Strategy Pattern

Strategy design pattern is one of the behavioral design pattern. Strategy pattern is used when we have multiple algorithm for a specific task and client decides the actual implementation to be used at runtime

// Strategy interface
interface PaymentStrategy {
    void pay(double amount);
}

// Concrete strategies

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

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

class CashPayment implements PaymentStrategy {
    @Override
    public void pay(double amount) {
        System.out.println("Paid $" + amount + " in cash.");
    }
}

// Context

class Checkout {
    private PaymentStrategy paymentStrategy;

    public void setPaymentStrategy(PaymentStrategy paymentStrategy) {
        this.paymentStrategy = paymentStrategy;
    }

    public void processOrder(double amount) {
        if (paymentStrategy == null) {
            throw new IllegalStateException("Payment strategy not set");
        }
        paymentStrategy.pay(amount);
    }
}

// Client

public class Main {
    public static void main(String[] args) {
        Checkout checkout = new Checkout();

        checkout.setPaymentStrategy(new CreditCardPayment());
        checkout.processOrder(25.50);

        checkout.setPaymentStrategy(new PayPalPayment());
        checkout.processOrder(13.75);

        checkout.setPaymentStrategy(new CashPayment());
        checkout.processOrder(7.00);
    }
}

Output will be :

Paid $25.5 using Credit Card.
Paid $13.75 using PayPal.
Paid $7.0 in cash.