Strategy Design pattern in java

              A design pattern that selects the algorithm’s behavior dynamically. Basically,this is a behavioral design pattern. It follows the runtime instruction to choose the algorithm. So it is more flexible.

How to implement the strategy design pattern?

It includes the three major components.

Strategy Interface: It is the basic for all implementation.

Concrete Strategies: It implements the interface with specific functionality.

Context class: this is thee one which allows the strategy to choose at runtime.

A shopping Application is developed here.

Strategy interface: PayBillStrategy

CreditCardPay ,PayPalPay : concrete strategy

Billpay() is used to pay the bill. It is overridden for two strategy.

Context class: Shopping Cart

setPayBillStrategy() : Select the strategy at runtime.

Checkout() : used to send cart to bill payment.

Shop App : It is the class with main function. It creates the objects and call the context class member functions.

#Java Program to implement the strategy design pattern

interface PayBillStrategy {

    void billpay(int amount);

}

class CreditCardPay implements PayBillStrategy {

    private String cardNo;

    public CreditCardPay(String cardNo) {

        this.cardNo = cardNo;

    }

     @Override

    public void billpay(int amt) {

        System.out.println("Paid " + amt + " using Credit Card Payment: " + cardNo);

    }

}

 class PayPalPay implements PayBillStrategy {

    private String emailid;

    public PayPalPay(String emailid) {

        this.emailid = emailid;

    }

    @Override

    public void billpay(int amt) {

        System.out.println("Paid " + amt + " using PayPal payment: " + emailid);

    }

}

class ShoppingCart {

    private PayBillStrategy paybillStrategy;

    // Setting the strategy at runtime

    public void setPayBillStrategy(PayBillStrategy paybillStrategy) {

        this.paybillStrategy = paybillStrategy;

    }

    public void checkout(int amt) {

        paybillStrategy.billpay(amt);

    }

}

public class ShopApp {

    public static void main(String[] args) {

        ShoppingCart cart1 = new ShoppingCart();

        System.out.println("Shopping Application");

        System.out.println("Your Billing deatails:");

        // Using Credit Card Pay bill strategy

        cart1.setPayBillStrategy(new CreditCardPay("9876-5432-0129-8765"));

        cart1.checkout(230);

         // Using PayPal Pay bill strategy

        cart1.setPayBillStrategy(new PayPalPay("customer@rajeevas.com"));

        cart1.checkout(180);

    }

}

While executing this program, you get the below output.

C:\raji\blog>javac ShopApp.java

C:\raji\blog>java ShopApp

Shopping Application

Your Billing deatails:

Paid 230 using Credit Card: 9876-5432-0129-8765

Paid 180 using PayPal: customer@rajeevas.com

This strategy is flexible and maintainable. Thus the way of strategy design pattern in java was implemented using a shopping application.