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Java Cheat Sheets

This Java cheat sheet includes symbols, ranges, grouping, assertions, and sample patterns.

Getting Started

// Hello World

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

Basic print statement

// Basic Commands

Single-line Comment

// This is a comment

Multi-line Comment

/* This is a multi-line comment */

Variable Declaration

int x = 5;

Output

System.out.println(x);

Input

Scanner sc = new Scanner(System.in); String s = sc.nextLine();

// Getting Started Resources

Java Getting Started docs.oracle.com W3Schools Java w3schools.com Baeldung Java Tutorials baeldung.com

Data Types

// Primitive Data Types

byte

8-bit integer (-128 to 127)

short

16-bit integer (-32,768 to 32,767)

int

32-bit integer (-2^31 to 2^31-1)

long

64-bit integer (-2^63 to 2^63-1)

float

32-bit floating point

double

64-bit floating point

char

16-bit Unicode character

boolean

true or false

// Type Conversion

int i = (int) 3.14; // 3
double d = 5; // 5.0

Casting between types

// Data Types Resources

Java Data Types Docs docs.oracle.com Baeldung: Java Data Types baeldung.com

Operators

// Arithmetic Operators

Addition

Subtraction

Multiplication

Division

Modulus

// Comparison Operators

Equal to

Not equal to

Greater than

Less than

Greater than or equal to

Less than or equal to

// Logical Operators

Logical AND

Logical OR

Logical NOT

// Operators Resources

Java Operators Docs docs.oracle.com W3Schools: Java Operators w3schools.com

Control Flow

// if-else Statement

int x = 10;
if (x > 5) {
    System.out.println("Greater");
} else {
    System.out.println("Smaller or equal");
}

Conditional branching

// switch Statement

int day = 2;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    default:
        System.out.println("Other");
}

Switch-case branching

// for Loop

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

Classic for loop

// Enhanced for Loop

int[] arr = {1,2,3};
for (int n : arr) {
    System.out.println(n);
}

For-each loop

// while Loop

int i = 0;
while (i < 3) {
    System.out.println(i);
    i++;
}

While loop

// do-while Loop

int i = 0;
do {
    System.out.println(i);
    i++;
} while (i < 3);

Do-while loop

// Loop Control Keywords

break

Exit the loop

continue

Skip to next iteration

// Control Flow Resources

Java Control Flow Docs docs.oracle.com Baeldung: Java Control Flow baeldung.com

Arrays

// Array Declaration

int[] arr = new int[3];
arr[0] = 1;
arr[1] = 2;
arr[2] = 3;

Declare and initialize array

// Array Initialization

int[] arr = {1, 2, 3};

Inline initialization

// Iterate Array

for (int n : arr) {
    System.out.println(n);
}

Loop through array

// Arrays Resources

Java Arrays Docs docs.oracle.com W3Schools: Java Arrays w3schools.com

Collections

// Common Collections

List

Ordered, allows duplicates

Set

Unordered, no duplicates

Map

Key-value pairs

Queue

FIFO structure

Stack

LIFO structure

// ArrayList Example

import java.util.ArrayList;
ArrayList<String> list = new ArrayList<>();
list.add("A");
list.add("B");
System.out.println(list.get(0));

Using ArrayList

// HashMap Example

import java.util.HashMap;
HashMap<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
System.out.println(map.get("A"));

Using HashMap

// Collections Resources

Java Collections Docs docs.oracle.com Baeldung: Java Collections baeldung.com

OOP Basics

// Class Definition

public class Car {
    private String brand;
    public Car(String brand) {
        this.brand = brand;
    }
    public void start() {
        System.out.println(brand + " is starting");
    }
}

Defining a class

// Object Instantiation

Car car = new Car("Toyota");
car.start();

Creating an object

// Inheritance

public class Animal {
    public void speak() {
        System.out.println("Animal speaks");
    }
}
public class Dog extends Animal {
    @Override
    public void speak() {
        System.out.println("Woof!");
    }
}

Extending a class

// Interface

public interface Drawable {
    void draw();
}
public class Circle implements Drawable {
    public void draw() {
        System.out.println("Drawing Circle");
    }
}

Defining and implementing interface

// Abstract Class

public abstract class Shape {
    abstract void draw();
}
public class Square extends Shape {
    void draw() {
        System.out.println("Drawing Square");
    }
}

Defining abstract class

// OOP Principles

Encapsulation

Hiding internal state and requiring all interaction to be performed through an object’s methods

Inheritance

Mechanism for a new class to use features of another class

Polymorphism

Ability to present the same interface for different data types

Abstraction

Hiding complex reality while exposing only the necessary parts

// OOP Resources

Java OOP Docs docs.oracle.com Baeldung: OOP in Java baeldung.com

Design Patterns

// Creational Patterns

Singleton

Only one instance exists

Factory

Creates objects without specifying the exact class

Builder

Constructs complex objects step by step

Prototype

Creates new objects by copying existing ones

// Singleton Pattern

public class Singleton {
    private static Singleton instance;
    private Singleton() {}
    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

Singleton implementation

// Factory Pattern

public interface Product { }
public class ConcreteProductA implements Product { }
public class ConcreteProductB implements Product { }
public class Factory {
    public Product createProduct(String type) {
        if (type.equals("A")) return new ConcreteProductA();
        else return new ConcreteProductB();
    }
}

Factory implementation

// Design Patterns Resources

Refactoring Guru: Design Patterns refactoring.guru Baeldung: Design Patterns baeldung.com

Java 8+ and Functional Programming

// Lambda Expression

List<String> list = Arrays.asList("a", "b", "c");
list.forEach(s -> System.out.println(s));

Basic lambda syntax

// Stream API

List<Integer> nums = Arrays.asList(1,2,3,4,5);
int sum = nums.stream().filter(n -> n % 2 == 0).mapToInt(Integer::intValue).sum();

Stream operations

// Optional

Optional<String> opt = Optional.ofNullable(null);
System.out.println(opt.orElse("default"));

Avoiding nulls with Optional

// Method Reference

list.forEach(System.out::println);

Reference to a method

// Functional Interfaces

Predicate<T>

boolean test(T t)

Consumer<T>

void accept(T t)

Supplier<T>

T get()

Function<T,R>

R apply(T t)

BiFunction<T,U,R>

R apply(T t, U u)

// Functional Programming Resources

Java Lambda Expressions docs.oracle.com Baeldung: Java 8 Streams baeldung.com Java Optional baeldung.com

Exception Handling

// try-catch-finally

try {
    int x = 1 / 0;
} catch (ArithmeticException e) {
    System.out.println("Error: " + e.getMessage());
} finally {
    System.out.println("Always runs");
}

Basic exception handling

// Custom Exception

public class MyException extends Exception {
    public MyException(String message) {
        super(message);
    }
}

Defining your own exception

// try-with-resources

try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    String line = br.readLine();
}

Automatic resource management

// Common Exceptions

NullPointerException

Accessing a null object

ArrayIndexOutOfBoundsException

Invalid array index

ClassCastException

Invalid type cast

IOException

I/O operation failed

FileNotFoundException

File not found

// Exception Handling Resources

Java Exception Handling docs.oracle.com Baeldung: Java Exceptions baeldung.com

File I/O and Serialization

// Reading a File

try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    String line;
    while ((line = br.readLine()) != null) {
        System.out.println(line);
    }
}

Read file with BufferedReader

// Writing to a File

try (BufferedWriter bw = new BufferedWriter(new FileWriter("file.txt"))) {
    bw.write("Hello!");
}

Write file with BufferedWriter

// NIO Files API

List<String> lines = Files.readAllLines(Paths.get("file.txt"));

Read all lines with NIO

// Serialization

ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("obj.ser"));
out.writeObject(obj);
out.close();
ObjectInputStream in = new ObjectInputStream(new FileInputStream("obj.ser"));
MyClass obj2 = (MyClass) in.readObject();
in.close();

Serialize and deserialize object

// File I/O & Serialization Resources

Java File I/O docs.oracle.com Java Serialization docs.oracle.com

Multithreading & Concurrency

// Thread Creation

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread running");
    }
}
new MyThread().start();
Runnable r = () -> System.out.println("Runnable running");
new Thread(r).start();

Extending Thread and implementing Runnable

// ExecutorService

ExecutorService executor = Executors.newFixedThreadPool(2);
executor.submit(() -> System.out.println("Task 1"));
executor.shutdown();

Thread pool management

// Synchronization

synchronized(this) {
    // critical section
}

Synchronized block

// Locks & Atomic

Lock lock = new ReentrantLock();
lock.lock();
try {
    // critical section
} finally {
    lock.unlock();
}
AtomicInteger ai = new AtomicInteger(0);
ai.incrementAndGet();

ReentrantLock and AtomicInteger

// Concurrency Resources

Java Concurrency docs.oracle.com Baeldung: Java Concurrency baeldung.com

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