The Strategy design pattern in Java allows defining a family of algorithms, encapsulating each one, and making them interchangeable to change behavior at runtime. 1. Define a strategy interface that declares the method(s) all strategies must implement. 2. Implement concrete strategies that provide specific behavior. 3. Create a context class that uses a strategy reference to delegate work. 4. Use the pattern in your application by dynamically setting strategies at runtime, promoting flexibility and maintainability while avoiding conditional logic.
Implementing the Strategy design pattern in Java is a clean way to define a family of algorithms, encapsulate each one, and make them interchangeable. It lets you change behavior at runtime without modifying the class that uses those behaviors. This is especially useful when you want to avoid messy conditional logic (like long if-else chains) and promote code reuse.
What Is the Strategy Pattern?
The Strategy pattern involves three main components:
- Context – the class that uses a strategy.
- Strategy Interface – defines the contract for all supported algorithms.
- Concrete Strategies – implementations of the strategy interface.
This separation allows you to plug in different behaviors dynamically.
Step-by-Step Implementation
Let’s walk through how to implement this with a simple example: payment methods.
1. Define the Strategy Interface
Start by creating an interface that declares the method(s) your strategies will implement.
public interface PaymentStrategy {
void pay(int amount);
}This sets the contract: any class implementing PaymentStrategy must provide a pay() method.
2. Implement Concrete Strategies
Now create classes that implement this interface. Each represents a different payment method.
public class CreditCardPayment implements PaymentStrategy {
@Override
public void pay(int amount) {
System.out.println("Paid " amount " using Credit Card");
}
}
public class PayPalPayment implements PaymentStrategy {
@Override
public void pay(int amount) {
System.out.println("Paid " amount " via PayPal");
}
}These are two concrete strategies. You can add more later without affecting existing code.
3. Create the Context Class
The context holds a reference to a strategy and delegates work to it.
public class ShoppingCart {
private PaymentStrategy paymentStrategy;
public void setPaymentStrategy(PaymentStrategy paymentStrategy) {
this.paymentStrategy = paymentStrategy;
}
public void checkout(int totalAmount) {
if (paymentStrategy == null) {
throw new IllegalStateException("No payment strategy set");
}
paymentStrategy.pay(totalAmount);
}
}This class doesn't care which payment method is used — it just calls the strategy's pay() method.
4. Use It in Your Application
Finally, wire everything together in your main application.
public class MainApp {
public static void main(String[] args) {
ShoppingCart cart = new ShoppingCart();
// Choose strategy at runtime
cart.setPaymentStrategy(new CreditCardPayment());
cart.checkout(150);
cart.setPaymentStrategy(new PayPalPayment());
cart.checkout(200);
}
}You can switch strategies on the fly, making your system flexible and easy to extend.
Tips When Using Strategy Pattern
Here are some practical things to keep in mind:
- Use interfaces or abstract classes for strategy definitions — it makes switching easier.
- Avoid tight coupling between the context and concrete strategies.
- Encapsulate variations in behavior behind strategy classes — this keeps your code cleaner.
- Consider using dependency injection to pass strategies into the context.
- If strategies need access to data from the context, you can pass it explicitly through method parameters.
Also, be careful not to overuse this pattern. If your strategies are very simple or rarely change, it might be overkill.
Summary
So, implementing the Strategy pattern in Java boils down to defining an interface, creating multiple implementations, and letting a context object use them interchangeably. It helps reduce conditionals, improves maintainability, and supports open/closed principle.
That’s basically it. Not too hard once you get the structure right.
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