Design Patterns – Strategy

Introduction to the Strategy Pattern

The Strategy pattern is a behavioral design pattern that defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows a client to choose an appropriate algorithm from a family of algorithms at runtime. This pattern promotes flexibility and enables the selection of algorithms without altering the client’s code.

Key Components of the Strategy Pattern

The Strategy pattern consists of the following key components:

• Context: The context is the class that maintains a reference to the chosen strategy object and uses it to execute an algorithm. It is often responsible for configuring and initializing the strategy.
• Strategy: The strategy is an interface or an abstract class that defines the algorithm to be used. Concrete strategy classes implement this interface with specific algorithms.
• Concrete Strategies: Concrete strategy classes are implementations of the strategy interface. They encapsulate specific algorithms and can be swapped within the context without affecting the client code.

Implementing the Strategy Pattern

Let’s see an example of how the Strategy pattern can be implemented in Java:

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

// Concrete strategy: Credit Card
class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

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

    public void pay(int amount) {
        System.out.println("Paid " + amount + " dollars using credit card " + cardNumber);
    }
}

// Concrete strategy: PayPal
class PayPalPayment implements PaymentStrategy {
    private String email;

    public PayPalPayment(String email) {
        this.email = email;
    }

    public void pay(int amount) {
        System.out.println("Paid " + amount + " dollars using PayPal with email " + email);
    }
}

// Context
class ShoppingCart {
    private PaymentStrategy paymentStrategy;

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

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

In this example, we have a PaymentStrategy interface, two concrete strategies (CreditCardPayment and PayPalPayment), and a ShoppingCart context. The ShoppingCart can be configured with different payment strategies, allowing customers to choose their preferred payment method at runtime.

Usage of the Strategy Pattern

The Strategy pattern is widely used in Java to support various algorithms or behaviors. It’s helpful in scenarios where you need to dynamically switch between different algorithms or implement runtime configuration. Common use cases include sorting algorithms, compression methods, and payment processing systems.

Benefits of the Strategy Pattern

The Strategy pattern offers several advantages:

• Flexibility: It allows you to change algorithms or behaviors at runtime without modifying the client’s code, promoting flexibility and maintainability.
• Reusability: Strategies can be reused in different contexts, as they are decoupled from the context and other strategies.
• Testability: Testing individual strategies is straightforward, making it easier to verify their correctness.

Comparison with Other Patterns

The Strategy pattern is often compared to other design patterns, such as the State pattern and the Command pattern. While all of these patterns deal with encapsulating behavior, the Strategy pattern focuses on allowing clients to choose from a family of interchangeable algorithms, whereas the State pattern deals with state-specific behavior and the Command pattern focuses on decoupling requests from objects.

Conclusion

The Strategy pattern is a valuable design pattern for promoting flexibility and maintainability in Java applications. It allows you to encapsulate algorithms and behaviors, making it easy to switch between different strategies at runtime. By implementing the Strategy pattern, you can create more versatile and adaptable software systems.