
ToIntFunction<String> length = String::length;
IntBinaryOperator sum = (a, b) -> a + b;

import java.util.*;

List<String> names = List.of("alice", "bob", "charlie");

// Lambda form
names.forEach(s -> System.out.println(s));
// Method reference (shorthand)
names.forEach(System.out::println);

// Four kinds of method references:

// 1. Static method: ClassName::staticMethod
names.stream().map(String::toUpperCase);

// 2. Instance method of particular object: instance::method
var printer = System.out;
names.forEach(printer::println);

// 3. Instance method of arbitrary object: ClassName::instanceMethod
List<String> upper = names.stream()
    .map(String::toUpperCase)
    .toList();

// 4. Constructor: ClassName::new
Supplier<ArrayList<String>> factory = ArrayList::new;
ArrayList<String> list = factory.get();

import java.util.function.*;

int x = 10;
// Capturing a local variable — must be final or effectively final
Supplier<Integer> getter = () -> x * 2;
System.out.println(getter.get());  // 20

// x = 20;  // ERROR: would break the lambda capture
// Local variables captured by lambdas must be final/effectively final

// Workaround: use an array or wrapper (mutable container)
int[] counter = {0};
Runnable inc = () -> counter[0]++;
inc.run();
inc.run();
System.out.println(counter[0]);  // 2

// Instance/static fields CAN be modified (no restriction)
class Holder {
    int value = 0;
    Runnable bump = () -> value++;  // OK, field access
}

// 'this' inside a lambda refers to the enclosing instance
class Outer {
    String name = "Outer";
    Runnable r = () -> System.out.println(this.name);  // "Outer"
}

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

// Supplier constructor reference
Supplier<StringBuilder> sbFactory = StringBuilder::new;
StringBuilder sb = sbFactory.get();

// Function constructor reference (with one arg)
Function<String, StringBuilder> sbFromString = StringBuilder::new;
StringBuilder named = sbFromString.apply("Hello");

// In streams: collect into a specific collection
List<String> names = List.of("a", "b", "c");
ArrayList<String> copy = names.stream()
    .collect(Collectors.toCollection(ArrayList::new));

// Array constructor reference
IntFunction<String[]> arrayFactory = String[]::new;
String[] arr = arrayFactory.apply(5);  // new String[5]

// Copying via constructor
record Point(int x, int y) {}
Function<Point, Point> copyCtor = Point::new;
Point p = copyCtor.apply(new Point(1, 2));

import java.util.Optional;

// empty() — no value
Optional<String> empty = Optional.empty();

// of() — value must be non-null (throws NPE if null)
Optional<String> present = Optional.of("hello");

// ofNullable() — accepts null safely
Optional<String> maybe = Optional.ofNullable(getName());

// From a stream that may produce 0 or 1 elements
Optional<Integer> first = List.of(1, 2, 3).stream().findFirst();

// Common helper pattern
public Optional<User> findUser(long id) {
    User u = db.lookup(id);
    return Optional.ofNullable(u);
}

String getName() { return Math.random() > 0.5 ? "Alice" : null; }
class User {}
class Db { User lookup(long id) { return null; } }
Db db = new Db();

import java.util.Optional;

Optional<String> name = Optional.of("Alice");

// isPresent / isEmpty (Java 11+)
if (name.isPresent()) {
    System.out.println(name.get());  // "Alice"
}
// Avoid .get() without checking — throws NoSuchElementException

// ifPresent: run action only if value exists
name.ifPresent(System.out::println);

// ifPresentOrElse (Java 9+)
name.ifPresentOrElse(
    System.out::println,
    () -> System.out.println("No name")
);

// orElse: provide default
String s1 = name.orElse("Anonymous");

// orElseGet: lazy default (computed only if needed)
String s2 = name.orElseGet(() -> expensiveDefault());

// orElseThrow: throw if absent
String s3 = name.orElseThrow(() -> new IllegalStateException("missing"));

String expensiveDefault() { return "computed"; }

import java.util.Optional;

Optional<String> name = Optional.of("Alice");

// map: transform the value if present
Optional<Integer> length = name.map(String::length);  // Optional[5]
Optional<String> upper = name.map(String::toUpperCase);

// flatMap: when the transform itself returns Optional
// (avoids Optional<Optional<T>>)
public Optional<String> findEmail(long id) {
    return Optional.ofNullable(db.get(id));
}
Optional<String> email = Optional.of(1L)
    .flatMap(this::findEmail);  // Optional<email> not Optional<Optional<email>>

// filter: keep only if predicate matches
Optional<Integer> adultAge = Optional.of(25)
    .filter(a -> a >= 18);  // Optional[25]
Optional<Integer> kid = Optional.of(10)
    .filter(a -> a >= 18);  // Optional.empty

// Chain transformations
String label = Optional.of("alice")
    .map(String::strip)
    .filter(s -> !s.isEmpty())
    .map(s -> s.substring(0, 1).toUpperCase() + s.substring(1))
    .orElse("unknown");  // "Alice"

class Db { String get(long id) { return "[email protected]"; } }
Db db = new Db();
Optional<String> findEmail(long id) { return Optional.ofNullable(db.get(id)); }

import java.util.Optional;

// BAD: using Optional for fields (not serializable, wastes memory)
class Bad {
    private Optional<String> name;  // DON'T
}

// GOOD: use plain field, return Optional from accessor
class Good {
    private String name;
    public Optional<String> getName() { return Optional.ofNullable(name); }
}

// BAD: Optional as method parameter (clutters API)
public void process(Optional<String> input) {}  // DON'T

// GOOD: method overloading or nullable param
public void process(String input) {}
public void process() { process(null); }

// BAD: .get() without check
String x = findName().get();  // throws if empty

// BAD: .isPresent() + .get() — defeats the purpose
Optional<String> opt = findName();
if (opt.isPresent()) {
    use(opt.get());  // just use ifPresent or map instead
}

// BAD: returning Optional from collections
public Optional<Item> find(...) {
    // Collections already express emptiness — return empty List, not Optional<List>
    return Optional.ofNullable(items);
}

import java.util.*;
import java.util.stream.*;

// stream() on Optional: 0 or 1 element stream
Optional<String> opt = Optional.of("hi");
opt.stream().forEach(System.out::println);

// Useful: flatMap Optional out of a stream
class User {
    String email;  // may be null
    User(String e) { email = e; }
    Optional<String> getEmail() { return Optional.ofNullable(email); }
}

List<User> users = List.of(
    new User("[email protected]"),
    new User(null),
    new User("[email protected]")
);

// Extract emails, skipping nulls — clean with Optional::stream
List<String> emails = users.stream()
    .flatMap(u -> u.getEmail().stream())
    .toList();  // [[email protected], [email protected]]

// Without Optional::stream you'd need filter+map
List<String> emails2 = users.stream()
    .map(User::getEmail)
    .filter(Optional::isPresent)
    .map(Optional::get)
    .toList();

import java.util.*;
import java.util.stream.*;

// From collections
Stream<String> s1 = List.of("a", "b").stream();
Stream<String> s2 = Set.of("x").stream();

// From arrays
int[] nums = {1, 2, 3};
IntStream s3 = Arrays.stream(nums);
Stream<String> s4 = Arrays.stream(new String[]{"a", "b"});

// Static factory methods
Stream<Integer> s5 = Stream.of(1, 2, 3);
Stream<Integer> s6 = Stream.empty();
Stream<Integer> s7 = Stream.iterate(1, n -> n * 2);  // infinite
Stream<Integer> s8 = Stream.iterate(1, n -> n < 100, n -> n + 1);  // bounded (Java 9+)
Stream<Double> s9 = Stream.generate(Math::random);  // infinite
Stream<String> s10 = Stream.ofNullable(null);  // 0 or 1 element (Java 9+)

// From functions (infinite, must limit)
List<Integer> powers = Stream.iterate(1, n -> n * 2)
    .limit(10)
    .toList();

// Numeric ranges
IntStream range = IntStream.range(0, 5);       // 0,1,2,3,4
IntStream closed = IntStream.rangeClosed(1, 5); // 1,2,3,4,5

import java.util.*;
import java.util.stream.*;

List<Integer> nums = List.of(3, 1, 4, 1, 5, 9, 2, 6, 5);

// filter: keep matching
List<Integer> evens = nums.stream()
    .filter(n -> n % 2 == 0).toList();

// map: transform
List<String> labels = nums.stream()
    .map(n -> "n=" + n).toList();

// flatMap: one-to-many
List<Integer> expanded = List.of(List.of(1, 2), List.of(3))
    .stream().flatMap(List::stream).toList();  // [1,2,3]

// distinct: remove duplicates
List<Integer> uniq = nums.stream().distinct().toList();

// sorted
List<Integer> asc = nums.stream().sorted().toList();
List<Integer> desc = nums.stream().sorted(Comparator.reverseOrder()).toList();

// peek: inspect (mainly for debugging)
nums.stream().peek(n -> System.out.println("seen " + n)).count();

// limit / skip
List<Integer> first3 = nums.stream().limit(3).toList();
List<Integer> after2 = nums.stream().skip(2).toList();

// takeWhile / dropWhile (Java 9+)
List<Integer> lt5 = nums.stream().takeWhile(n -> n < 5).toList();

import java.util.*;
import java.util.stream.*;

record Person(String name, String city, int age) {}

List<Person> people = List.of(
    new Person("Alice", "NYC", 30),
    new Person("Bob", "LA", 25),
    new Person("Carol", "NYC", 35),
    new Person("Dave", "LA", 40)
);

// groupingBy: Map<key, List<item>>
Map<String, List<Person>> byCity = people.stream()
    .collect(Collectors.groupingBy(Person::city));
// {NYC=[Alice,Carol], LA=[Bob,Dave]}

// groupingBy with downstream collector
Map<String, Long> countByCity = people.stream()
    .collect(Collectors.groupingBy(Person::city, Collectors.counting()));

Map<String, Integer> sumAgeByCity = people.stream()
    .collect(Collectors.groupingBy(Person::city,
        Collectors.summingInt(Person::age)));

Map<String, List<String>> namesByCity = people.stream()
    .collect(Collectors.groupingBy(Person::city,
        Collectors.mapping(Person::name, Collectors.toList())));

// partitioningBy: Map<Boolean, List> (2 buckets)
Map<Boolean, List<Person>> byAge = people.stream()
    .collect(Collectors.partitioningBy(p -> p.age() >= 30));

// groupingBy with TreeMap for sorted keys
Map<String, List<Person>> sorted = people.stream()
    .collect(Collectors.groupingBy(Person::city, TreeMap::new, Collectors.toList()));

import java.util.*;
import java.util.stream.*;

List<Integer> nums = List.of(1, 2, 3, 4, 5);

// reduce: combine all into one
int sum = nums.stream().reduce(0, Integer::sum);          // 15
Optional<Integer> product = nums.stream().reduce((a, b) -> a * b);  // Optional[120]

// Built-in summary collectors
IntSummaryStatistics stats = nums.stream()
    .mapToInt(Integer::intValue)
    .summaryStatistics();
// stats.getCount(), getSum(), getMin(), getMax(), getAverage()

// Common terminal collectors
long count = nums.stream().collect(Collectors.counting());
double avg = nums.stream().collect(Collectors.averagingInt(Integer::intValue));
int max = nums.stream().collect(Collectors.maxBy(Comparator.naturalOrder())).orElse(0);

// join strings
String joined = List.of("a", "b", "c").stream()
    .collect(Collectors.joining(", ", "[", "]"));  // "[a, b, c]"

// teeing (Java 12+): two collectors, merge results
record Range(int min, int max) {}
Range range = nums.stream().collect(Collectors.teeing(
    Collectors.minBy(Comparator.naturalOrder()),
    Collectors.maxBy(Comparator.naturalOrder()),
    (mn, mx) -> new Range(mn.orElse(0), mx.orElse(0))
));

import java.util.stream.*;
import java.util.*;

// IntStream / LongStream / DoubleStream avoid boxing
IntStream range = IntStream.rangeClosed(1, 100);
int sum = range.sum();                       // 5050
double avg = IntStream.of(1, 2, 3).average().orElse(0);

// mapToInt / mapToLong / mapToDouble from object stream
int totalAge = people().stream().mapToInt(Person::age).sum();

// boxed: convert primitive stream back to object stream
List<Integer> list = IntStream.range(0, 5).boxed().toList();

// mapToObj: primitive -> objects
List<String> labels = IntStream.range(1, 4)
    .mapToObj(i -> "item-" + i).toList();

// asLongStream / asDoubleStream
LongStream longs = IntStream.range(0, 5).asLongStream();

// Common numeric operations
int max = IntStream.of(3, 1, 4, 1, 5).max().orElse(Integer.MIN_VALUE);
boolean anyEven = IntStream.of(1, 3, 5).anyMatch(n -> n % 2 == 0);

// Iterate to build numeric sequences
List<Integer> fib = Stream.iterate(new int[]{0, 1}, a -> new int[]{a[1], a[0] + a[1]})
    .limit(10).mapToInt(a -> a[0]).boxed().toList();

List<Person> people() { return List.of(new Person("a", "c", 30)); }
record Person(String name, String city, int age) {}

import java.util.*;
import java.util.stream.*;

// parallelStream: uses common ForkJoinPool
long sum = List.of(1, 2, 3, 4, 5).parallelStream()
    .mapToInt(Integer::intValue).sum();

// Convert sequential to parallel
long count = IntStream.range(0, 1_000_000).parallel()
    .filter(n -> n % 2 == 0).count();

// Order may differ — use forEachOrdered if order matters
List.of(1, 2, 3, 4).parallelStream()
    .forEachOrdered(System.out::println);

// Collecting preserves encounter order (but work is parallel)
List<Integer> doubled = IntStream.range(0, 1000).parallel()
    .map(n -> n * 2).boxed().toList();

// Custom thread pool (avoid blocking the common pool)
import java.util.concurrent.ForkJoinPool;
ForkJoinPool pool = new ForkJoinPool(8);
int result = pool.submit(() ->
    IntStream.range(0, 1000).parallel().sum()
).get();

// WHEN to use parallel: large dataset, CPU-heavy per element,
// order-independent, stateless operations
// WHEN NOT: small data, I/O-bound, shared mutable state, ordered ops

// Generic class
public class Box<T> {
    private T value;
    public void set(T v) { value = v; }
    public T get() { return value; }
}

Box<String> strBox = new Box<>();
strBox.set("hello");
String s = strBox.get();  // no cast needed

// Multiple type parameters
public class Pair<K, V> {
    private final K key;
    private final V value;
    public Pair(K k, V v) { key = k; value = v; }
    public K key() { return key; }
    public V value() { return value; }
}

Pair<String, Integer> p = new Pair<>("age", 30);

// Generic method (independent of class type params)
public static <T> T first(List<T> list) {
    return list.get(0);
}

// Generic method with multiple type params
public static <K, V> Map<K, V> zip(List<K> keys, List<V> values) {
    Map<K, V> m = new HashMap<>();
    for (int i = 0; i < keys.size(); i++) m.put(keys.get(i), values.get(i));
    return m;
}

// Upper bound: T must be a subtype of Number
public static <T extends Number> double sum(List<T> nums) {
    double total = 0;
    for (Number n : nums) total += n.doubleValue();
    return total;
}

sum(List.of(1, 2, 3));        // Integer is a Number
sum(List.of(1.0, 2.5));       // Double is a Number
// sum(List.of("a"));          // compile error

// Multiple bounds: T must extend all (first is class, rest interfaces)
interface Comparable<T> { int compareTo(T o); }
interface Serializable {}

public static <T extends Number & Comparable<T> & Serializable>
    T max(List<T> list) {
    T best = list.get(0);
    for (T t : list) if (t.compareTo(best) > 0) best = t;
    return best;
}

// Bound lets you call methods of the bound
public static <T extends CharSequence> int totalLength(List<T> items) {
    int len = 0;
    for (CharSequence c : items) len += c.length();  // can call .length()
    return len;
}

import java.util.*;

// ? (unbounded wildcard) — any type
void printAll(List<?> list) {
    for (Object o : list) System.out.println(o);
}

// ? extends T (upper-bounded / covariant) — producer (PECS: Producer Extends)
double sum(List<? extends Number> nums) {
    double total = 0;
    for (Number n : nums) total += n.doubleValue();
    return total;
}
sum(List.of(1, 2, 3));     // List<Integer> OK
sum(List.of(1.0, 2.0));    // List<Double> OK
// nums.add(5);  // ERROR: can't add (don't know exact type)

// ? super T (lower-bounded / contravariant) — consumer (PECS: Consumer Super)
void addNumbers(List<? super Integer> list) {
    list.add(1); list.add(2); list.add(3);  // OK to add Integer
}
addNumbers(new ArrayList<Number>());  // OK
addNumbers(new ArrayList<Object>());  // OK
// Number n = list.get(0);  // only safe to read as Object

// PECS rule: Producer Extends, Consumer Super
// If you read from a collection, use ? extends T
// If you write to a collection, use ? super T
public static <T> void copy(List<? super T> dest, List<? extends T> src) {
    for (T t : src) dest.add(t);
}

import java.util.*;

// At runtime, generic types are erased to their bounds (or Object)
// List<String>, List<Integer>, List<?> all become List at runtime

List<String> strings = new ArrayList<>();
List<Integer> ints = new ArrayList<>();
// Runtime: both are just ArrayList

// You CANNOT do these due to erasure:
// new T()              — can't instantiate type param
// new T[]              — can't create generic array
// instanceof List<String>  — only instanceof List (raw)
// class MyException<T> extends Exception  — can't extend Throwable generically
// static T field       — no static generic fields

// Erasure means overloads clash:
// void process(List<String> list) {}
// void process(List<Integer> list) {}  // ERROR: same erasure

// Checking types at runtime requires Class<T>
public static <T> List<T> filter(List<?> items, Class<T> type) {
    List<T> result = new ArrayList<>();
    for (Object o : items) {
        if (type.isInstance(o)) result.add(type.cast(o));
    }
    return result;
}

// Heap pollution: when unchecked warnings lead to runtime ClassCastException
List<String> polluted = (List<String>)(List) List.of(1, 2);  // unchecked
// String s = polluted.get(0);  // ClassCastException at runtime

import java.util.*;

// Type inference: compiler figures out T from arguments
public static <T> T pick(T a, T b) { return Math.random() > 0.5 ? a : b; }
String s = pick("hello", "world");      // T inferred as String
Number n = pick(1, 2.0);                // T inferred as Number (common supertype)

// Target typing: inference uses the expected type
List<String> list = Collections.emptyList();  // T inferred from target

// Inference with method chains
List<Integer> nums = List.of(1, 2, 3);
String joined = nums.stream()
    .map(Object::toString)              // Stream<String>
    .collect(Collectors.joining(","));  // inferred

// Explicit type witness (rarely needed)
Collections.<String>emptyList();

// Generic constructor
class Holder<T> {
    private T value;
    <U extends T> Holder(U init) { value = init; }  // constructor type param
    T get() { return value; }
}
Holder<Number> h = new Holder<>(42);  // U=Integer, T=Number

import java.util.stream.Collectors;
class Math { static double random() { return 0.5; } }

// Generic interface
interface Repository<T, ID> {
    Optional<T> findById(ID id);
    List<T> findAll();
    void save(T entity);
}

// Implement with concrete types
class UserRepository implements Repository<User, Long> {
    public Optional<User> findById(Long id) { /* ... */ return Optional.empty(); }
    public List<User> findAll() { return List.of(); }
    public void save(User entity) {}
}

// Generic interface with self-referencing bound (Comparable pattern)
interface Comparable<T> {
    int compareTo(T other);
}
class Temperature implements Comparable<Temperature> {
    private final double celsius;
    Temperature(double c) { celsius = c; }
    public int compareTo(Temperature other) {
        return Double.compare(celsius, other.celsius);
    }
}

// Generic builder pattern
class Builder<T> {
    private T value;
    public Builder<T> set(T v) { value = v; return this; }
    public T build() { return value; }
}

// Type token pattern for runtime type safety
class TypeSafeMap {
    private final Map<Class<?>, Object> map = new HashMap<>();
    public <T> void put(Class<T> type, T value) { map.put(type, value); }
    public <T> T get(Class<T> type) { return type.cast(map.get(type)); }
}

record User(String name) {}
import java.util.Optional;

import java.util.*;

// @Override: declares intent to override (compiler checks)
class Animal {
    public void sound() { System.out.println("..."); }
}
class Dog extends Animal {
    @Override
    public void sound() { System.out.println("Woof"); }
}

// @Deprecated: marks API as outdated
class OldApi {
    @Deprecated(since = "1.5", forRemoval = true)
    public void legacy() {}
}

// @SuppressWarnings: silence compiler warnings
@SuppressWarnings("unchecked")
List<String> list = (List<String>) new ArrayList();

// @FunctionalInterface: enforces single abstract method
@FunctionalInterface
interface Op { int apply(int a, int b); }

// Common warning keys: unchecked, deprecation, rawtypes, null

// Java 17+ sealed-related
@Deprecated
class ToRemove {}

import java.lang.annotation.*;

// Define an annotation
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Test {
    String value() default "";
    long timeout() default 0L;
}

// Use it
class MyTests {
    @Test
    public void quickCheck() {}

    @Test(timeout = 5000)
    public void slowCheck() {}

    @Test("custom-name")
    public void named() {}
}

// Annotation with default values
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.FIELD})
public @interface Entity {
    String table() default "";
    String[] columns() default {};
}

@Entity(table = "users", columns = {"id", "name"})
class User {}

// Marker annotation (no members)
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Service {}

import java.lang.annotation.*;

// RetentionPolicy.SOURCE: discarded by compiler (e.g., @Override)
@Retention(RetentionPolicy.SOURCE)
@interface CompileOnly {}

// RetentionPolicy.CLASS: kept in .class but not loaded (default)
@Retention(RetentionPolicy.CLASS)
@interface BytecodeOnly {}

// RetentionPolicy.RUNTIME: available via reflection at runtime
@Retention(RetentionPolicy.RUNTIME)
@interface RuntimeVisible {}

// ElementType targets
@Target(ElementType.TYPE)         // classes, interfaces, enums
@interface ForType {}

@Target(ElementType.METHOD)
@interface ForMethod {}

@Target(ElementType.FIELD)
@interface ForField {}

@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@interface ForTypeUse {}

// Java 8+ type-use annotations (annotate any type occurrence)
@RuntimeVisible String[] names;  // example usage
List<@RuntimeVisible String> typed;

import java.lang.annotation.*;
import java.lang.reflect.*;
import java.util.*;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface Route {
    String path();
    String method() default "GET";
}

class Api {
    @Route(path = "/users", method = "GET")
    public void listUsers() {}

    @Route(path = "/users", method = "POST")
    public void createUser() {}
}

// Scan methods for @Route at runtime
for (Method m : Api.class.getDeclaredMethods()) {
    if (m.isAnnotationPresent(Route.class)) {
        Route r = m.getAnnotation(Route.class);
        System.out.println(r.method() + " " + r.path() + " -> " + m.getName());
    }
}
// Output:
// GET /users -> listUsers
// POST /users -> createUser

// Reading annotations on a class
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface Table { String name(); }

@Table(name = "orders")
class Order {}
Table t = Order.class.getAnnotation(Table.class);
System.out.println(t.name());  // "orders"

import java.lang.annotation.*;

// Repeatable: allow same annotation multiple times
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface Schedule {
    String cron();
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface Schedules {
    Schedule[] value();  // container annotation
}

// Make Schedule repeatable
@Repeatable(Schedules.class)
@interface Schedule2 {
    String cron();
}

// Now you can repeat it (Java 8+)
class Job {
    @Schedule2(cron = "0 0 * * *")
    @Schedule2(cron = "0 30 * * *")
    public void run() {}
}

// Meta-annotations: annotations on annotations
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)  // can only annotate other annotations
@interface TestCategory {}

@TestCategory
@Retention(RetentionPolicy.RUNTIME)
@interface UnitTest {}

// Inherited: subclass inherits the annotation
@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface Persistent {}
@Persistent class Base {}
class Child extends Base {}  // Child also has @Persistent

// Documented: appears in Javadoc
@Documented
@interface PublicApi {}

import java.lang.reflect.*;

// Three ways to get a Class object
Class<String> c1 = String.class;                  // class literal
Class<?> c2 = "hello".getClass();                 // from instance
Class<?> c3 = Class.forName("java.lang.String");  // by name (throws checked)

// Basic introspection
Class<?> c = String.class;
System.out.println(c.getName());          // "java.lang.String"
System.out.println(c.getSimpleName());    // "String"
System.out.println(c.getPackage());       // "package java.lang"
System.out.println(c.getSuperclass());    // "class java.lang.Object"
System.out.println(Modifier.toString(c.getModifiers()));  // "public final"

// Check type relationships
System.out.println(c.isInterface());      // false
System.out.println(c.isArray());          // false
System.out.println(c.isEnum());           // false
System.out.println(c.isRecord());         // false (Java 16+)
System.out.println(CharSequence.class.isAssignableFrom(c));  // true

// Primitive class objects
Class<?> intClass = int.class;
Class<?> intArrayClass = int[].class;
System.out.println(intClass.isPrimitive());  // true

import java.lang.reflect.*;
import java.util.*;

class Sample {
    public String name;
    private int count;
    public Sample() {}
    public Sample(String n) { name = n; }
    private void secret() {}
    public int compute(int x) { return x * 2; }
}

Class<?> c = Sample.class;

// Fields: getDeclaredFields includes private; getFields only public
for (Field f : c.getDeclaredFields()) {
    System.out.println(f.getName() + " : " + f.getType().getSimpleName()
        + " (" + Modifier.toString(f.getModifiers()) + ")");
}

// Methods
for (Method m : c.getDeclaredMethods()) {
    System.out.println(m.getName()
        + " params=" + Arrays.toString(m.getParameterTypes())
        + " returns=" + m.getReturnType().getSimpleName());
}

// Constructors
for (Constructor<?> ctor : c.getConstructors()) {
    System.out.println("ctor params=" + Arrays.toString(ctor.getParameterTypes()));
}

// Lookup specific member
Field nameField = c.getDeclaredField("name");
Method compute = c.getDeclaredMethod("compute", int.class);
Constructor<?> ctor = c.getDeclaredConstructor(String.class);

import java.lang.reflect.*;

class Greeter {
    public String greet(String name) { return "Hello, " + name; }
    private String secret() { return "hidden"; }
}

Object obj = new Greeter();
Class<?> c = obj.getClass();

// Invoke public method
Method greet = c.getMethod("greet", String.class);
String result = (String) greet.invoke(obj, "Alice");  // "Hello, Alice"

// Invoke private method
Method sec = c.getDeclaredMethod("secret");
sec.setAccessible(true);  // bypass access check
String s = (String) sec.invoke(obj);  // "hidden"

// Create instances via constructor
Constructor<?> noArg = c.getConstructor();
Object o1 = noArg.newInstance();

// With args
class Person {
    String name;
    public Person(String n) { name = n; }
    public String toString() { return "Person(" + name + ")"; }
}
Constructor<?> ctor = Person.class.getConstructor(String.class);
Object p = ctor.newInstance("Bob");
System.out.println(p);  // "Person(Bob)"

// Array creation via reflection
Object strArray = Array.newInstance(String.class, 5);
Array.set(strArray, 0, "first");
String v = (String) Array.get(strArray, 0);

import java.lang.reflect.*;

class Config {
    public String env = "dev";
    private int retries = 3;
    public static String VERSION = "1.0";
}

Config cfg = new Config();
Class<?> c = cfg.getClass();

// Read public field
Field env = c.getField("env");
String e = (String) env.get(cfg);  // "dev"

// Read private field
Field retries = c.getDeclaredField("retries");
retries.setAccessible(true);
int r = retries.getInt(cfg);  // 3
// For objects: Object val = field.get(instance);

// Modify fields
env.set(cfg, "prod");
retries.setInt(cfg, 5);
System.out.println(cfg.env);     // "prod"

// Static fields: pass null as the instance
Field version = c.getField("VERSION");
String v = (String) version.get(null);  // "1.0"
version.set(null, "2.0");

// Type-specific getters/setters avoid boxing
// getInt/setInt, getLong/setLong, getBoolean/setBoolean, etc.
// For reference types, use get()/set()

import java.lang.reflect.*;
import java.util.*;

// JDK dynamic proxy: implements interfaces at runtime
interface UserService {
    String getUser(long id);
    void deleteUser(long id);
}

// InvocationHandler intercepts every method call
class LoggingHandler implements InvocationHandler {
    private final Object target;
    LoggingHandler(Object t) { target = t; }

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("Calling " + method.getName() + "(" + Arrays.toString(args) + ")");
        long start = System.nanoTime();
        Object result = method.invoke(target, args);
        System.out.println("  -> " + result + " (" + (System.nanoTime() - start) + "ns)");
        return result;
    }
}

UserService real = id -> "user-" + id;
UserService proxied = (UserService) Proxy.newProxyInstance(
    UserService.class.getClassLoader(),
    new Class<?>[]{UserService.class},
    new LoggingHandler(real)
);
proxied.getUser(42);  // logs the call and result

// Common reflection use cases:
// - Frameworks: Spring DI, JPA entities, Jackson serialization
// - AOP/proxies: transactions, logging, security
// - Annotation processing at runtime
// - Test frameworks: JUnit discovers @Test methods
// - Code generation tools

// Caution: reflection is slower, bypasses compile-time checks,
// and can break under modules (--add-opens). Prefer alternatives when possible.

import java.sql.*;

// Basic connection (try-with-resources auto-closes)
String url = "jdbc:postgresql://localhost:5432/mydb";
try (Connection conn = DriverManager.getConnection(url, "user", "pass")) {
    System.out.println("Connected: " + conn.getSchema());
    // ... use connection
}

// Modern: no need for Class.forName() with JDBC 4+ (auto-discovery)
// Legacy: Class.forName("org.postgresql.Driver");

// Common URL patterns:
// jdbc:postgresql://host:5432/db
// jdbc:mysql://host:3306/db
// jdbc:oracle:thin:@host:1521:db
// jdbc:sqlite:/path/to/db.sqlite
// jdbc:h2:mem:test  (in-memory H2)

// Connection properties
import java.util.Properties;
Properties props = new Properties();
props.setProperty("user", "user");
props.setProperty("password", "pass");
props.setProperty("ssl", "true");
try (Connection c = DriverManager.getConnection(url, props)) {
    // ...
}

import java.sql.*;

// Statement: plain SQL (vulnerable to injection — avoid for user input)
try (Connection c = DriverManager.getConnection("jdbc:h2:mem:", "sa", "");
     Statement st = c.createStatement()) {

    st.execute("CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(100))");
    st.execute("INSERT INTO users VALUES (1, 'Alice')");

    // NEVER do this with user input — SQL injection!
    // st.execute("SELECT * FROM users WHERE name = '" + userInput + "'");
}

// PreparedStatement: parameterized, safe from injection, can be reused
String sql = "INSERT INTO users (id, name) VALUES (?, ?)";
try (Connection c = DriverManager.getConnection("jdbc:h2:mem:", "sa", "");
     PreparedStatement ps = c.prepareStatement(sql)) {

    // Set parameters by index (1-based!)
    ps.setInt(1, 1);
    ps.setString(2, "Alice");
    ps.executeUpdate();

    // Reuse with different params
    ps.setInt(1, 2);
    ps.setString(2, "Bob");
    ps.executeUpdate();

    // Batch inserts
    for (int i = 3; i <= 100; i++) {
        ps.setInt(1, i);
        ps.setString(2, "user" + i);
        ps.addBatch();
    }
    ps.executeBatch();
}

import java.sql.*;
import java.util.*;

record User(int id, String name, String email) {}

// Execute query and map rows
String sql = "SELECT id, name, email FROM users WHERE active = ?";
List<User> users = new ArrayList<>();
try (Connection c = getConnection();
     PreparedStatement ps = c.prepareStatement(sql)) {
    ps.setBoolean(1, true);
    try (ResultSet rs = ps.executeQuery()) {
        while (rs.next()) {  // advance to next row
            users.add(new User(
                rs.getInt("id"),       // by column name
                rs.getString("name"),
                rs.getString("email")  // by column name (preferred)
            ));
        }
    }
}

// Column access by index (1-based) or name (more readable)
// rs.getInt(1), rs.getString(2), rs.getBoolean("active")

// Handle NULLs
String nick = rs.getString("nickname");
if (rs.wasNull()) nick = "anonymous";  // distinguish NULL from real null

// Scrollable/updatable ResultSet (needs specific flags)
Statement st = c.createStatement(
    ResultSet.TYPE_SCROLL_INSENSITIVE,
    ResultSet.CONCUR_UPDATABLE
);
ResultSet rs = st.executeQuery("SELECT * FROM users");
rs.absolute(5);  // jump to row 5
rs.updateString("name", "newname");
rs.updateRow();  // persist change

Connection getConnection() throws SQLException {
    return DriverManager.getConnection("jdbc:h2:mem:", "sa", "");
}

import java.sql.*;

// Transactions: disabled by default (auto-commit = true)
try (Connection c = getConnection()) {
    c.setAutoCommit(false);  // start transaction
    try (PreparedStatement ps = c.prepareStatement(
            "UPDATE accounts SET balance = balance - ? WHERE id = ?")) {
        ps.setInt(1, 100); ps.setInt(2, 1); ps.executeUpdate();  // debit
        ps.setInt(1, -100); ps.setInt(2, 2); ps.executeUpdate();  // credit
    }
    c.commit();  // commit both

    // If any step fails, rollback
} catch (SQLException e) {
    // connection auto-closed; rollback happens implicitly on close if not committed
}

// Explicit rollback pattern
try (Connection c = getConnection()) {
    c.setAutoCommit(false);
    try {
        // ... multiple statements
        c.commit();
    } catch (SQLException e) {
        c.rollback();  // undo all changes in this transaction
        throw e;
    }
}

// Savepoints: partial rollback
Statement st = c.createStatement();
st.execute("INSERT INTO log VALUES (1)");
Savepoint sp = c.setSavepoint("before-risky");
st.execute("INSERT INTO log VALUES (2)");
c.rollback(sp);  // undo only after savepoint
c.commit();

// Transaction isolation levels
c.setTransactionIsolation(Connection.TRANSACTION_READ_COMMITTED);
// Levels: NONE, READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, SERIALIZABLE

Connection getConnection() throws SQLException {
    return DriverManager.getConnection("jdbc:h2:mem:", "sa", "");
}

import com.zaxxer.hikari.*;
import java.sql.*;
import java.util.*;

// HikariCP: high-performance JDBC connection pool
HikariConfig config = new HikariConfig();
config.setJdbcUrl("jdbc:postgresql://localhost:5432/mydb");
config.setUsername("user");
config.setPassword("pass");
config.setMaximumPoolSize(10);
config.setMinimumIdle(2);
config.setConnectionTimeout(30_000);   // ms to wait for a connection
config.setIdleTimeout(600_000);        // ms before idle connections close
config.setMaxLifetime(1_800_000);      // ms max connection lifetime
config.setPoolName("app-pool");

HikariDataSource ds = new HikariDataSource(config);

// Borrow a connection, use it, return it (auto via try-with-resources)
try (Connection c = ds.getConnection();
     PreparedStatement ps = c.prepareStatement("SELECT * FROM users")) {
    try (ResultSet rs = ps.executeQuery()) {
        while (rs.next()) System.out.println(rs.getString("name"));
    }
}  // connection returns to pool here

// Shutdown pool when app stops
ds.close();

// Why pool? Opening a TCP+auth connection is expensive (~10-100ms).
// A pool keeps connections warm and reuses them across requests,
// dramatically reducing latency under load.

// Alternative pools: Apache DBCP, c3p0, Tomcat JDBC, Agroal

import java.io.*;

// Read bytes from a file
try (InputStream in = new FileInputStream("input.bin")) {
    int b;
    while ((b = in.read()) != -1) {  // -1 = end of stream
        // process byte
    }
}

// Buffered for performance (8KB default buffer)
try (InputStream in = new BufferedInputStream(new FileInputStream("big.bin"))) {
    byte[] buffer = new byte[8192];
    int read;
    while ((read = in.read(buffer)) != -1) {
        // process buffer[0..read]
    }
}

// readAllBytes (small files only — loads everything into memory)
byte[] all = new FileInputStream("small.bin").readAllBytes();

// Write bytes
try (OutputStream out = new BufferedOutputStream(new FileOutputStream("out.bin"))) {
    out.write(65);  // single byte
    out.write(new byte[]{66, 67, 68});
    out.flush();  // force buffered data to disk
}

// Copy streams (Java 9+)
try (InputStream in = new FileInputStream("src.bin");
     OutputStream out = new FileOutputStream("dst.bin")) {
    in.transferTo(out);  // efficient bulk copy
}

// Standard streams
System.in.read();   // stdin (InputStream)
System.out.write(65);  // stdout (PrintStream)

import java.nio.*;
import java.nio.channels.*;
import java.nio.file.*;
import java.io.*;

// Channel: high-performance, block-oriented I/O
try (FileChannel ch = FileChannel.open(Path.of("data.bin"),
        StandardOpenOption.READ, StandardOpenOption.WRITE)) {

    // ByteBuffer: fixed-capacity block of bytes
    ByteBuffer buf = ByteBuffer.allocate(1024);
    int read = ch.read(buf);  // fill buffer from channel
    buf.flip();  // switch from write mode to read mode

    while (buf.hasRemaining()) {
        byte b = buf.get();
    }
    buf.clear();  // reset for next read (or compact() to preserve unread)
}

// Direct buffer: outside JVM heap, faster for large I/O (no copy)
ByteBuffer direct = ByteBuffer.allocateDirect(64 * 1024);

// Scatter/gather: read into multiple buffers / write from multiple
ByteBuffer header = ByteBuffer.allocate(128);
ByteBuffer body = ByteBuffer.allocate(1024);
ch.read(new ByteBuffer[]{header, body});  // scatter

// Transfer between channels (zero-copy on some OSes)
try (FileChannel src = FileChannel.open(Path.of("a.bin"));
     FileChannel dst = FileChannel.open(Path.of("b.bin"), StandardOpenOption.WRITE)) {
    src.transferTo(0, src.size(), dst);
}

// ByteOrder
buf.order(ByteOrder.LITTLE_ENDIAN);
int value = buf.getInt();

import java.nio.file.*;
import java.nio.file.attribute.*;
import java.io.IOException;

// Creating Path objects
Path p1 = Path.of("a/b/c.txt");          // relative
Path p2 = Path.of("/usr/local/bin");     // absolute
Path p3 = Paths.get("config.json");      // older API, same thing

// Path manipulation (doesn't touch filesystem)
System.out.println(p1.getFileName());    // "c.txt"
System.out.println(p1.getParent());      // "a/b"
System.out.println(p1.getRoot());        // "" (relative)
System.out.println(p2.getRoot());        // "/"
System.out.println(p1.toAbsolutePath()); // "/cwd/a/b/c.txt"
System.out.println(p1.normalize());      // removes . and ..
System.out.println(p1.resolve("d.txt")); // "a/b/c.txt/d.txt"
System.out.println(p1.resolveSibling("x.txt")); // "a/b/x.txt"
System.out.println(p1.relativize(Path.of("a/b")));  // "../.."

// File metadata
Path file = Path.of("notes.txt");
System.out.println(Files.size(file));            // bytes
System.out.println(Files.getLastModifiedTime(file));
System.out.println(Files.isReadable(file));
System.out.println(Files.isWritable(file));

// PosixFilePermissions (Linux/macOS)
String perms = "rwxr-xr--";
Set<PosixFilePermission> set = PosixFilePermissions.fromString(perms);
Files.setPosixFilePermissions(file, set);

// Symbolic links
Path link = Path.of("link.txt");
Files.createSymbolicLink(link, Path.of("target.txt"));
Path target = Files.readSymbolicLink(link);

import java.nio.file.*;
import java.io.IOException;
import java.util.stream.*;

// List directory entries (one level)
try (Stream<Path> entries = Files.list(Path.of("."))) {
    entries.filter(Files::isRegularFile)
           .map(Path::getFileName)
           .forEach(System.out::println);
}

// Walk file tree recursively (depth-first)
try (Stream<Path> walk = Files.walk(Path.of("/project"))) {
    walk.filter(Files::isRegularFile)
        .filter(p -> p.toString().endsWith(".java"))
        .forEach(System.out::println);
}

// Walk with depth limit
try (Stream<Path> walk = Files.walk(Path.of("/project"), 3)) {
    walk.forEach(System.out::println);
}

// Find with BiPredicate (path + attributes)
try (Stream<Path> found = Files.find(Path.of("/logs"), 10,
        (path, attrs) -> attrs.isRegularFile()
            && attrs.size() > 1_000_000
            && path.toString().endsWith(".log"))) {
    found.forEach(p -> System.out.println("Large log: " + p));
}

// FileVisitor for full control (pre/post visit, skip subtrees)
Files.walkFileTree(Path.of("/project"), new SimpleFileVisitor<>() {
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
        System.out.println("Visiting " + file);
        return FileVisitResult.CONTINUE;
    }
    public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {
        if (dir.getFileName().toString().equals("target")) {
            return FileVisitResult.SKIP_SUBTREE;  // skip target dirs
        }
        return FileVisitResult.CONTINUE;
    }
});

import java.nio.file.attribute.BasicFileAttributes;
import java.nio.file.attribute.BasicFileAttributes;

import java.nio.file.*;
import static java.nio.file.StandardWatchEventKinds.*;
import java.io.IOException;

// Watch a directory for changes
try (WatchService watcher = FileSystems.getDefault().newWatchService()) {
    Path dir = Path.of("/var/log");
    dir.register(watcher,
        ENTRY_CREATE,
        ENTRY_DELETE,
        ENTRY_MODIFY);

    System.out.println("Watching " + dir + "...");
    while (true) {
        WatchKey key = watcher.take();  // blocks until event
        for (WatchEvent<?> event : key.pollEvents()) {
            Path changed = dir.resolve((Path) event.context());
            System.out.println(event.kind() + " -> " + changed);

            if (event.kind() == OVERFLOW) continue;  // events lost

            // React to change
            if (event.kind() == ENTRY_CREATE) {
                System.out.println("New file: " + changed);
            }
        }
        if (!key.reset()) break;  // key no longer valid (dir deleted)
    }
} catch (InterruptedException e) {
    Thread.currentThread().interrupt();
}

// Note: WatchService watches directories, not individual files.
// To watch a file, watch its parent dir and filter by context().
// Events may be coalesced (rapid modifications may yield one event).
// For recursive watching, register each subdirectory manually
// or use a library like Apache Commons IO's FileAlterationMonitor.

// Record: concise immutable data carrier (Java 16+)
public record Point(int x, int y) {}

// Equivalent to writing:
// - final class with private final int x, y
// - constructor Point(int x, int y)
// - accessors x(), y()
// - equals, hashCode, toString (all auto-generated)

Point p = new Point(3, 4);
System.out.println(p.x());          // 3 (accessor, not getX())
System.out.println(p.y());          // 4
System.out.println(p);              // "Point[x=3, y=4]"
System.out.println(p.equals(new Point(3, 4)));  // true

// Immutable: no setters
// p.setX(5);  // no such method

// Custom record with validation
public record Age(int value) {
    public Age {
        if (value < 0 || value > 150) {
            throw new IllegalArgumentException("Invalid age: " + value);
        }
    }
}

// Record with multiple components
public record Employee(String name, int id, double salary, String dept) {}

// Records implement interfaces
public interface Named { String name(); }
public record Customer(String name, long id) implements Named {}

// Compact constructor: validation without reassigning
public record Email(String address) {
    public Email {
        if (!address.contains("@")) {
            throw new IllegalArgumentException("Bad email: " + address);
        }
        address = address.toLowerCase().strip();  // normalize
    }
}

// Add custom methods (but not mutable fields)
public record Money(double amount, String currency) {
    public Money plus(Money other) {
        if (!currency.equals(other.currency)) {
            throw new IllegalArgumentException("Currency mismatch");
        }
        return new Money(amount + other.amount, currency);
    }
    public Money times(int factor) {
        return new Money(amount * factor, currency);
    }
    public static Money usd(double amt) { return new Money(amt, "USD"); }
}

// Canonical + non-canonical constructors
public record Range(int start, int end) {
    // Compact canonical (validation)
    public Range {
        if (start > end) throw new IllegalArgumentException();
    }
    // Non-canonical convenience constructor
    public Range(int end) {
        this(0, end);
    }
}

// Static factories are common on records
public record Point(int x, int y) {
    public static Point origin() { return new Point(0, 0); }
    public static Point of(int x, int y) { return new Point(x, y); }
}

// Sealed class: restricts which classes can extend it (Java 17+)
public sealed interface Shape
    permits Circle, Square, Triangle {}

public record Circle(double radius) implements Shape {}
public record Square(double side) implements Shape {}
public record Triangle(double base, double height) implements Shape {}

// Every permitted subtype must be final, sealed, or non-sealed
public non-sealed class WeirdShape implements Shape {}  // open again

// Why sealed? Enables exhaustive pattern matching
public double area(Shape s) {
    return switch (s) {
        case Circle c -> Math.PI * c.radius() * c.radius();
        case Square sq -> sq.side() * sq.side();
        case Triangle t -> 0.5 * t.base() * t.height();
        // No default needed — compiler knows all subtypes
    };
}

// Sealed class hierarchy with records = algebraic data types
// Combines: closed inheritance (sealed) + immutable data (record)

// Sealed classes also work with classes (not just interfaces)
public sealed abstract class Result permits Success, Failure {}
public final class Success<T>(T value) extends Result {}
public final class Failure(String error) extends Result {}

// Old way: cast after instanceof
Object obj = "hello";
if (obj instanceof String) {
    String s = (String) obj;  // redundant cast
    System.out.println(s.length());
}

// Java 16+: pattern variable
if (obj instanceof String s) {
    System.out.println(s.length());  // s is in scope, no cast
}

// The variable is scoped by the pattern's truth
if (obj instanceof String s && s.length() > 3) {
    System.out.println("Long string: " + s);
}
// s is NOT in scope here if the && short-circuited

// Negation scoping
if (!(obj instanceof String s)) {
    // s NOT in scope here
    return;
}
// s IS in scope here (the instanceof must have been true)

// Combining with other patterns
Object data = 42;
String desc = switch (data) {
    case Integer i when i > 0 -> "positive int: " + i;
    case Integer i -> "non-positive int: " + i;
    case String s -> "string: " + s;
    case null -> "null";
    default -> "other";
};

// Java 21: switch pattern matching with type patterns and guards
sealed interface Event permits Login, Logout, Message, Error {}
record Login(String user) implements Event {}
record Logout(String user) implements Event {}
record Message(String from, String text) implements Event {}
record Error(int code, String detail) implements Event {}

String describe(Event e) {
    return switch (e) {
        case Login l -> l.user() + " logged in";
        case Logout l -> l.user() + " logged out";
        case Message m when m.from().equals("system") -> "[system] " + m.text();
        case Message m -> m.from() + ": " + m.text();
        case Error(int code, String detail) when code >= 500 -> "SERVER ERROR " + code;
        case Error(int code, String detail) -> "error " + code + ": " + detail;
        case null -> "no event";  // explicit null handling
    };
}

// Record patterns: destructure records in one go
record Point(int x, int y) {}
String classify(Object o) {
    return switch (o) {
        case Point(int x, int y) when x == y -> "diagonal";
        case Point(int x, int y) -> "point at (" + x + "," + y + ")";
        default -> "not a point";
    };
}

// Nested patterns
record Box(Point p) {}
String describe2(Object o) {
    return switch (o) {
        case Box(Point(int x, int y)) -> "box at " + x + "," + y;
        default -> "unknown";
    };
}

// File: src/com.example.app/module-info.java
module com.example.app {
    requires java.sql;           // depends on java.sql module
    requires transitive com.example.lib;  // re-export (consumers see it too)
    requires static java.annotation;  // compile-time only (optional at runtime)

    exports com.example.app.api;       // public API visible to all
    exports com.example.app.internal to com.example.test;  // qualified export

    opens com.example.app.model to com.fasterxml.jackson.databind;  // reflection only
    opens com.example.app.dynamic;  // open to all for reflection

    uses com.example.app.spi.Plugin;  // service consumer
    provides com.example.app.spi.Plugin with com.example.app.plugins.DefaultPlugin;  // service provider
}

// Key directives:
// requires: depends on another module
// exports: makes packages public to other modules
// opens: allows deep reflection (for frameworks like Jackson, Hibernate)
// uses/provides: service loader integration

// A module without module-info.java is an "unnamed module" (classpath behavior)

// Module A: com.example.library
module com.example.library {
    // Public API anyone can use
    exports com.example.library.api;

    // Internal package: only visible to specific modules
    exports com.example.library.internal to com.example.app;

    // Allow reflection for frameworks (Jackson, JPA)
    opens com.example.library.model;

    // Only specific module can reflect
    opens com.example.library.config to com.example.app;

    // Dependencies
    requires java.logging;
    requires transitive java.sql;  // consumers of A also get java.sql
}

// Module B: com.example.app (consumer)
module com.example.app {
    requires com.example.library;  // use A's exported packages
    // Note: transitive means java.sql is also available here

    requires com.fasterxml.jackson.databind;
}

// 'requires transitive X' means: any module requiring this one
// also reads X. Use when your exported API exposes X's types.

// 'opens' vs 'exports':
// exports: compile-time + runtime access to public members
// opens: runtime reflective access to ALL members (including private)

// Reflective access without 'opens' fails with InaccessibleObjectException
// in Java 16+ (strong encapsulation enforced by default).

// SPI (Service Provider Interface) pattern with modules

// 1. Define the service interface in an API module
module com.example.spi {
    exports com.example.spi;
}
package com.example.spi;
public interface Plugin {
    String name();
    void run();
}

// 2. Implement in a provider module
module com.example.plugin.impl {
    requires com.example.spi;
    provides com.example.spi.Plugin with com.example.plugin.impl.DefaultPlugin;
}
package com.example.plugin.impl;
import com.example.spi.Plugin;
public class DefaultPlugin implements Plugin {
    public String name() { return "default"; }
    public void run() { System.out.println("running"); }
}

// 3. Consume in an app module
module com.example.app {
    requires com.example.spi;
    uses com.example.spi.Plugin;  // declares intent to load services
}

// Loading services at runtime
import java.util.ServiceLoader;
ServiceLoader<Plugin> loader = ServiceLoader.load(Plugin.class);
for (Plugin p : loader) {
    System.out.println("Found: " + p.name());
    p.run();
}

// ServiceLoader is how JDBC drivers, SLF4J backends, and many
// plugin systems are discovered without compile-time dependencies.

// Compile a module
//   javac -d out/com.example.app \
//         --module-source-path src \
//         --module com.example.app

// Run a modular app
//   java --module-path out --module com.example.app/com.example.app.Main

// Package as a modular JAR (includes module-info.class)
//   jar --create --file app.jar --main-class com.example.app.Main -C out/com.example.app .

// Module path vs classpath:
// --module-path: modules with module-info, strong encapsulation enforced
// --class-path (or -cp): legacy "unnamed module", everything public, no encapsulation

// Mixing: modular JARs can be used on the classpath too (automatic module)
//   java -cp lib/app.jar:lib/dep.jar com.example.app.Main
// An automatic module: a JAR without module-info on the module path.
// Its name is derived from the JAR filename (Automatic-Module-Name in MANIFEST.MF
// gives an explicit name).

// Inspect a module JAR
//   jar --describe-module --file app.jar

// List observable modules
//   java --list-modules
//   java --describe-module java.sql

// Add opens at runtime for legacy reflection (escape hatch)
//   java --add-opens com.example.app/com.example.app.internal=ALL-UNNAMED

# jdeps: analyze dependencies (find unused, list required modules)

# List dependencies of a JAR
jdeps --module-path lib app.jar

# Generate module-info.java for an existing JAR
jdeps --generate-module-info ./out app.jar

# Show which JDK modules a JAR uses
jdeps --print-module-deps --ignore-missing-deps app.jar
# Output: java.base,java.logging,java.sql

# jlink: create a custom stripped-down JRE containing only needed modules
jlink \
  --module-path "$JAVA_HOME/jmods:./out" \
  --add-modules com.example.app \
  --output ./custom-jre \
  --strip-debug \
  --compress=zip-6 \
  --no-header-files \
  --no-man-pages \
  --launcher app=com.example.app/com.example.app.Main

# The custom JRE is self-contained:
#   ./custom-jre/bin/app   # launches the app
#   ./custom-jre/bin/java  # the stripped JVM

# Benefits of jlink:
# - Smaller distribution (only needed modules)
# - Faster startup (less to load)
# - No need to install Java on target machine
# - Can cross-target (different OS/arch) with matching jmods

# Common workflow:
# 1. jdeps to find required modules
# 2. jlink to build a custom runtime
# 3. Package app + custom JRE together (Docker image, installer)

import java.util.concurrent.locks.*;

// ReentrantLock: more flexible than synchronized
class Counter {
    private final ReentrantLock lock = new ReentrantLock();
    private int count = 0;

    public void increment() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock();  // MUST be in finally
        }
    }

    public int get() {
        lock.lock();
        try { return count; }
        finally { lock.unlock(); }
    }
}

// tryLock with timeout (avoids deadlock-induced hangs)
if (lock.tryLock(1, java.util.concurrent.TimeUnit.SECONDS)) {
    try { /* work */ } finally { lock.unlock(); }
}

// Fair lock (FIFO ordering, slower)
ReentrantLock fair = new ReentrantLock(true);

// ReadWriteLock: many readers OR one writer
class Cache {
    private final ReentrantReadWriteLock rw = new ReentrantReadWriteLock();
    private final Lock read = rw.readLock();
    private final Lock write = rw.writeLock();
    private java.util.Map<String, String> data = new java.util.HashMap<>();

    public String get(String key) {
        read.lock();
        try { return data.get(key); }
        finally { read.unlock(); }
    }
    public void put(String key, String val) {
        write.lock();
        try { data.put(key, val); }
        finally { write.unlock(); }
    }
}

import java.util.concurrent.*;
import java.util.*;

// ConcurrentHashMap: thread-safe HashMap (no null keys/values)
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.putIfAbsent("a", 2);  // only if absent
map.compute("a", (k, v) -> v == null ? 1 : v + 1);  // atomic update
map.merge("a", 1, Integer::sum);  // add 1 atomically
Integer val = map.getOrDefault("a", 0);

// CopyOnWriteArrayList: snapshot semantics, fast reads, slow writes
CopyOnWriteArrayList<String> listeners = new CopyOnWriteArrayList<>();
listeners.add("x");  // copies entire array
for (String l : listeners) { /* safe iteration, no ConcurrentModificationException */ }

// BlockingQueue: producer-consumer pattern
BlockingQueue<String> queue = new ArrayBlockingQueue<>(100);
// Producer
queue.put("task");        // blocks if full
boolean added = queue.offer("task", 1, TimeUnit.SECONDS);  // timed
// Consumer
String task = queue.take();  // blocks if empty
String polled = queue.poll(1, TimeUnit.SECONDS);

// ConcurrentLinkedQueue: non-blocking, unbounded
Queue<String> q = new ConcurrentLinkedQueue<>();
q.offer("a");

// SkipListMap / SkipListSet: concurrent sorted collections
ConcurrentNavigableMap<Integer, String> sorted = new ConcurrentSkipListMap<>();

import java.util.concurrent.TimeUnit;

import java.util.concurrent.*;
import java.util.*;

// CountDownLatch: one-shot gate, N threads must arrive before proceeding
CountDownLatch ready = new CountDownLatch(3);
List<String> results = Collections.synchronizedList(new ArrayList<>());

for (int i = 0; i < 3; i++) {
    final int id = i;
    new Thread(() -> {
        try { Thread.sleep(id * 100); } catch (InterruptedException e) {}
        results.add("worker-" + id);
        ready.countDown();  // signal done
    }).start();
}
ready.await();  // main thread blocks until count reaches 0
System.out.println("All done: " + results);

// CyclicBarrier: reusable barrier, threads wait for each other
CyclicBarrier barrier = new CyclicBarrier(3, () ->
    System.out.println("--- phase complete ---"));

Runnable worker = () -> {
    try {
        System.out.println(Thread.currentThread().getName() + " phase 1");
        barrier.await();  // wait for all 3
        System.out.println(Thread.currentThread().getName() + " phase 2");
        barrier.await();  // reusable for next phase
    } catch (Exception e) {}
};
for (int i = 0; i < 3; i++) new Thread(worker).start();

// Phaser: more flexible (variable parties, multiple phases)
Phaser phaser = new Phaser(3);
phaser.register();  // dynamically add a party
phaser.arriveAndAwaitAdvance();

import java.util.concurrent.*;

// Semaphore: limit concurrent access to N permits
Semaphore pool = new Semaphore(5);  // 5 concurrent allowed

pool.acquire();  // blocks until a permit is available
try {
    // critical section (at most 5 threads here at once)
    System.out.println("Working, permits left: " + pool.availablePermits());
} finally {
    pool.release();  // return the permit
}

// Try-acquire (non-blocking)
if (pool.tryAcquire()) {
    try { /* work */ } finally { pool.release(); }
} else {
    System.out.println("Too busy, try later");
}

// Timed acquire
if (pool.tryAcquire(1, TimeUnit.SECONDS)) {
    try { /* work */ } finally { pool.release(); }
}

// Use case: rate limiting, connection pools, parking lots

// Exchanger: two threads swap values
Exchanger<String> ex = new Exchanger<>();
new Thread(() -> {
    try {
        String got = ex.exchange("from-A");  // gives "from-A", receives "from-B"
        System.out.println("A got: " + got);
    } catch (InterruptedException e) {}
}).start();
String got = ex.exchange("from-B");  // gives "from-B", receives "from-A"
System.out.println("B got: " + got);

import java.util.concurrent.TimeUnit;

import java.util.concurrent.*;
import java.util.*;

ExecutorService pool = Executors.newFixedThreadPool(4);

// Async composition (like JS Promises)
CompletableFuture<String> future = CompletableFuture
    .supplyAsync(() -> "hello", pool)            // async supplier
    .thenApply(String::toUpperCase)              // transform
    .thenCompose(s -> CompletableFuture.supplyAsync(  // flatMap
        () -> s + " world"))
    .thenApply(s -> s + "!");

System.out.println(future.join());  // "HELLO world!"

// Combine two independent futures
CompletableFuture<Integer> a = CompletableFuture.supplyAsync(() -> 10);
CompletableFuture<Integer> b = CompletableFuture.supplyAsync(() -> 20);
CompletableFuture<Integer> sum = a.thenCombine(b, Integer::sum);
System.out.println(sum.join());  // 30

// Run multiple and wait for all (or any)
List<CompletableFuture<Integer>> futures = List.of(
    CompletableFuture.supplyAsync(() -> 1, pool),
    CompletableFuture.supplyAsync(() -> 2, pool),
    CompletableFuture.supplyAsync(() -> 3, pool)
);
CompletableFuture<Void> all = CompletableFuture.allOf(
    futures.toArray(new CompletableFuture[0]));
all.join();
List<Integer> results = futures.stream().map(CompletableFuture::join).toList();

// Error handling
CompletableFuture<String> safe = CompletableFuture
    .supplyAsync(() -> { throw new RuntimeException("boom"); })
    .exceptionally(ex -> "fallback: " + ex.getMessage());
System.out.println(safe.join());  // "fallback: boom"

// handle: both success and failure
CompletableFuture<String> handled = CompletableFuture
    .supplyAsync(() -> "ok")
    .handle((val, ex) -> ex == null ? val : "error");

// Timeout (Java 9+)
CompletableFuture<String> timed = CompletableFuture
    .supplyAsync(() -> { Thread.sleep(5000); return "slow"; })
    .orTimeout(1, TimeUnit.SECONDS)
    .exceptionally(ex -> "timed out");

import java.util.concurrent.*;
import java.util.*;

// Virtual threads: lightweight, cheap, millions possible
// Java 21 LTS feature — "Project Loom"

// Start a virtual thread
Thread vt = Thread.ofVirtual().start(() -> {
    System.out.println("Running on: " + Thread);
});

// Builder pattern
Thread vt2 = Thread.ofVirtual().name("worker-1").start(() -> {
    // blocking I/O here is fine — virtual thread yields, not the OS thread
});

// Per-thread factory
ThreadFactory factory = Thread.ofVirtual().factory();

// ExecutorService for virtual threads (Java 21+)
try (ExecutorService es = Executors.newVirtualThreadPerTaskExecutor()) {
    // Submit a million tasks — each gets its own virtual thread
    List<Future<String>> futures = new ArrayList<>();
    for (int i = 0; i < 1_000_000; i++) {
        final int id = i;
        futures.add(es.submit(() -> {
            Thread.sleep(100);  // blocking call is cheap on virtual threads
            return "done-" + id;
        }));
    }
    // virtual threads yield when blocking, so 1M concurrent is feasible
}

// Why virtual threads?
// - Platform threads (OS threads) are heavy (~1MB stack, kernel scheduling)
// - Virtual threads are user-mode, ~KB, scheduled by JVM on a small carrier pool
// - Blocking I/O on a virtual thread doesn't block a platform thread
// - Lets you write straightforward blocking code at scale (no reactive complexity)

// Best practice: use virtual threads for I/O-bound work,
// NOT for CPU-bound work (use platform threads / parallelStream).

import java.util.*;
import java.util.stream.*;

record Person(String name, int age) {}

List<Person> people = List.of(
    new Person("Alice", 30),
    new Person("Bob", 25),
    new Person("Carol", 30),
    new Person("Dave", 25)
);

// Comparator.comparing (key extractor)
people.stream().sorted(Comparator.comparing(Person::name)).toList();
people.stream().sorted(Comparator.comparingInt(Person::age)).toList();

// Reversed
people.stream().sorted(Comparator.comparing(Person::age).reversed()).toList();

// Chained (thenComparing): sort by age, then by name
List<Person> sorted = people.stream().sorted(
    Comparator.comparingInt(Person::age)
              .thenComparing(Person::name)
).toList();
// [Bob(25), Dave(25), Alice(30), Carol(30)]

// Nulls handling
Comparator<String> cmp = Comparator.nullsFirst(Comparator.naturalOrder());
List.of("b", null, "a").stream().sorted(cmp).toList();  // [null, a, b]

// Custom comparator
Comparator<Person> byNameLen = (a, b) -> a.name().length() - b.name().length();

// Mutable list sort
List<String> names = new ArrayList<>(List.of("charlie", "alice", "bob"));
names.sort(Comparator.naturalOrder());
// names = [alice, bob, charlie]

// Comparing with primitive specializations avoids boxing
Comparator<Person> byAge = Comparator.comparingInt(Person::age);

import java.util.*;

// Java 9+ immutable factories (List.of, Set.of, Map.of)
List<String> immutable = List.of("a", "b", "c");
Set<Integer> set = Set.of(1, 2, 3);
Map<String, Integer> map = Map.of("a", 1, "b", 2);
Map<String, Integer> bigMap = Map.ofEntries(
    Map.entry("x", 1), Map.entry("y", 2), Map.entry("z", 3)
);
// immutable.add("d");  // throws UnsupportedOperationException
// immutable.set(0, "z");  // throws
// nulls not allowed in these immutable collections

// Unmodifiable view (wraps an existing collection)
List<String> mutable = new ArrayList<>(List.of("a", "b"));
List<String> view = Collections.unmodifiableList(mutable);
// view.add("c");  // throws
mutable.add("c");  // but changes to backing list ARE visible in view
System.out.println(view);  // [a, b, c]

// CopyOf (Java 10+): creates immutable copy
List<String> copy = List.copyOf(mutable);  // independent immutable copy

// Arrays.asList: fixed-size view of an array
String[] arr = {"a", "b"};
List<String> fixed = Arrays.asList(arr);
fixed.set(0, "x");  // OK (writes through to array)
// fixed.add("c");  // throws (size fixed)

// To make a truly mutable copy:
List<String> mut = new ArrayList<>(Arrays.asList(arr));

import java.util.*;

// Queue: FIFO (offer/poll/peek)
Queue<String> queue = new LinkedList<>();
queue.offer("a"); queue.offer("b");
System.out.println(queue.peek());  // "a" (head)
System.out.println(queue.poll());  // "a" (remove head)

// Deque: double-ended (add/remove at both ends)
Deque<String> deque = new ArrayDeque<>();
deque.addFirst("a"); deque.addLast("b");
System.out.println(deque.peekFirst());  // "a"
System.out.println(deque.peekLast());   // "b"
deque.removeFirst(); deque.removeLast();

// ArrayDeque as a stack (push/pop/peek)
Deque<String> stack = new ArrayDeque<>();
stack.push("first");  // addFirst
stack.push("second");
System.out.println(stack.pop());  // "second" (LIFO)
System.out.println(stack.peek()); // "first"

// PriorityQueue: orders by Comparator (not insertion order)
PriorityQueue<Integer> pq = new PriorityQueue<>();
pq.offer(5); pq.offer(1); pq.offer(3);
System.out.println(pq.poll());  // 1 (smallest first)
System.out.println(pq.poll());  // 3

// With custom comparator
PriorityQueue<String> byLen = new PriorityQueue<>(Comparator.comparingInt(String::length));
byLen.offer("aaa"); byLen.offer("a"); byLen.offer("aa");
System.out.println(byLen.poll());  // "a"

// BlockingQueue implementations (concurrency): see concurrency-deep section
// ArrayDeque is faster than Stack/LinkedList for stack/queue use

import java.util.*;

Map<String, Integer> counts = new HashMap<>();

// getOrDefault: safe read with default
int n = counts.getOrDefault("missing", 0);  // 0, no null

// putIfAbsent: only set if not present
counts.putIfAbsent("a", 1);  // sets to 1
counts.putIfAbsent("a", 2);  // no change (already present)

// compute: recompute value for a key
counts.compute("a", (k, v) -> v == null ? 1 : v + 1);  // increment

// computeIfAbsent: lazy initialization (cache pattern)
Map<String, List<String>> groups = new HashMap<>();
groups.computeIfAbsent("key", k -> new ArrayList<>()).add("item");
// creates list if absent, then adds — atomic, no race in single thread

// computeIfPresent: update only if present
counts.computeIfPresent("a", (k, v) -> v > 0 ? v - 1 : null);  // decrement, remove at 0

// merge: combine existing and new value (great for counting)
counts.merge("word", 1, Integer::sum);  // word count pattern
counts.merge("word", 1, Integer::sum);  // now 2
counts.merge("word", 1, Integer::sum);  // now 3

// merge with removal: function returns null -> entry removed
counts.merge("word", 1, (old, v) -> old > 1 ? old - 1 : null);

// replaceAll: transform all values
Map<String, Integer> doubled = new HashMap<>(counts);
doubled.replaceAll((k, v) -> v * 2);

// Word frequency counter (idiomatic)
String text = "the cat the dog the bird";
Map<String, Integer> freq = new HashMap<>();
for (String w : text.split(" ")) {
    freq.merge(w, 1, Integer::sum);
}

import java.util.*;
import java.util.stream.*;

List<Integer> nums = new ArrayList<>(List.of(3, 1, 4, 1, 5, 9, 2, 6));

// Sorting
Collections.sort(nums);                    // in-place, natural order
Collections.sort(nums, Comparator.reverseOrder());
nums.sort(Comparator.naturalOrder());      // List.sort (preferred)

// Searching (list must be sorted first)
int idx = Collections.binarySearch(nums, 4);  // index, or negative if absent

// Shuffling & reversing
Collections.shuffle(nums);
Collections.reverse(nums);

// Frequency & disjoint
int freq = Collections.frequency(nums, 1);  // count occurrences
boolean dis = Collections.disjoint(List.of(1, 2), List.of(3, 4));  // true

// Min/max
int min = Collections.min(nums);
int max = Collections.max(nums, Comparator.reverseOrder());

// Singleton collections (immutable, single element)
Set<String> one = Collections.singleton("only");
List<Integer> oneList = Collections.singletonList(42);
Map<String, Integer> oneMap = Collections.singletonMap("k", 1);

// Empty collections (prefer over returning null)
List<Object> empty = Collections.emptyList();
Set<Object> emptySet = Collections.emptySet();

// nCopies (immutable list of n copies)
List<String> padding = Collections.nCopies(5, "x");  // [x,x,x,x,x]

// Synchronized wrappers (legacy — prefer concurrent collections)
List<String> sync = Collections.synchronizedList(new ArrayList<>());

// Checked wrappers (catch heap pollution at runtime)
List<String> checked = Collections.checkedList(new ArrayList<>(), String.class);
// checked.add(123);  // throws ClassCastException at the add site

import org.junit.jupiter.api.*;
import static org.junit.jupiter.api.Assertions.*;

class CalculatorTest {
    private Calculator calc;

    @BeforeAll
    static void setUpOnce() {
        // runs once before all tests (must be static)
        System.out.println("Starting CalculatorTest");
    }

    @BeforeEach
    void setUp() {
        // runs before each test
        calc = new Calculator();
    }

    @Test
    @DisplayName("2 + 2 should equal 4")
    void addsTwoNumbers() {
        assertEquals(4, calc.add(2, 2));
    }

    @Test
    void dividesByZeroThrows() {
        ArithmeticException ex = assertThrows(
            ArithmeticException.class,
            () -> calc.divide(1, 0)
        );
        assertEquals("Division by zero", ex.getMessage());
    }

    @AfterEach
    void tearDown() {
        // runs after each test (cleanup)
        calc = null;
    }

    @AfterAll
    static void tearDownOnce() {
        // runs once after all tests
    }

    @Disabled("until bug #42 is fixed")
    @Test
    void skippedTest() {}
}

class Calculator {
    int add(int a, int b) { return a + b; }
    int divide(int a, int b) {
        if (b == 0) throw new ArithmeticException("Division by zero");
        return a / b;
    }
}

import org.junit.jupiter.api.*;
import static org.junit.jupiter.api.Assertions.*;

class AssertionsTest {

    @Test
    void equality() {
        assertEquals(4, 2 + 2);
        assertEquals(4, 2 + 2, "math is broken");
        assertNotEquals(5, 2 + 2);
    }

    @Test
    void booleans() {
        assertTrue(5 > 3);
        assertFalse(5 < 3, "5 should not be less than 3");
    }

    @Test
    void nullness() {
        assertNull(null);
        assertNotNull(new Object());
    }

    @Test
    void sameInstance() {
        String a = "x";
        assertSame(a, a);          // ==
        assertNotSame(a, new String("x"));
    }

    @Test
    void collections() {
        assertIterableEquals(List.of(1, 2, 3), List.of(1, 2, 3));
        assertLinesMatch(List.of("a.*", "b"), List.of("abc", "b"));
    }

    @Test
    void groupedAssertions() {
        // All run even if one fails — reports all failures
        assertAll("person",
            () -> assertEquals("Alice", "Alice"),
            () -> assertEquals(30, 30),
            () -> assertNotNull("x")
        );
    }

    @Test
    void timeout() {
        // Fails if it takes longer than 100ms
        assertTimeout(Duration.ofMillis(100), () -> {
            Thread.sleep(10);
        });
        // assertTimeoutPreemptively: stops the task early (in another thread)
    }

    @Test
    void customMessage() {
        int result = 5;
        assertEquals(4, result, () -> "expected 4 but got " + result);
        // Supplier<String> — message built lazily only on failure
    }

    import java.time.Duration;
    import java.util.List;
}

import org.junit.jupiter.params.*;
import org.junit.jupiter.params.provider.*;
import static org.junit.jupiter.api.Assertions.*;

class ParameterizedTests {

    @ParameterizedTest
    @ValueSource(ints = {1, 2, 3, 4, 5})
    void positiveNumbersArePositive(int n) {
        assertTrue(n > 0);
    }

    @ParameterizedTest
    @ValueSource(strings = {"", "  ", "\t"})
    void blankStrings(String s) {
        assertTrue(s.isBlank());
    }

    @ParameterizedTest
    @NullAndEmptySource
    @ValueSource(strings = {"  ", "\t"})
    void blankOrNull(String s) {
        assertTrue(s == null || s.isBlank());
    }

    @ParameterizedTest
    @CsvSource({
        "1, 1, 2",
        "2, 3, 5",
        "10, -5, 5"
    })
    void addition(int a, int b, int expected) {
        assertEquals(expected, a + b);
    }

    @ParameterizedTest
    @CsvFileSource(resources = "/testdata.csv", numLinesToSkip = 1)
    void fromCsv(int a, int b, int expected) {
        assertEquals(expected, a + b);
    }

    @ParameterizedTest
    @MethodSource("additionProvider")
    void fromMethod(int a, int b, int expected) {
        assertEquals(expected, a + b);
    }
    static java.util.stream.Stream<Arguments> additionProvider() {
        return java.util.stream.Stream.of(
            Arguments.of(1, 1, 2),
            Arguments.of(2, 3, 5)
        );
    }

    @ParameterizedTest
    @EnumSource(TimeUnit.class)
    void allEnums(TimeUnit unit) {
        assertNotNull(unit);
    }
}

import org.junit.jupiter.api.*;
import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.condition.*;

class LifecycleTest {

    @Test
    void topLevel() {}

    @Nested
    @DisplayName("when stack is empty")
    class WhenEmpty {
        @BeforeEach
        void setUp() { /* stack is empty here */ }

        @Test
        void throwsOnPop() {
            assertThrows(Exception.class, () -> {});
        }

        @Nested
        @DisplayName("after pushing one element")
        class AfterPush {
            @BeforeEach
            void push() { /* push one */ }

            @Test
            void popReturnsElement() {
                // ...
            }
        }
    }

    // Conditional execution
    @Test
    @EnabledOnOs(OS.LINUX)
    void onlyOnLinux() {}

    @Test
    @EnabledIfSystemProperty(named = "env", matches = "ci")
    void onlyInCi() {}

    @Test
    @EnabledIfEnvironmentVariable(named = "DATABASE", matches = "postgres")
    void onlyWithPostgres() {}

    @Test
    @DisabledIf("customCondition")
    void conditional() {}
    static boolean customCondition() { return java.time.LocalTime.now().getHour() < 9; }

    // Tagging for selective runs
    @Test
    @Tag("slow")
    void slowIntegrationTest() {}

    @Test
    @Tag("fast")
    void fastUnitTest() {}

    // Run only fast: mvn test -Dgroups=fast
    import org.junit.jupiter.api.condition.OS;
    import java.util.concurrent.TimeUnit;

import org.junit.jupiter.api.*;
import org.mockito.*;
import static org.mockito.Mockito.*;
import static org.junit.jupiter.api.Assertions.*;

class UserServiceTest {

    @Mock
    UserRepository repo;  // mock dependency

    @InjectMocks
    UserService service;  // real service with mocks injected

    @BeforeEach
    void setUp() {
        MockitoAnnotations.openMocks(this);
    }

    @Test
    void returnsUserWhenFound() {
        // Arrange: stub the mock
        User alice = new User(1, "Alice");
        when(repo.findById(1)).thenReturn(alice);

        // Act
        String name = service.getUserName(1);

        // Assert
        assertEquals("Alice", name);
        verify(repo).findById(1);       // called once
        verify(repo, never()).findById(2);
    }

    @Test
    void throwsWhenNotFound() {
        when(repo.findById(99)).thenReturn(null);
        // or: when(repo.findById(99)).thenThrow(new RuntimeException());

        assertThrows(RuntimeException.class, () -> service.getUserName(99));
    }

    @Test
    void argumentMatchers() {
        when(repo.findById(anyInt())).thenReturn(new User(0, "default"));
        // matchers: eq(), any(), anyInt(), contains(), argThat()

        service.getUserName(42);
        verify(repo).findById(intThat(n -> n > 0));
    }

    @Test
    void verifyInteractionDetails() {
        service.getUserName(1);
        verify(repo, times(1)).findById(1);
        verify(repo, atLeastOnce()).findById(anyInt());
        verifyNoMoreInteractions(repo);
    }

    @Test
    void spy_partialMock() {
        List<String> spy = spy(new ArrayList<>());
        spy.add("real");
        when(spy.size()).thenReturn(100);  // stub one method
        assertEquals(100, spy.size());     // stubbed
        assertEquals(1, spy.size());       // wait, this would be 100 too
    }
}

interface UserRepository { User findById(int id); }
record User(int id, String name) {}
class UserService {
    UserRepository repo;
    UserService(UserRepository r) { repo = r; }
    String getUserName(int id) {
        User u = repo.findById(id);
        if (u == null) throw new RuntimeException("not found");
        return u.name();
    }
}

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class CalcTest {
    @Test
    void testAdd() {
        assertEquals(5, calc.add(2, 3));
    }
}

@ParameterizedTest
@ValueSource(ints = {1, 2, 3, 4})
void testPositive(int n) { assertTrue(n > 0); }

@ParameterizedTest
@CsvSource({"1,2,3", "4,5,9"})
void testAdd(int a, int b, int expected) {
    assertEquals(expected, calc.add(a, b));
}

class DbTest {
    @BeforeAll static void setupAll() { /* once before all */ }
    @AfterAll static void tearDownAll() { /* once after all */ }
    @BeforeEach void setup() { /* before each test */ }
    @AfterEach void tearDown() { /* after each test */ }
}

@Test
void testAll() {
    assertAll("person",
        () -> assertEquals("Alice", p.getName()),
        () -> assertEquals(30, p.getAge())
    );
}
@Test
void testException() {
    assertThrows(IllegalArgumentException.class, () -> service.process(-1));
}

@ExtendWith(MockitoExtension.class)
class UserServiceTest {
    @Mock UserRepository repo;
    @InjectMocks UserService service;
    @Test
    void testFind() {
        when(repo.findById(1)).thenReturn(Optional.of(new User("Alice")));
        assertEquals("Alice", service.findUser(1).getName());
        verify(repo).findById(1);
    }
}

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example</groupId>
  <artifactId>myapp</artifactId>
  <version>1.0.0</version>
  <dependencies>
    <dependency>
      <groupId>org.junit.jupiter</groupId>
      <artifactId>junit-jupiter</artifactId>
      <version>5.10.0</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</project>

mvn clean          # Remove target/
mvn compile        # Compile sources
mvn test           # Run tests
mvn package        # Build JAR
mvn install        # Install to local repo
mvn dependency:tree  # Show dependency tree

plugins { id 'java'; id 'application' }
repositories { mavenCentral() }
dependencies {
    implementation 'com.google.guava:guava:32.1.3-jre'
    testImplementation 'org.junit.jupiter:junit-jupiter:5.10.0'
}
application { mainClass = 'com.example.App' }

gradle clean       # Clean build
gradle build       # Build + test
gradle test        # Run tests
gradle run         # Run application
gradle bootRun     # Run Spring Boot
gradle dependencies  # Show dependency tree

// settings.gradle
include 'core', 'web', 'api'
// build.gradle (root)
subprojects {
    apply plugin: 'java'
    repositories { mavenCentral() }
}
// In web/build.gradle
dependencies { implementation project(':core') }

@SpringBootApplication
public class App {
    public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }
}

@RestController
@RequestMapping("/api/users")
public class UserController {
    @GetMapping("/{id}")
    public User getUser(@PathVariable Long id) {
        return userService.findById(id);
    }
    @PostMapping
    public User create(@RequestBody User user) {
        return userService.save(user);
    }
}

@Service
public class UserService {
    private final UserRepository repo;
    @Autowired  // Constructor injection (recommended)
    public UserService(UserRepository repo) {
        this.repo = repo;
    }
}

@Configuration
public class AppConfig {
    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }
    @Bean @Primary
    public DataSource primaryDataSource() {
        return DataSourceBuilder.create().build();
    }
}

# application.properties
server.port=8080
spring.datasource.url=jdbc:postgresql://localhost/mydb
spring.jpa.hibernate.ddl-auto=update
# Profile-specific
# application-dev.properties
server.port=9090

Connection conn = DriverManager.getConnection(url, user, pass);
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT * FROM users");
while (rs.next()) {
    System.out.println(rs.getString("name"));
}
rs.close(); stmt.close(); conn.close();

String sql = "INSERT INTO users (name, email) VALUES (?, ?)";
try (PreparedStatement ps = conn.prepareStatement(sql)) {
    ps.setString(1, "Alice");
    ps.setString(2, "[email protected]");
    ps.executeUpdate();
}

conn.setAutoCommit(false);
try {
    stmt.executeUpdate("UPDATE accounts SET bal = bal - 100 WHERE id = 1");
    stmt.executeUpdate("UPDATE accounts SET bal = bal + 100 WHERE id = 2");
    conn.commit();
} catch (SQLException e) {
    conn.rollback();
}

HikariConfig config = new HikariConfig();
config.setJdbcUrl(url);
config.setMaximumPoolSize(10);
HikariDataSource ds = new HikariDataSource(config);
Connection conn = ds.getConnection();

ResultSet rs = stmt.executeQuery("SELECT * FROM users");
ResultSetMetaData meta = rs.getMetaData();
int cols = meta.getColumnCount();
for (int i = 1; i <= cols; i++)
    System.out.println(meta.getColumnName(i) + ": " + meta.getColumnTypeName(i));

ExecutorService executor = Executors.newFixedThreadPool(4);
Future<String> future = executor.submit(() -> {
    Thread.sleep(1000);
    return "Result";
});
String result = future.get(5, TimeUnit.SECONDS);
executor.shutdown();

CompletableFuture.supplyAsync(() -> fetchData())
    .thenApply(data -> process(data))
    .thenAccept(result -> System.out.println(result))
    .exceptionally(ex -> { ex.printStackTrace(); return null; });

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.computeIfAbsent("b", k -> k.length());
CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
BlockingQueue<Task> queue = new LinkedBlockingQueue<>(100);

CountDownLatch latch = new CountDownLatch(3);
for (int i = 0; i < 3; i++)
    new Thread(() -> { work(); latch.countDown(); }).start();
latch.await();  // Wait for all

CyclicBarrier barrier = new CyclicBarrier(3, () -> System.out.println("All ready"));

AtomicInteger counter = new AtomicInteger(0);
counter.incrementAndGet();
counter.compareAndSet(0, 1);
LongAdder adder = new LongAdder();
adder.increment();

// Heap: objects and arrays (shared)
// - Young Gen: Eden, S0, S1
// - Old Gen: long-lived objects
// Stack: method frames (per thread)
// Metaspace: class metadata (Java 8+)
// JVM flags: -Xms512m -Xmx2g

// Bootstrap -> Extension -> Application classloaders
class MyClass {
    static { System.out.println("Static init"); }
}
// Class.forName("MyClass") triggers loading
// -verbose:class shows class loading

// javap -c MyClass.class
// Method int add(int, int):
//   iload_1      // Load local var 1
//   iload_2      // Load local var 2
//   iadd         // Add
//   ireturn      // Return int

// JIT compiles hot methods to native code
// -XX:+PrintCompilation  // Show JIT activity
// -XX:CompileThreshold=10000  // Method call count
// Tiered: Interpreter -> C1 -> C2

// Get thread dump
jstack <pid>
// Or: kill -3 <pid>
// Deadlock detection
jstack -l <pid> | grep -A 20 "Found deadlock"

# Serial GC (single-threaded)
-XX:+UseSerialGC
# Parallel GC (throughput)
-XX:+UseParallelGC
# G1 GC (balanced, default in Java 9+)
-XX:+UseG1GC
# ZGC (low-latency)
-XX:+UseZGC

# Set heap
-Xms4g -Xmx4g
# Max GC pause target
-XX:MaxGCPauseMillis=200
# Region size (1-32MB)
-XX:G1HeapRegionSize=16m
# Enable GC logging
-Xlog:gc*:file=gc.log:time,uptime

// Common leak: static collections
static Map<String, Object> cache = new HashMap<>();
// Objects never removed -> leak
// Detect with:
jmap -histo <pid> | head -20
// Heap dump:
jmap -dump:format=b,file=heap.hprof <pid>

WeakReference<Object> weakRef = new WeakReference<>(new Object());
SoftReference<byte[]> softRef = new SoftReference<>(new byte[1024]);
WeakHashMap<Object, String> map = new WeakHashMap<>();
map.put(key, "value");  // Entry removed when key is GC'd

// Cleaner API (Java 9+)
class Resource implements AutoCloseable {
    private final Cleaner.Cleanable cleanable;
    Resource() {
        cleanable = Cleaner.create().register(this, () -> cleanup());
    }
    public void close() { cleanable.clean(); }
}

Map<String, List<Person>> byCity =
    people.stream().collect(Collectors.groupingBy(Person::getCity));
Map<String, Long> countByCity =
    people.stream().collect(Collectors.groupingBy(
        Person::getCity, Collectors.counting()));

Map<Boolean, List<Person>> partition =
    people.stream().collect(Collectors.partitioningBy(p -> p.getAge() >= 18));
// {false=[minors], true=[adults]}
Map<Boolean, Long> count =
    people.stream().collect(Collectors.partitioningBy(
        p -> p.getAge() >= 18, Collectors.counting()));

String names = people.stream()
    .map(Person::getName)
    .collect(Collectors.joining(", "));
// "Alice, Bob, Charlie"
String csv = people.stream()
    .map(p -> p.getName() + "=" + p.getAge())
    .collect(Collectors.joining("\n", "[", "]"));

int totalAge = people.stream()
    .collect(Collectors.reducing(0, Person::getAge, Integer::sum));
Optional<Person> oldest = people.stream()
    .collect(Collectors.reducing((p1, p2) ->
        p1.getAge() > p2.getAge() ? p1 : p2));

Collector<Person, ?, String> toJson = Collector.of(
    StringBuilder::new,
    (sb, p) -> sb.append(`{"name":"${p.getName()}"}`),
    StringBuilder::append,
    StringBuilder::toString
);

Integer a = 127; Integer b = 127;
System.out.println(a == b);  // true (cached)
Integer c = 128; Integer d = 128;
System.out.println(c == d);  // false (not cached)
System.out.println(c.equals(d));  // true

String s = "Hello";
s.concat(" World");  // Returns new string, s unchanged
System.out.println(s);  // "Hello"
// Use StringBuilder for mutation
StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");  // Mutates in place

class Person {
    String name;
    public boolean equals(Object o) {
        if (!(o instanceof Person)) return false;
        return name.equals(((Person)o).name);
    }
    public int hashCode() { return name.hashCode(); }
}

// Checked: must catch or declare
try { Thread.sleep(1000); }
catch (InterruptedException e) { /* handle */ }
// Unchecked: RuntimeException
throw new IllegalArgumentException("bad input");

// BAD: resource leak
FileInputStream fis = new FileInputStream("file.txt");
// If exception here, fis never closed
// GOOD: try-with-resources
try (FileInputStream fis = new FileInputStream("file.txt")) {
    // Use resource
}  // Auto-closed even on exception

public class Singleton {
    private static volatile Singleton instance;
    private Singleton() {}
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) instance = new Singleton();
            }
        }
        return instance;
    }
}

public class Pizza {
    private final String size;
    private final boolean cheese;
    private Pizza(Builder b) { size = b.size; cheese = b.cheese; }
    public static class Builder {
        private String size; private boolean cheese;
        public Builder size(String s) { size = s; return this; }
        public Builder cheese(boolean c) { cheese = c; return this; }
        public Pizza build() { return new Pizza(this); }
    }
}

interface PaymentStrategy { void pay(double amount); }
class CreditCard implements PaymentStrategy {
    public void pay(double amount) { System.out.println("Card: " + amount); }
}
class Cart { private PaymentStrategy strategy; void checkout() { strategy.pay(100); } }

interface Observer { void update(String event); }
class Subject {
    private List<Observer> observers = new ArrayList<>();
    void subscribe(Observer o) { observers.add(o); }
    void notify(String event) { observers.forEach(o -> o.update(event)); }
}

abstract class Document { abstract void open(); }
class PDF extends Document { void open() { /* ... */ } }
abstract class DocFactory { abstract Document create(); }
class PDFFactory extends DocFactory { Document create() { return new PDF(); } }