Java Cheat Sheet

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

// Single line comment
/* Multi-line
   comment */
/** Javadoc comment */

Scanner scanner = new Scanner(System.in);
String name = scanner.nextLine();

Integer.parseInt("42")  // String to int
String.valueOf(42)      // int to String
Double.parseDouble("3.14")

obj instanceof String  // true/false
obj.getClass().getName()

String name = "John";

int age = 25;

double price = 19.99;

boolean isActive = true;

int[] nums = {1, 2, 3, 4, 5};

ArrayList<String> list = new ArrayList<>();

HashMap<String, Integer> map = new HashMap<>();

final double PI = 3.14159;

5 + 3   // 8 (addition)
5 - 3   // 2 (subtraction)
5 * 3   // 15 (multiplication)
5 / 3   // 1 (integer division)
5 % 3   // 2 (modulus)
int x = 5; x++  // 6 (increment)

5 == 5  // true (equal)
5 != 3  // true (not equal)
5 > 3   // true (greater)
5 < 3   // false (less)
5 >= 5  // true
"a".equals("a")  // true (strings)

true && false   // false (AND)
true || false   // true (OR)
!true           // false (NOT)

// Short-circuit: right side not evaluated
x != 0 && 10/x > 1

int x = 5    // Assign
x += 3       // x = 8 (add)
x -= 2       // x = 6 (subtract)
x *= 2       // x = 12 (multiply)
x /= 3       // x = 4 (divide)
x %= 3       // x = 1 (modulus)

int a = 10, b = 5;
int max = (a > b) ? a : b;  // max = 10

Object obj = "Hello";
if (obj instanceof String s) {
    System.out.println(s.length());  // 5
}

5 & 3   // 1 (AND: 101 & 011)
5 | 3   // 7 (OR: 101 | 011)
5 ^ 3   // 6 (XOR)
~5      // -6 (NOT)
5 << 1  // 10 (left shift)
5 >> 1  // 2 (right shift)

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

for (String item : list) {
    System.out.println(item);
}

while (count < 10) {
    count++;
}

do {
    count++;
} while (count < 10);

for (int i = 0; i < 10; i++) {
    if (i == 5) break;
}

for (int i = 0; i < 10; i++) {
    if (i == 5) continue;
    System.out.println(i);
}

outer: for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        if (j == 1) break outer;
    }
}

list.forEach(item -> {
    System.out.println(item);
});

Iterator<String> it = list.iterator();
while (it.hasNext()) {
    System.out.println(it.next());
}

map.forEach((key, value) -> {
    System.out.println(key + ": " + value);
});

if (x > 0) {
    System.out.println("positive");
} else if (x < 0) {
    System.out.println("negative");
} else {
    System.out.println("zero");
}

String result = switch (day) {
    case "Mon" -> "Monday";
    case "Tue" -> "Tuesday";
    default -> "Other";
};

switch (day) {
    case "Mon":
        System.out.println("Monday");
        break;
    default:
        System.out.println("Other");
}

if (obj instanceof String s && s.length() > 5) {
    System.out.println(s);
}

String msg = (age >= 18) ? "adult" : "minor";

public String greet(String name) {
    return "Hello, " + name + "!";
}

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

public void printMessage(String msg) {
    System.out.println(msg);
}

public int sum(int... nums) {
    return Arrays.stream(nums).sum();
}

public class Dog {
    private String name;

    public Dog(String name) {
        this.name = name;
    }

    public String bark() {
        return name + " says woof!";
    }
}

public class Puppy extends Dog {
    public Puppy(String name) {
        super(name);
    }
}

public interface Animal {
    void speak();
}

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

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Error: " + e.getMessage());
}

try {
    riskyOperation();
} catch (Exception e) {
    e.printStackTrace();
} finally {
    cleanup();
}

throw new IllegalArgumentException("Invalid input");

list.stream().filter(x -> x > 0).toList();

list.stream().map(String::toUpperCase).toList();

nums.stream().reduce(0, Integer::sum);

stream.collect(Collectors.toList());