Design Patterns

Design Patterns are reusable solutions to common software design problems.

Creational Patterns

Singleton

Ensures only one instance of a class exists.

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

Factory

Creates objects without specifying the exact class.

interface Shape {
    void draw();
}

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

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

class ShapeFactory {
    public Shape getShape(String type) {
        if (type.equals("CIRCLE")) return new Circle();
        if (type.equals("SQUARE")) return new Square();
        return null;
    }
}

Structural Patterns

Adapter

Converts one interface to another.

interface MediaPlayer {
    void play(String file);
}

class MP3Player implements MediaPlayer {
    public void play(String file) {
        System.out.println("Playing MP3: " + file);
    }
}

class MP4Player {
    public void playMP4(String file) {
        System.out.println("Playing MP4: " + file);
    }
}

class MediaAdapter implements MediaPlayer {
    MP4Player mp4Player = new MP4Player();
    
    public void play(String file) {
        mp4Player.playMP4(file);
    }
}

Behavioral Patterns

Observer

Notifies multiple objects about state changes.

interface Observer {
    void update(String message);
}

class Subject {
    private List<Observer> observers = new ArrayList<>();
    
    public void attach(Observer observer) {
        observers.add(observer);
    }
    
    public void notifyAllObservers(String message) {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }
}

Strategy

Defines a family of algorithms and makes them interchangeable.

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using Credit Card");
    }
}

class PayPalPayment implements PaymentStrategy {
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using PayPal");
    }
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;
    
    public void setPaymentStrategy(PaymentStrategy strategy) {
        this.paymentStrategy = strategy;
    }
    
    public void checkout(int amount) {
        paymentStrategy.pay(amount);
    }
}

Quiz