Class Adapter vs. Object Adapter: Understanding and Implementing Adapter Patterns in Java

3.1.2 Class Adapter vs. Object Adapter

The Adapter Pattern is a structural design pattern that allows incompatible interfaces to work together. It acts as a bridge between two incompatible interfaces, enabling them to communicate. In Java, there are two primary forms of the Adapter Pattern: Class Adapter and Object Adapter. Each has its own unique approach and use cases, which we will explore in this section.

Class Adapter

Definition and Implementation

A Class Adapter uses inheritance to adapt one interface to another. By extending the class whose interface needs to be adapted, the class adapter can override methods and provide the necessary interface to the client.

Key Characteristics:

• Inheritance: Class adapters use inheritance to adapt interfaces. This means the adapter class must inherit from the class it adapts.
• Single Inheritance Limitation: Due to Java’s single inheritance constraint, a class adapter can only adapt one class at a time.

Java Code Example

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

// Target interface expected by the client
interface Target {
    void request();
}

// Class Adapter using inheritance
class ClassAdapter extends Adaptee implements Target {
    @Override
    public void request() {
        // Adapting the specificRequest to the request method
        specificRequest();
    }
}

// Client code
public class Client {
    public static void main(String[] args) {
        Target target = new ClassAdapter();
        target.request(); // Outputs: Specific request from Adaptee
    }
}

Object Adapter

Definition and Implementation

An Object Adapter uses composition to adapt one interface to another. Instead of inheriting from the class it adapts, it holds an instance of the class and delegates calls to it.

Key Characteristics:

• Composition: Object adapters use composition, holding a reference to the adaptee object and forwarding requests to it.
• Multiple Adaptation: Can adapt multiple classes or interfaces since it does not rely on inheritance.

Java Code Example

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

// Target interface expected by the client
interface Target {
    void request();
}

// Object Adapter using composition
class ObjectAdapter implements Target {
    private Adaptee adaptee;

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

    @Override
    public void request() {
        // Adapting the specificRequest to the request method
        adaptee.specificRequest();
    }
}

// Client code
public class Client {
    public static void main(String[] args) {
        Adaptee adaptee = new Adaptee();
        Target target = new ObjectAdapter(adaptee);
        target.request(); // Outputs: Specific request from Adaptee
    }
}

UML Diagrams

To better understand the differences between class and object adapters, let’s look at UML diagrams for each approach.

Class Adapter UML

    classDiagram
	    class Target {
	        +request()
	    }
	    class Adaptee {
	        +specificRequest()
	    }
	    class ClassAdapter {
	        +request()
	    }
	    Target <|.. ClassAdapter
	    ClassAdapter --|> Adaptee

Object Adapter UML

    classDiagram
	    class Target {
	        +request()
	    }
	    class Adaptee {
	        +specificRequest()
	    }
	    class ObjectAdapter {
	        -Adaptee adaptee
	        +request()
	    }
	    Target <|.. ObjectAdapter
	    ObjectAdapter --> Adaptee

Advantages and Limitations

Class Adapter

Advantages:

• Access to Protected Members: Can access protected members of the adaptee class due to inheritance.
• Simplicity: Directly inherits and overrides methods, making it straightforward for simple adaptations.

Limitations:

• Tight Coupling: Inherits from the adaptee, leading to tight coupling and potential issues with future changes.
• Single Inheritance Restriction: Limited by Java’s single inheritance, restricting its use to adapting one class at a time.

Object Adapter

Advantages:

• Loose Coupling: Uses composition, promoting loose coupling and flexibility.
• Multiple Adaptation: Can adapt multiple classes or interfaces, providing greater versatility.

Limitations:

• Complexity: May introduce additional complexity due to delegation.
• Performance Overhead: Slight performance overhead due to additional object composition.

When to Use Each Adapter

Class Adapter

• Access to Protected Members: When access to protected members of the adaptee class is necessary.
• Simple Adaptation Needs: When the adaptation is straightforward and involves a single class.

Object Adapter

• Multiple Adaptation Needs: When adapting multiple classes or interfaces is required.
• Loose Coupling Requirement: When loose coupling is a priority for maintainability and extensibility.

Best Practices

• Evaluate Design Requirements: Carefully assess the design requirements to choose the appropriate adapter approach.
• Favor Composition Over Inheritance: Generally, prefer object adapters to promote loose coupling unless specific needs dictate otherwise.
• Consider Future Changes: Anticipate potential changes and choose the adapter type that offers the best balance between flexibility and complexity.

Conclusion

Choosing between a class adapter and an object adapter depends on the specific needs and constraints of your project. Understanding the trade-offs and benefits of each approach is crucial for effective design pattern implementation in Java.

Quiz Time!