Tutorial

Strategy Design Pattern in Java - Example Tutorial

Published on August 3, 2022
author

Pankaj

Strategy Design Pattern in Java - Example Tutorial

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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 Pattern

strategy pattern, strategy design pattern Strategy pattern is also known as Policy Pattern. We define multiple algorithms and let client application pass the algorithm to be used as a parameter. One of the best example of strategy pattern is Collections.sort() method that takes Comparator parameter. Based on the different implementations of Comparator interfaces, the Objects are getting sorted in different ways. For our example, we will try to implement a simple Shopping Cart where we have two payment strategies - using Credit Card or using PayPal. First of all we will create the interface for our strategy pattern example, in our case to pay the amount passed as argument. PaymentStrategy.java

package com.journaldev.design.strategy;

public interface PaymentStrategy {

	public void pay(int amount);
}

Now we will have to create concrete implementation of algorithms for payment using credit/debit card or through paypal. CreditCardStrategy.java

package com.journaldev.design.strategy;

public class CreditCardStrategy implements PaymentStrategy {

	private String name;
	private String cardNumber;
	private String cvv;
	private String dateOfExpiry;
	
	public CreditCardStrategy(String nm, String ccNum, String cvv, String expiryDate){
		this.name=nm;
		this.cardNumber=ccNum;
		this.cvv=cvv;
		this.dateOfExpiry=expiryDate;
	}
	@Override
	public void pay(int amount) {
		System.out.println(amount +" paid with credit/debit card");
	}

}

PaypalStrategy.java

package com.journaldev.design.strategy;

public class PaypalStrategy implements PaymentStrategy {

	private String emailId;
	private String password;
	
	public PaypalStrategy(String email, String pwd){
		this.emailId=email;
		this.password=pwd;
	}
	
	@Override
	public void pay(int amount) {
		System.out.println(amount + " paid using Paypal.");
	}

}

Now our strategy pattern example algorithms are ready. We can implement Shopping Cart and payment method will require input as Payment strategy. Item.java

package com.journaldev.design.strategy;

public class Item {

	private String upcCode;
	private int price;
	
	public Item(String upc, int cost){
		this.upcCode=upc;
		this.price=cost;
	}

	public String getUpcCode() {
		return upcCode;
	}

	public int getPrice() {
		return price;
	}
	
}

ShoppingCart.java

package com.journaldev.design.strategy;

import java.text.DecimalFormat;
import java.util.ArrayList;
import java.util.List;

public class ShoppingCart {

	//List of items
	List<Item> items;
	
	public ShoppingCart(){
		this.items=new ArrayList<Item>();
	}
	
	public void addItem(Item item){
		this.items.add(item);
	}
	
	public void removeItem(Item item){
		this.items.remove(item);
	}
	
	public int calculateTotal(){
		int sum = 0;
		for(Item item : items){
			sum += item.getPrice();
		}
		return sum;
	}
	
	public void pay(PaymentStrategy paymentMethod){
		int amount = calculateTotal();
		paymentMethod.pay(amount);
	}
}

Notice that payment method of shopping cart requires payment algorithm as argument and doesn’t store it anywhere as instance variable. Let’s test our strategy pattern example setup with a simple program. ShoppingCartTest.java

package com.journaldev.design.strategy;

public class ShoppingCartTest {

	public static void main(String[] args) {
		ShoppingCart cart = new ShoppingCart();
		
		Item item1 = new Item("1234",10);
		Item item2 = new Item("5678",40);
		
		cart.addItem(item1);
		cart.addItem(item2);
		
		//pay by paypal
		cart.pay(new PaypalStrategy("myemail@example.com", "mypwd"));
		
		//pay by credit card
		cart.pay(new CreditCardStrategy("Pankaj Kumar", "1234567890123456", "786", "12/15"));
	}

}

Output of above program is:

50 paid using Paypal.
50 paid with credit/debit card

Strategy Design Pattern Class Diagram

Strategy Pattern, Strategy Design pattern in java

Strategy Design Pattern Important Points

  • We could have used composition to create instance variable for strategies but we should avoid that as we want the specific strategy to be applied for a particular task. Same is followed in Collections.sort() and Arrays.sort() method that take comparator as argument.
  • Strategy Pattern is very similar to State Pattern. One of the difference is that Context contains state as instance variable and there can be multiple tasks whose implementation can be dependent on the state whereas in strategy pattern strategy is passed as argument to the method and context object doesn’t have any variable to store it.
  • Strategy pattern is useful when we have multiple algorithms for specific task and we want our application to be flexible to chose any of the algorithm at runtime for specific task.

That’s all for Strategy Pattern in java, I hope you liked it.

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Pankaj

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JournalDev
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February 17, 2022

its not duplicate //pay by paypal cart.pay(new PaypalStrategy(“myemail@example.com”, “mypwd”)); //pay by credit card cart.pay(new CreditCardStrategy(“Pankaj Kumar”, “1234567890123456”, “786”, “12/15”)); } can i have diference between strategy run time Polymorphism

- sudhakar

    JournalDev
    DigitalOcean Employee
    DigitalOcean Employee badge
    July 21, 2021

    Great examples and explaination, thanks to you my doubts about this patter are now gone, tahnks!

    - Oscar Montenegro

      JournalDev
      DigitalOcean Employee
      DigitalOcean Employee badge
      May 18, 2020

      Hi everyone I found this video helpful explained about design pattern while taking a real-time example of the clash of clans game I guarantee it will worth watching the video https://youtu.be/qKssS\_hbRm0

      - adarsh gupta

        JournalDev
        DigitalOcean Employee
        DigitalOcean Employee badge
        February 6, 2020

        Excelent explanation, other sites fail to demonstrate a real use case other than uml diagrams.

        - Hector

          JournalDev
          DigitalOcean Employee
          DigitalOcean Employee badge
          January 26, 2020

          Awsome Explanation

          - Satya Prakash Maurya

            JournalDev
            DigitalOcean Employee
            DigitalOcean Employee badge
            November 2, 2019

            I wish if there is an option for a standing ovation. The examples were very nice.

            - Sagar

              JournalDev
              DigitalOcean Employee
              DigitalOcean Employee badge
              September 17, 2019

              Item item1 = new Item(“1234”,10); What does this statement mean? And how would it know which parameter is the price either “1234” or 10. Can you please explain in detail

              - Kalpesh

                JournalDev
                DigitalOcean Employee
                DigitalOcean Employee badge
                May 9, 2019

                This is wrong implementation of strategy, you either put an if else loop based on a string which says which payment method to use and then call that objects pay method, or pass the interface reference of payment Strategy in the pay method.

                - sas

                  JournalDev
                  DigitalOcean Employee
                  DigitalOcean Employee badge
                  April 2, 2019

                  How this is different from Facade design pattern?

                  - GAURAV PANT

                    JournalDev
                    DigitalOcean Employee
                    DigitalOcean Employee badge
                    January 25, 2019

                    Really nice example with explanation.

                    - rup

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