Java Cheat Sheet

// Package declaration (optional) package mypackage; // Import statements import java.util.*; // Main class public class Main { public static void main(String[] args) { // This is a single-line comment /* Multi-line comment: Declaring a string variable and printing it */ String greeting = "Hello, Mie!"; System.out.println(greeting); } } 

public class Main { public static void main(String[] args) { // Explicit type int x = 42; String name = "Alice"; // Type inference (Java 10+) var y = 10; var greeting = "Hello"; // Multiple variables of the same type int a = 1, b = 2, c = 3; // Constants (final) final double Pi = 3.14159; final String Greeting = "Hello World"; // Print examples System.out.println(x + ", " + name); System.out.println(y + ", " + greeting); System.out.println(a + ", " + b + ", " + c); System.out.println(Pi + ", " + Greeting); System.out.println(StatusOK + ", " + StatusNotFound); } } 

public class Main { public static void main(String[] args) { // Boolean boolean b = true; // String String s = "Hello"; // Integers byte i8 = -128; // 8-bit short i16 = -32768; // 16-bit int i32 = -2147483648; // 32-bit long i64 = -9223372036854775808L; // 64-bit // Note: Java does NOT have unsigned types (except char) char u16 = 65535; // 16-bit unsigned, Unicode character // Floating Point float f32 = 3.14f; // 32-bit double f64 = 3.14159265359; // 64-bit // Complex numbers not built-in in Java; need libraries like Apache Commons Math // Char (alias for Unicode code point, 16-bit unsigned) char r = '世'; // Byte (8-bit signed) byte by = 'A'; // Print examples System.out.println("Boolean: " + b); System.out.println("String: " + s); System.out.println("Integers: " + i8 + ", " + i16 + ", " + i32 + ", " + i64); System.out.println("Char: " + r); System.out.println("Byte: " + by); System.out.println("Floating Point: " + f32 + ", " + f64); } } 

public class Main { public static void main(String[] args) { // --- Arithmetic Operators --- int a = 10, b = 3; System.out.println(a + b); // addition → 13 System.out.println(a - b); // subtraction → 7 System.out.println(a * b); // multiplication → 30 System.out.println(a / b); // division → 3 System.out.println(a % b); // modulus → 1 a++; // increment b--; // decrement // --- Relational (Comparison) Operators --- System.out.println(a == b); // equal to System.out.println(a != b); // not equal to System.out.println(a > b); // greater than System.out.println(a < b); // less than System.out.println(a >= b); // greater or equal System.out.println(a <= b); // less or equal // --- Logical Operators --- boolean x = true, y = false; System.out.println(x && y); // AND → false System.out.println(x || y); // OR → true System.out.println(!x); // NOT → false // --- Bitwise Operators --- int m = 6, n = 3; // 6 = 110, 3 = 011 System.out.println(m & n); // AND → 2 System.out.println(m | n); // OR → 7 System.out.println(m ^ n); // XOR → 5 System.out.println(~m); // NOT → -7 System.out.println(m << 1); // left shift → 12 System.out.println(m >> 1); // right shift → 3 System.out.println(m >>> 1); // unsigned right shift → 3 // --- Assignment Operators --- int c = 5; c += 3; // c = 8 c -= 2; // c = 6 c *= 4; // c = 24 c /= 6; // c = 4 c %= 3; // c = 1 System.out.println(c); // --- Other Operators --- int max = (a > b) ? a : b; // ternary operator System.out.println(max); String str = "Hello"; boolean check = str instanceof String; // type check System.out.println(check); // true } } 

public class Main { public static void main(String[] args) { // --- If / Else --- int x = 10; if (x > 5) { // if condition System.out.println("x > 5"); } else if (x == 5) { // else-if condition System.out.println("x == 5"); } else { // else System.out.println("x < 5"); } // --- Switch --- int day = 3; switch (day) { case 1: System.out.println("Monday"); break; // break to prevent fall-through case 2: case 3: System.out.println("Tue or Wed"); break; default: System.out.println("Other day"); } // Expression-less switch alternative: use if-else int num = 15; if (num % 2 == 0) { System.out.println("Even"); } else { System.out.println("Odd"); } // --- Classic For Loop --- for (int i = 0; i < 3; i++) { // initialize; condition; increment System.out.println(i); } // --- While Loop --- int j = 0; while (j < 3) { // condition System.out.println(j); j++; // increment inside loop } // --- Do-While Loop --- int k = 0; do { System.out.println(k); k++; } while (k < 3); // check condition after loop // --- Infinite Loop --- // for (;;) { System.out.println("loop"); } // runs forever // --- For-each Loop (like Go range) --- int[] nums = {10, 20, 30}; // array for (int val : nums) { // for-each iteration System.out.println(val); } java.util.Map<String, Integer> map = java.util.Map.of("a", 1, "b", 2); // map for (java.util.Map.Entry<String, Integer> entry : map.entrySet()) { // iterate map System.out.println(entry.getKey() + ": " + entry.getValue()); } String str = "Java"; // string iteration for (int i = 0; i < str.length(); i++) { char r = str.charAt(i); // get character at index System.out.println(i + ":" + r); } // --- Break / Continue --- for (int i = 0; i < 5; i++) { if (i == 2) continue; // skip current iteration if (i == 4) break; // exit loop System.out.println(i); } } } 

import java.util.ArrayList; import java.util.HashMap; import java.util.Map; public class Main { public static void main(String[] args) { // --- Array --- // Fixed size, elements of the same type int[] arr = {1, 2, 3}; // create array System.out.println(arr[0]); // access element → 1 // --- ArrayList --- // Dynamic size, elements of the same type ArrayList<String> list = new ArrayList<>(); list.add("Java"); // add element list.add("Python"); list.add("C++"); list.add("Rust"); System.out.println(list.get(0)); // access element → "Java" // --- HashMap --- // Key-value pairs (unordered) HashMap<String, Integer> map = new HashMap<>(); map.put("Alice", 25); map.put("Bob", 30); System.out.println(map.get("Alice")); // access value → 25 // --- Iteration examples --- // Array for (int val : arr) { System.out.println(val); } // ArrayList for (String val : list) { System.out.println(val); } // HashMap for (Map.Entry<String, Integer> entry : map.entrySet()) { System.out.println(entry.getKey() + ": " + entry.getValue()); } } } 

import java.util.function.BiFunction; import java.util.function.Function; public class Main { public static void main(String[] args) { // --- Basic method --- int sum = add(3, 5); // call method System.out.println(sum); // 8 // --- Multiple return values (simulated using array) --- int[] result = divide(10, 3); // returns quotient and remainder System.out.println("Quotient: " + result[0] + ", Remainder: " + result[1]); // 3, 1 // --- Variadic method --- int total = sumAll(1, 2, 3, 4); // pass multiple arguments System.out.println(total); // 10 // --- Method as value (using functional interface) --- BiFunction<Integer, Integer, Integer> f = Main::add; // reference to method int res = f.apply(10, 20); System.out.println(res); // 30 // --- Anonymous function / Lambda --- Function<String, String> greet = (name) -> "Hello " + name; String msg = greet.apply("Java"); System.out.println(msg); // Hello Java } // Basic method static int add(int a, int b) { return a + b; } // Simulate multiple return values using array static int[] divide(int a, int b) { int q = a / b; int r = a % b; return new int[]{q, r}; } // Variadic method static int sumAll(int... nums) { int total = 0; for (int n : nums) { total += n; } return total; } }