Java Cheatsheet

int x = 5;
String s = "hello";
boolean flag = true;

if (x > 0) {
  System.out.println("Positive");
} else if (x < 0) {
  System.out.println("Negative");
} else {
  System.out.println("Zero");
}

switch (day) {
  case 1:
    System.out.println("Mon");
    break;
  case 2:
    System.out.println("Tue");
    break;
  default:
    System.out.println("Other");
}

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

while (x < 5) {
  x++;
}
do {
  x--;
} while (x > 0);

for (int i = 0; i < 10; i++) {
  if (i == 5) break;
  if (i % 2 == 0) continue;
  System.out.println(i);
}

int add(int a, int b) {
  return a + b;
}
void greet(String name) {
  System.out.println("Hello, " + name);
}

int sum(int a, int b) { return a + b; }
double sum(double a, double b) { return a + b; }

class Animal {
  String name;
  Animal(String n) { name = n; }
  void speak() { System.out.println("Hi, I'm " + name); }
}
Animal a = new Animal("Dog");
a.speak();

class MathUtil {
  static int square(int x) { return x * x; }
}
int y = MathUtil.square(5);

class Animal {
  void speak() { System.out.println("Animal"); }
}
class Dog extends Animal {
  void speak() { System.out.println("Woof"); }
}
Dog d = new Dog();
d.speak();

Animal a = new Dog();
a.speak(); // Woof

abstract class Shape {
  abstract double area();
}
class Circle extends Shape {
  double r;
  Circle(double r) { this.r = r; }
  double area() { return Math.PI * r * r; }
}

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

enum Day { MON, TUE, WED }
Day d = Day.MON;

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

import java.io.*;
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
  String line;
  while ((line = br.readLine()) != null) {
    System.out.println(line);
  }
} catch (IOException e) {
  e.printStackTrace();
}

Runnable r = () -> System.out.println("Run");
r.run();

import java.util.*;
List nums = Arrays.asList(1,2,3);
nums.stream().filter(n -> n > 1).forEach(System.out::println);

import java.util.Optional;
Optional o = Optional.of("hi");
o.ifPresent(System.out::println);

int n = 42;
double d = 3.14;
Math.max(1, 2);
Math.pow(2, 3);
Math.sqrt(16);
Math.random();

String s = "hello";
s.length();
s.charAt(0);
s.substring(1, 3);
s.toUpperCase();
s.equals("hi");
s.replace("h", "H");
s.split(",");

int[] arr = {1, 2, 3};
arr[0] = 10;
System.out.println(arr.length);
Arrays.sort(arr);
for (int n : arr) {
  System.out.println(n);
}

import java.util.List;
import java.util.ArrayList;
List list = new ArrayList<>();
list.add("a");
list.get(0);
list.size();
list.remove("a");
for (String s : list) {
  System.out.println(s);
}

import java.util.HashMap;
HashMap map = new HashMap<>();
map.put("a", 1);
map.get("a");
map.containsKey("a");
map.remove("a");
for (String k : map.keySet()) {
  System.out.println(k + ": " + map.get(k));
}

import java.util.Arrays;
import java.util.Collections;
int[] arr = {3, 1, 2};
Arrays.sort(arr);
Collections.reverse(Arrays.asList(arr));
StringBuilder sb = new StringBuilder();
sb.append("hi");
sb.reverse();

package myapp;
import java.util.*;
// ... your code ...

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