Strategy Design pattern - Implementation [Payment]

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Why Use Strategy Pattern in Payment Processing?

E-commerce and billing systems require support for diverse payment methods such as Credit Card, PayPal, and UPI / Net Banking.

Instead of writing messy if-else or switch blocks within a shopping cart or checkout service, the Strategy Design Pattern enables you to encapsulate each payment gateway into its own dedicated class.

Key Advantages of This Approach

• Open/Closed Principle: Add new payment modes (e.g., Apple Pay, Crypto) without modifying existing checkout code.
• Loose Coupling: The checkout component does not care how individual payments are authorized.
• Runtime Flexibility: Customers can freely change their payment choice right at checkout.

Core Architecture Elements

• PaymentStrategy (Interface): Declares the common method signature (e.g., pay(int amount)).
• Concrete Strategies: Implementation classes such as CreditCardStrategy and PaypalStrategy.
• ShoppingCart / PaymentContext: Maintains a reference to the selected PaymentStrategy and initiates payment processing.

Java Code Implementation

1. Define the Strategy Interface:

public interface PaymentStrategy {
    public void pay(int amount);
}

2. Create Concrete Implementations:

public class CreditCardStrategy implements PaymentStrategy {
    private String cardNumber;

    public CreditCardStrategy(String cardNumber) {
        this.cardNumber = cardNumber;
    }

    @Override
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using Credit Card.");
    }
}

3. Configure the Context:

public class ShoppingCart {
    private PaymentStrategy paymentStrategy;

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

    public void checkout(int amount) {
        paymentStrategy.pay(amount);
    }
}

Related Design Pattern Tutorials to Watch Next

• Strategy Design Pattern in Action: Real-Time Payment Processing
• Strategy Design Pattern Overview: A Beginner's Introduction
• Strategy Design Pattern: Sequence Diagram Walkthrough

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Click here to watch in Youtube : https://www.youtube.com/watch?v=o-rSyfmBqh0

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Strategy Design pattern - Implementation [Payment]

Strategy Design pattern - Implementation [Payment] - Class Diagram

PaymentStrategy.java

public interface PaymentStrategy
{
public void pay(String amount);

}


DebitCardPaymentStrategy.java

public class DebitCardPaymentStrategy implements PaymentStrategy
{

public void pay(String amount)
{
System.out.println("Customer pays the money " +amount+ "Rs. using Debit Card");
}

}

CreditCardPaymentStrategy.java

public class CreditCardPaymentStrategy implements PaymentStrategy
{

public void pay( String amount )
{
System.out.println("Customer pays the money " + amount + "Rs. using Credit Card");
}

}

ByCashPaymentStrategy.java

public class ByCashPaymentStrategy implements PaymentStrategy
{

public void pay( String amount )
{
System.out.println("Customer pays the money " + amount + "Rs. as a Cash");
}

}


PaymentContext.java

public class PaymentContext
{
private PaymentStrategy paymentStrategy;

// Client will set what PaymentStrategy to use by calling this method
public void setPaymentStrategy(PaymentStrategy strategy)
{
this.paymentStrategy = strategy;
}

public PaymentStrategy getPaymentStrategy()
{
return paymentStrategy;
}

public void pay(String amount)
{
paymentStrategy.pay(amount);
}

}


Customer.java

import java.util.Scanner;

public class Customer
{

public static void main( String[] args )
{

System.out.println("Please enter Payment Type : 'CreditCard' or 'DebitCard' or 'ByCash'");
Scanner scanner = new Scanner(System.in);
String paymentType = scanner.next();
System.out.println("Payment type is : " + paymentType);

System.out.println("\nPlease enter Amount to Pay : ");
Scanner scanner1 = new Scanner(System.in);
String amount = scanner1.next();
System.out.println("amount is : " + amount);

PaymentContext ctx = null;
ctx = new PaymentContext();

if( "CreditCard".equalsIgnoreCase(paymentType) )
{
ctx.setPaymentStrategy(new CreditCardPaymentStrategy());
}
else if( "DebitCard".equalsIgnoreCase(paymentType) )
{
ctx.setPaymentStrategy(new DebitCardPaymentStrategy());
}
else if( "ByCash".equalsIgnoreCase(paymentType) )
{
ctx.setPaymentStrategy(new ByCashPaymentStrategy());
}

System.out.println("PaymentContext has :"+ctx.getPaymentStrategy());

ctx.pay(amount);

}
}

Output

Please enter Payment Type : 'CreditCard' or 'DebitCard' or 'ByCash'
CreditCard
Payment type is : CreditCard

Please enter Amount to Pay :
90000
amount is : 90000
PaymentContext has :CreditCardPaymentStrategy@1e4cbc4
Customer pays the money 90000Rs. using Credit Card


Please enter Payment Type : 'CreditCard' or 'DebitCard' or 'ByCash'
DebitCard
Payment type is : DebitCard

Please enter Amount to Pay :
90000
amount is : 90000
PaymentContext has :DebitCardPaymentStrategy@1e4cbc4
Customer pays the money 90000Rs. using Debit Card