In what situations is it recommended to use interfaces in development?

Interfaces in Java are recommended to be used in the following situations:

• To define a contract that different classes must implement, ensuring polymorphism.
• When you need to separate the declaration of behavior from its implementation, so that implementations can be changed without modifying the code that uses the interface.
• To simplify testing by allowing real implementations to be replaced with mocks or stubs.
• When designing architecture using SOLID principles, especially the Interface Segregation Principle (ISP) and Dependency Inversion Principle (DIP).
• To implement event-driven models, callbacks, and other patterns where abstraction is important.

Example:

public interface PaymentProcessor {
    void processPayment(double amount);
}

public class PaypalProcessor implements PaymentProcessor {
    public void processPayment(double amount) {
        // implementation via PayPal
    }
}

public class OrderService {
    private PaymentProcessor paymentProcessor;

    public OrderService(PaymentProcessor paymentProcessor) {
        this.paymentProcessor = paymentProcessor;
    }

    public void checkout(double amount) {
        paymentProcessor.processPayment(amount);
    }
}