Java Cheat Sheets

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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();
    }
}

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

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

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

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

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

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

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

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

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

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

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();

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();

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

synchronized(this) {
    // critical section
}

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