Java

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.

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Showing Output

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

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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);    }}

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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)

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Comments

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

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Constants

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

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Arithmetic Operators

+ - * / % ++ --

• Division between integers truncates result.
• % gives remainder.

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Assignment Operators

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

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Comparison Operators

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

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Logical Operators

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

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Escape Sequences

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

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

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Type Casting

Widening (automatic)

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

Narrowing (manual)

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

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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");}

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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

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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

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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);}

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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");

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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)