What is an adapter in the context of Java programming?

Adapter is a structural design pattern that allows objects with incompatible interfaces to work together. It acts as a mediator, transforming the interface of one class into the interface expected by another class.

It is used when:

• You need to use an existing class, but its interface does not match your needs.
• You need to create a class that interacts with several existing classes with different interfaces.

There are two main types of implementation:

• Class Adapter (via inheritance): The adapter inherits from the class being adapted and implements the target interface.
• Object Adapter (via composition): The adapter contains an instance of the class being adapted and implements the target interface, delegating calls to the adapted object.

Example of an object adapter:

// Target interface
interface Target {
    void request();
}

// Adaptee with incompatible interface
class Adaptee {
    public void specificRequest() {
        System.out.println("Specific request from Adaptee");
    }
}

// Adapter implementing Target and using Adaptee
class Adapter implements Target {
    private Adaptee adaptee;

    public Adapter(Adaptee adaptee) {
        this.adaptee = adaptee;
    }

    @Override
    public void request() {
        // Transforming request() call to specificRequest() of Adaptee
        adaptee.specificRequest();
    }
}

// Usage example
public class Client {
    public static void main(String[] args) {
        Adaptee adaptee = new Adaptee();
        Target target = new Adapter(adaptee); // Using the adapter
        target.request(); // Calling method via the target interface
    }
}

Advantages of the adapter:

• Allows using existing classes without modifying their source code.
• Improves code reusability.
• Simplifies integration of classes with incompatible interfaces.