Java Cheatsheet

Basics

Boilerplate

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

public → Access modifier (needed for JVM to call main).
static → Allows JVM to call without creating an object.
void → Method returns nothing.
String[] args → Command-line arguments.

Showing Output

System.out.print("Hello");     // prints without newline
System.out.println(" World");  // prints with newline
System.out.printf("Age: %d", 20); // formatted output

Taking Input (Scanner)

import java.util.Scanner;
 
class InputExample {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
 
        String name = sc.nextLine();   // String
        int age = sc.nextInt();        // int
        float f = sc.nextFloat();      // float
        double d = sc.nextDouble();    // double
        boolean b = sc.nextBoolean();  // boolean
        char c = sc.next().charAt(0);  // char (first char of input)
 
        System.out.println("Name: " + name);
    }
}

Primitive Types

Eight primitives: byte, short, int, long, float, double, boolean, char.

byte (8-bit, -128 to 127)
short (16-bit, -32,768 to 32,767)
int (32-bit, default for integers)
long (64-bit, append L for literals)
float (32-bit, append f)
double (64-bit, default for decimals)
boolean (true or false)
char (16-bit Unicode)

Comments

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

Constants

final double PI = 3.14159;
static final int MAX = 100;

Arithmetic Operators

+ - * / % ++ --

Division between integers truncates result.
% gives remainder.

Assignment Operators

=, +=, -=, *=, /=, %=

Comparison Operators

==, !=, >, <, >=, <=

Logical Operators

&& (AND), || (OR), ! (NOT)

Escape Sequences

\n → newline
\t → tab
\\ → backslash
\' → single quote
\" → double quote
\r → carriage return
\b → backspace

(\? is not valid in Java; use "?" literally.)

Type Casting

Widening (automatic)

int x = 45;
double y = x; // OK

Narrowing (manual)

double d = 45.9;
int n = (int) d; // truncates decimal

Control Flow

if / else if / else

if (x > 0) { ... } 
else if (x == 0) { ... } 
else { ... }

Ternary

String result = (x > 0) ? "Positive" : "Negative";

switch (Java 7+ supports Strings)

switch(day) {
    case 1: System.out.println("Mon"); break;
    ...
    default: System.out.println("Invalid");
}

Loops

while

while (i < 5) {
    i++;
}

do-while

do {
    i++;
} while (i < 5);

for

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

for-each

for (int n : arr) { ... }

break / continue

break → exits loop
continue → skips iteration

Arrays

int[] nums = new int[5];
String[] names = {"Harry", "Rohan"};
System.out.println(names.length);

Multi-dimensional

int[][] matrix = {
  {1, 2, 3},
  {4, 5, 6}
};
System.out.println(matrix[1][2]); // 6

Methods

static int add(int a, int b) {
    return a + b;
}
 
public static void main(String[] args) {
    System.out.println(add(5, 10));
}

Method Overloading

void sum(int a, int b) { ... }
void sum(double a, double b) { ... }

Recursion

int fact(int n) {
    if (n == 0) return 1;
    return n * fact(n - 1);
}

Strings

Strings are objects of java.lang.String.

String s = "Hello";
int len = s.length();
s.toUpperCase();
s.toLowerCase();
s.indexOf("l");
s.contains("He");
s.equals("Hello");
s.equalsIgnoreCase("hello");
s.substring(0, 3); // "Hel"
s.replace("H", "J");

Math Class

Math.max(5, 10);
Math.min(5, 10);
Math.sqrt(16);
Math.pow(2, 3);
Math.abs(-10);
Math.random(); // [0.0, 1.0)

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