Java Adapter Pattern: Class vs Object Adapters with Code Examples

Java Adapter Pattern Overview

The Adapter pattern is a structural design pattern that converts the interface of a class into another interface that clients expect. It enables incompatible interfaces to work together.

Key Roles

Target : The interface the client expects.

Adapter : Implements the Target interface and holds a reference to the Adaptee.

Adaptee : The existing interface that needs adaptation.

Common Use Cases

Refactoring legacy code: converting old interfaces to new ones.

System integration: bridging interfaces between two systems.

Adopting new interfaces: making a new incompatible interface work with existing code.

Implementation Approaches

1. Class Adapter (Inheritance)

The adapter extends the Adaptee and implements the Target interface.

public interface Target {
    void request();
}

public interface Adaptee {
    void specificRequest();
}

public class Adapter extends Adaptee implements Target {
    @Override
    public void request() {
        specificRequest();
    }
}

2. Object Adapter (Composition)

The adapter implements the Target interface and contains an instance of Adaptee.

public interface Target {
    void request();
}

public interface Adaptee {
    void specificRequest();
}

public class AdapteeImpl implements Adaptee {
    @Override
    public void specificRequest() {
        // concrete implementation
    }
}

public class Adapter implements Target {
    private Adaptee adaptee;

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

    @Override
    public void request() {
        adaptee.specificRequest();
    }
}

The class adapter uses inheritance, while the object adapter uses composition, offering more flexibility.