Java Cheat Sheet

Free Java cheat sheet: the most-used Java syntax and methods at a glance — searchable and beginner-friendly.

Basics

Concept	Syntax	Example
Main method	`public static void main(String[] args)`	`Entry point of every Java program`
Print	`System.out.println()`	`System.out.println("Hello");`
Variable	`type name = value;`	`int age = 25;`
Array	`type[] name = new type[size]`	`int[] nums = {1, 2, 3};`

OOP

Concept	Syntax	Example
Class	`class Name { }`	`class Dog { String name; }`
Constructor	`ClassName(params) { }`	`Dog(String n) { name = n; }`
Inheritance	`class B extends A { }`	`class Puppy extends Dog { }`
Interface	`interface Name { method(); }`	`interface Runnable { void run(); }`
Abstract class	`abstract class Name { }`	`abstract class Shape { abstract double area(); }`

Collections

Concept	Syntax	Example
ArrayList	`List<T> list = new ArrayList<>()`	`List<String> names = new ArrayList<>();`
HashMap	`Map<K,V> map = new HashMap<>()`	`Map<String,Integer> ages = new HashMap<>();`
HashSet	`Set<T> set = new HashSet<>()`	`Set<String> unique = new HashSet<>();`
For-each	`for (T item : collection)`	`for (String s : list) { ... }`
Stream API	`.stream().filter().map().collect()`	`list.stream().filter(x -> x > 5).toList();`

Exception Handling

Concept	Syntax	Example
Try-catch	`try { } catch (Exception e) { }`	`try { int x = 1/0; } catch (ArithmeticException e) { ... }`
Throw	`throw new Exception(msg)`	`throw new IllegalArgumentException("Invalid");`
Finally	`try { } finally { }`	`finally { connection.close(); }`
Custom exception	`class MyEx extends Exception { }`	`class NotFoundEx extends RuntimeException { }`

Strings

Concept	Syntax	Example
Length	`str.length()`	`"hello".length() // 5`
Substring	`str.substring(start, end)`	`"hello".substring(0, 3) // "hel"`
Split	`str.split(regex)`	`"a,b,c".split(",") // ["a","b","c"]`
Format	`String.format("%s is %d", a, b)`	`String.format("Hi %s", name);`