Adapter Class in Java

Last Updated on December 28, 2023 by Ankit Kochar

Java’s Adapter class is a fundamental component that simplifies the implementation of interfaces by providing default implementations for all methods within an interface. It serves as a bridge between an interface and a class by allowing the class to use only the methods it needs, rather than implementing all methods of the interface. This flexibility streamlines the development process, enabling developers to focus on specific functionalities without the obligation to define all interface methods.

What is an Adapter Class in Java?

An adapter class in java is a class that provides a default implementation of all methods in a listener interface. It is used to make listener interface implementation easier by providing default methods that can be modified as per requirement. Adapter classes are found in the java.awt.event and javax.swing.event packages, which are used to handle events in java graphical user interfaces (GUI). With the help of the adapter class, developers can establish event listeners by implementing only the methods that are needed for their application. This decreases the amount of code needed to implement listener interfaces while also increasing code readability and maintainability.

Relationship Between Adapter Class in Java and Listener Interface

When we wish to use the majority of the methods given under the event listener interface, we implement event listeners in our code. But, if we implement any listener interface directly in a class, we must implement all of the methods within that interface, which increases the code size unnecessarily. This type of difficulty can be resolved with the help of an adapter class in java. When we only intend to use selected methods, an adapter class is the best solution to create an event listener.

To use an adapter class, we just need to create a subclass for it and override the required methods. To further understand this, consider the example of MouseListener interface. The primary function of this interface is to be informed whenever the state of the mouse changes. There are five methods available in this interface: mouse clicked, mouseExited, mouseEntered, mousePressed, and mouseReleased. When an event occurs, even if only one of these methods is required, all of them must be implemented which means that methods that are not required are left untouched. To avoid this wasteful implementation, an adapter class is implemented.

Types of Adapter Classes in Java

The adapter class in java can be found in three separate packages. The three packages are as follows:

java.awt. event | Adapter Class in Java
All of the classes required for event handling in java are contained in the package java.awt.event. Each event class includes a user interface via which you can create handlers for it. A java interface is solely composed of abstract methods. If you create a class that implements an interface, whether you utilize it or not, you must give an implementation for each of its methods (the standard practice is to provide an empty block if you don’t use it).

To make programming easier, the java.awt.event package offers classes that implement the interfaces and provide default implementations for their methods (usually doing nothing). In the Java API, an event-handling interface is referred to as a listener interface.

When the state of an object changes, an event occurs. For example, clicking a button, dragging a mouse, and so on. The java.awt.event package contains a number of event classes and Listener interfaces for handling events.

java.awt.dnd | Adapter Class in Java
The java.awt.dnd package, introduced in Java 1.2, consists of classes and interfaces that simplify data transfer via the drag-and-drop metaphor. This feature is based on the data transfer framework provided by the java.awt.datatransfer package. A DropTarget object is a proxy for an object that wants to receive drops, whereas a DragSource object is a proxy for an item that starts dragging. This package’s numerous event classes manage all inter-object communication throughout the drag operation.

java.swing. event | Adapter Class in Java
The javax.swing.event package adds Swing-specific event objects, listeners, and adapters to the java.awt.event package. Event types are specified by classes with names that end in "Event," and their fields and methods provide information about the occurrence. Event listeners are interfaces whose names end in "Listener."

The methods of these interfaces are used to notify interested objects when specific events occur. Listener interface implementations with names that end in "Adapter" are useful no-op implementations. Subclassing an adapter class is typically less difficult than creating the corresponding listener interface from scratch.

Difference Between the Adapter Class in Java and the Object Adapter.

There are two types of adapter class in java: Class Adapter and Object Adapter. The main difference between Class Adapter and Object Adapter is in the way they achieve the adaptation of interfaces.

Class Adapter:
In the Class Adapter pattern, the adapter class extends the adaptee class and implements the target interface. This way, the adapter inherits the functionality of the adaptee and can also use it to fulfill the target interface’s requirements.
java

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

In this example, the Adapter class extends the Adaptee class and implements the Target interface. The request() method of the Adapter class calls the specificRequest() method of the Adaptee class to fulfill the Target interface’s requirements.

Object Adapter:
In Object Adapter pattern, the adapter class contains an instance of the adaptee class and implements the target interface. This way, the adapter can use the adaptee’s functionality to fulfill the target interface’s requirements.

public class Adapter implements Target {
    private Adaptee adaptee;

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

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

In this example, the Adapter class contains an instance of the Adaptee class and implements the Target interface. The request() method of the Adapter class calls the specificRequest() method of the Adaptee class to fulfill the Target interface’s requirements.

So, the key difference between the two types of Adapter classes is that the Class Adapter uses inheritance to adapt the interfaces, whereas Object Adapter uses composition. In general, Object Adapter is preferred over Class Adapter because it is more flexible and allows for easier swapping of adaptee classes.

Advantages of Adapter Class in Java

Here are some advantages of adapter class in java:

• With the adapter class, even distinct or unrelated classes can work (function) together.
• With the help of adapter class in java, similar classes can be used in a variety of ways.
• It contributes to increasing the visibility of the classes.
• It makes working with disconnected classes easier.
• It greatly improves the reusability of the classes.
• Users get an option to use the pluggable kit to create apps, making it extremely handy.
• The adapter class also facilitates in grouping the patterns together in a class.

Conclusion
In Java, the Adapter class pattern proves to be an effective tool for implementing interfaces efficiently. By offering default implementations for interface methods, it reduces the effort required to create classes that conform to an interface’s contract. Developers benefit from its flexibility, as it allows selective implementation of interface methods, promoting cleaner and more maintainable code. The Adapter class design pattern significantly contributes to Java’s versatility, enabling developers to write more concise and focused code while ensuring adherence to interface specifications.

FAQs (Frequently Asked Questions) Based on Adapter Class in Java

Here are some frequently asked questions related to adapter class in java.

1. What is the purpose of the Adapter class in Java?
The Adapter class in Java serves as a bridge between interfaces and classes. It provides default implementations for all methods within an interface, allowing classes to selectively override only the methods they need. This simplifies the implementation of interfaces by reducing the mandatory requirement to define all methods.

2. How does the Adapter class enhance code reusability in Java?
The Adapter class promotes code reusability by providing default implementations for interface methods. This allows multiple classes to extend the Adapter class and selectively override only the required methods, avoiding redundant code and ensuring consistency across implementations.

3. Can multiple interfaces be implemented using the Adapter class in Java?
Yes, the Adapter class can be extended to implement multiple interfaces by selectively overriding the necessary methods from each interface. This flexibility allows a single Adapter class to provide functionalities from various interfaces to its subclasses.

4. When should one use the Adapter class in Java?
The Adapter class is beneficial when implementing interfaces with multiple methods, especially when a class needs to utilize only a subset of these methods. It’s useful in scenarios where developers seek a streamlined way to implement interfaces without being burdened by the necessity to define all methods.

5. Are there any limitations to using the Adapter class in Java?
While the Adapter class simplifies interface implementation, it might introduce some overhead, especially if many methods within an interface need to be overridden. Additionally, misuse of the Adapter pattern might lead to complex class hierarchies, which could reduce code readability and maintainability. Hence, it’s crucial to use the Adapter pattern judiciously, considering the specific requirements of the project.