Java Cheat Sheet

Primitive Types

int age = 25;
double price = 9.99;
char grade = 'A';
boolean isActive = true;

int number = 10;
final double PI = 3.14159; // constant

Arithmetic

int sum = 5 + 3;
int product = 5 * 2;

Relational

boolean isEqual = (a == b);

Logical

boolean result = (x > 5) && (y < 10);

int i = (int) 3.14; // explicit
float f = 10; // implicit

Scanner sc = new Scanner(System.in);
String name = sc.nextLine();
System.out.println("Hello " + name);

// Single line comment
/* Multi-line comment */

if / else / switch

if (x > 0) {
  System.out.println("Positive");
} else {
  System.out.println("Non-positive");
}

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

Loops

for (int i = 0; i < 5; i++) {}
while (x > 0) {}
do {} while (x > 0);

break / continue

for (int i = 0; i < 10; i++) {
  if (i == 5) continue;
  if (i == 8) break;
}

Classes and Objects

class Dog {
  String name;
  void bark() {
    System.out.println("Woof");
  }
}
Dog d = new Dog();
d.name = "Rex";

Constructors, this, Overloading

Dog(String name) {
  this.name = name;
}
Dog(String name, int age) { ... }

Access Modifiers, Encapsulation

private int id;
public void setId(int id) { this.id = id; }

Inheritance, Overriding, Polymorphism

class Animal {}
class Cat extends Animal {
  @Override void speak() {}
}

Abstract Classes, Interfaces

abstract class Shape { abstract void draw(); }
interface Drawable { void draw(); }

Static, final, Inner Classes

static int count;
final int MAX = 100;
class Outer { class Inner {} }

try {
  int result = 10 / 0;
} catch (ArithmeticException e) {
  System.out.println("Error: " + e.getMessage());
} finally {
  System.out.println("Cleanup");
}

void risky() throws IOException {}
throw new IllegalArgumentException("Bad input");

int[] nums = {1, 2, 3};
ArrayList list = new ArrayList<>();
LinkedList ll = new LinkedList<>();
HashMap map = new HashMap<>();
HashSet set = new HashSet<>();
Stack stack = new Stack<>();
Queue queue = new LinkedList<>();
Collections.sort(list);

String s = "hello";
s.length();
s.toUpperCase();
s.substring(1,3);

StringBuilder sb = new StringBuilder("hi");
sb.append(" there");

String a = "test";
String b = new String("test");
a == b // false
a.equals(b) // true

enum Color { RED, GREEN, BLUE; }

class Box { T value; }

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

List list = Arrays.asList("a", "b", "c");
list.stream().filter(x -> x.equals("a"));

@FunctionalInterface
interface MyFunc { void run(); }

@interface MyAnnotation {}

import java.util.*;
void printAll(String... args) {}

class MyThread extends Thread {
  public void run() {
    System.out.println("Running");
  }
}

Thread t = new MyThread();
t.start();

Runnable r = () -> {};
new Thread(r).start();

synchronized void syncMethod() {}

Thread.sleep(1000);
t.join();
wait(); notify();

FileReader fr = new FileReader("file.txt");
BufferedReader br = new BufferedReader(fr);

FileWriter fw = new FileWriter("out.txt");
BufferedWriter bw = new BufferedWriter(fw);

ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("data.obj"));
oos.writeObject(obj);

// java.util: Collections, Date, Scanner
// java.lang: Math, String, Object (auto-imported)
// java.io: File, BufferedReader, InputStream

// JVM: Executes bytecode
// JRE: JVM + standard libraries
// JDK: JRE + development tools

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

for (String arg : args) {
  System.out.println(arg);
}

System.gc();

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