Java Cheat Sheet

Complete reference guide for Java with interactive examples and live playground links

Basic Syntax

Hello World

The classic first program: outputting text to the console.

Java

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

Variables and Data Types

Variable declaration and basic data types in Java

Java

// Primitive types
int age = 30;
double height = 5.9;
boolean isActive = true;
char grade = 'A';

// Reference types
String name = "Java";
Integer count = 42;
Double price = 19.99;
Boolean isValid = true;

// Type conversion
int num = 42;
double decimal = (double) num;  // Explicit casting
int backToInt = (int) decimal;  // Explicit casting

String Operations

Common string operations and formatting techniques

Java

String text = "Hello World";

// String methods
System.out.println(text.toUpperCase());        // HELLO WORLD
System.out.println(text.toLowerCase());        // hello world
System.out.println(text.replace("World", "Java"));  // Hello Java
System.out.println(text.split(" "));           // [Hello, World]
System.out.println(text.length());             // 11

// String formatting
String name = "Alice";
int age = 25;
System.out.printf("My name is %s and I'm %d years old%n", name, age);
System.out.println(String.format("My name is %s and I'm %d years old", name, age));

Arrays and Collections

Working with arrays and collections

Java

// Arrays
int[] numbers = {1, 2, 3, 4, 5};
String[] fruits = new String[3];
fruits[0] = "apple";
fruits[1] = "banana";
fruits[2] = "cherry";

// ArrayList
ArrayList<String> list = new ArrayList<>();
list.add("apple");
list.add("banana");
list.add("cherry");
list.remove("banana");
list.set(0, "orange");

// HashMap
HashMap<String, Integer> map = new HashMap<>();
map.put("apple", 1);
map.put("banana", 2);
map.put("cherry", 3);
map.remove("banana");
int value = map.get("apple");

Input/Output

Basic input and output operations

Java

// Console output
System.out.println("Hello World");
System.out.printf("Name: %s, Age: %d%n", name, age);
System.out.print("No new line");

// Console input
Scanner scanner = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = scanner.nextLine();
System.out.print("Enter your age: ");
int age = scanner.nextInt();
scanner.close();

Comments

Different ways to add comments and documentation

Java

// Single line comment

/*
 * Multi-line comment
 * This is a block comment
 */

/**
 * Javadoc comment
 * Used for documentation
 * @param name the name parameter
 * @return the greeting string
 */
public String greet(String name) {
    return "Hello, " + name;
}

Control Flow

Conditional Statements

Conditional logic and decision making

Java

// If-else
int age = 18;
if (age >= 18) {
    System.out.println("Adult");
} else if (age >= 13) {
    System.out.println("Teenager");
} else {
    System.out.println("Child");
}

// Ternary operator
String status = (age >= 18) ? "Adult" : "Minor";

// Switch statement
switch (age) {
    case 18:
        System.out.println("Just became adult");
        break;
    case 13:
        System.out.println("Just became teenager");
        break;
    default:
        System.out.println("Other age");
}

Loops

Different types of loops and iterations

Java

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

// Enhanced for loop
String[] fruits = {"apple", "banana", "cherry"};
for (String fruit : fruits) {
    System.out.println(fruit);
}

// While loop
int count = 0;
while (count < 5) {
    System.out.println(count);
    count++;
}

// Do-while loop
int num = 0;
do {
    System.out.println(num);
    num++;
} while (num < 5);

Exception Handling

Error handling with try-catch blocks and custom exceptions

Java

try {
    // Risky code
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero!");
} catch (Exception e) {
    System.out.println("An error occurred: " + e.getMessage());
} finally {
    System.out.println("This always executes");
}

// Custom exception
class ValidationException extends Exception {
    public ValidationException(String message) {
        super(message);
    }
}

// Throwing exceptions
public void validate(int age) throws ValidationException {
    if (age < 0) {
        throw new ValidationException("Age cannot be negative");
    }
}

Object-Oriented Programming

Classes and Objects

Class definition, constructors, and methods

Java

public class Person {
    // Fields
    private String name;
    private int age;
    
    // Constructor
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    // Getters and Setters
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
    
    // Methods
    public void introduce() {
        System.out.printf("Hi, I'm %s and I'm %d years old%n", name, age);
    }
}

// Usage
Person person = new Person("Alice", 25);
person.introduce();

Inheritance and Interfaces

Inheritance, abstract classes, and interfaces

Java

// Abstract class
abstract class Animal {
    protected String name;
    
    public Animal(String name) {
        this.name = name;
    }
    
    public abstract void makeSound();
}

// Interface
interface Movable {
    void move();
    void stop();
}

// Class implementing interface and extending abstract class
class Dog extends Animal implements Movable {
    public Dog(String name) {
        super(name);
    }
    
    @Override
    public void makeSound() {
        System.out.println("Woof!");
    }
    
    @Override
    public void move() {
        System.out.println("Dog is running");
    }
    
    @Override
    public void stop() {
        System.out.println("Dog stopped");
    }
}

Collections Framework

Lists and Sets

Working with Lists and Sets

Java

// ArrayList
List<String> list = new ArrayList<>();
list.add("apple");
list.add("banana");
list.add("cherry");
list.remove("banana");
String first = list.get(0);

// LinkedList
List<Integer> linkedList = new LinkedList<>();
linkedList.add(1);
linkedList.add(2);
linkedList.add(3);

// HashSet
Set<String> set = new HashSet<>();
set.add("apple");
set.add("banana");
set.add("cherry");
set.add("apple");  // Duplicate, won't be added

// TreeSet
Set<Integer> treeSet = new TreeSet<>();
treeSet.add(3);
treeSet.add(1);
treeSet.add(2);
// Will be stored in order: 1, 2, 3

Maps

Map implementations and operations

Java

// HashMap
Map<String, Integer> map = new HashMap<>();
map.put("apple", 1);
map.put("banana", 2);
map.put("cherry", 3);

// Accessing values
int value = map.get("apple");
boolean exists = map.containsKey("banana");
map.remove("cherry");

// TreeMap
Map<String, Integer> treeMap = new TreeMap<>();
treeMap.put("cherry", 3);
treeMap.put("apple", 1);
treeMap.put("banana", 2);
// Keys will be stored in order: apple, banana, cherry

// Iterating
for (Map.Entry<String, Integer> entry : map.entrySet()) {
    System.out.printf("%s: %d%n", entry.getKey(), entry.getValue());
}

Streams and Lambdas

Stream Operations

Working with Java Streams API

Java

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

// Filter and map
List<Integer> evenSquares = numbers.stream()
    .filter(n -> n % 2 == 0)
    .map(n -> n * n)
    .collect(Collectors.toList());

// Reduce
int sum = numbers.stream()
    .reduce(0, Integer::sum);

// Grouping
Map<String, List<Person>> byAge = people.stream()
    .collect(Collectors.groupingBy(p -> p.getAge() >= 18 ? "Adult" : "Minor"));

// Parallel stream
int sum = numbers.parallelStream()
    .mapToInt(Integer::intValue)
    .sum();

Lambda Expressions

Lambda expressions and functional programming

Java

// Functional interfaces
@FunctionalInterface
interface Calculator {
    int calculate(int a, int b);
}

// Lambda expressions
Calculator add = (a, b) -> a + b;
Calculator subtract = (a, b) -> a - b;
Calculator multiply = (a, b) -> a * b;

// Method references
List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
names.forEach(System.out::println);

// Comparator
List<Person> people = new ArrayList<>();
people.sort((p1, p2) -> p1.getName().compareTo(p2.getName()));
// or
people.sort(Comparator.comparing(Person::getName));

Java - Interactive Developer Reference

Hover over code blocks to copy or run in live playground

Developer Cheatsheets

Java Cheat Sheet

Basic Syntax

Hello World

Variables and Data Types

String Operations

Arrays and Collections

Input/Output

Comments

Control Flow

Conditional Statements

Loops

Exception Handling

Object-Oriented Programming

Classes and Objects

Inheritance and Interfaces

Collections Framework

Lists and Sets

Maps

Streams and Lambdas

Stream Operations

Lambda Expressions