IT. Expert System.

Strategy 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 is also known as Policy Pattern. We defines multiple algorithms and let client application pass the algorithm to be used as a parameter. One of the best example of this 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, check this post for sorting objects in java using Java Comparable and Comparator.

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, in our case to pay the amount passed as argument.

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

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

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");
    }
 
}

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 algorithms are ready and we can implement Shopping Cart and payment method will require input as Payment 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;
    }
     
}

public class ShoppingCart {
 
    //List of items
    List items;
     
    public ShoppingCart(){
        this.items=new ArrayList();
    }
     
    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 setup with a simple program.

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

Class Diagram

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.