QUICK REFERENCE

Java OOP Cheat Sheet

The essential syntax, patterns, and boilerplate every Java LLD engineer needs at their fingertips.

Basics

Class Anatomy

The fundamental structure of a Java class.

public class Dog {
    static String species = "Canis"; // Class Var
    private String name; // Instance Var
    public Dog(String name) { // Constructor
        this.name = name;
    }
    public void bark() {
        System.out.println(this.name + " says Woof!");
    }
}
Core

Encapsulation

Using private fields and public getters/setters.

public class Account {
    private double balance; // Data Hiding
    public double getBalance() {
        return balance;
    }
    public void setBalance(double amount) {
        if (amount >= 0) { // Validation logic
            this.balance = amount;
        }
    }
}
Inheritance

Inheritance

Extending classes using the 'extends' keyword.

class Animal {
    void speak() { System.out.println("..."); }
}
class Dog extends Animal {
    @Override
    void speak() {
        super.speak(); // Call parent method
        System.out.println("Woof!");
    }
}
Polymorphism

Overloading

Compile-time polymorphism (Same method, diff params).

class Calculator {
    // Method Overloading
    int add(int a, int b) { 
        return a + b; 
    }
    
    double add(double a, double b) { 
        return a + b; 
    }
}
Abstraction

Interfaces

Contracts that define 'what' not 'how'. Supports multiple inheritance.

interface Flyable {
    void fly(); // Abstract by default
}
class Bird implements Flyable {
    @Override
    public void fly() {
        System.out.println("Flapping wings...");
    }
}
Abstraction

Abstract Classes

Partial implementation. Can have both abstract and concrete methods.

abstract class Vehicle {
    abstract void move(); // No body
    
    void fuel() { // Concrete method
        System.out.println("Refueling...");
    }
}
Modern Java

Java Records

Immutable data carriers (Java 14+). Concise data classes.

// Automatically generates constructor, 
// getters, equals(), hashCode(), toString()
public record User(int id, String username) {}
User u = new User(1, "admin");
System.out.println(u.username());
Utils

Collections

Essential data structures for LLD.

import java.util.*;
List<String> list = new ArrayList<>();
list.add("Item");
Map<String, Integer> map = new HashMap<>();
map.put("Key", 100);
Set<String> unique = new HashSet<>();
Patterns

Thread-Safe Singleton

Double-checked locking implementation.

public class Database {
    private static volatile Database instance;
    private Database() {} // Private constructor
    public static Database getInstance() {
        if (instance == null) {
            synchronized (Database.class) {
                if (instance == null) instance = new Database();
            }
        }
        return instance;
    }
}