Java 8 introduced the concept of default methods in interfaces. Default methods allow us to add new functionality to existing interfaces without breaking the implementation of implementing classes.

What are Default Methods?

A default method is a method in an interface that provides a default implementation. It allows us to define a method in an interface and provide a default implementation for it.

Syntax

The syntax to define a default method in an interface is as follows:

public interface MyInterface {
    // Regular abstract method
    void regularMethod();
    
    // Default method with a default implementation
    default void defaultMethod() {
        // Default implementation goes here
    }
}

Usage

Default methods are mainly used to add new functionality to an existing interface without breaking the implementations of the classes that implement the interface. They provide a way to extend the behavior of an interface without modifying the existing code.

For example, consider an interface called Drawable:

public interface Drawable {
    void draw();
    
    default void print() {
        System.out.println("Printing...");
    }
}

In the above code, the Drawable interface declares a draw method as an abstract method and a print method as a default method with a default implementation. Any class that implements the Drawable interface can choose to override the draw method but can also utilize the default implementation of the print method.

Benefits of Default Methods

Default methods provide several benefits:

1. Backward compatibility: Default methods allow us to add new methods to existing interfaces without breaking the code that implements those interfaces. This ensures backward compatibility and makes it easier to evolve existing APIs.

2. Code reuse: Default methods enable us to reuse common implementation logic across multiple classes that implement the same interface. This reduces code duplication.

3. Convenience: Default methods make it convenient to add new functionality to interfaces without requiring all implementing classes to update their code.

Caveats

Although default methods provide many benefits, they should be used with caution. Here are a few caveats to keep in mind:

1. Diamond Problem: Default methods can create conflicts if multiple interfaces provide default implementations for the same method. In such cases, the implementing class must provide its own implementation to resolve the conflict.

2. Default Method Overriding: When a class implements multiple interfaces that have default methods with the same name, the class needs to explicitly override the default method to provide its own implementation.

Conclusion

Default methods in interfaces are a powerful feature introduced in Java 8. They provide a way to add new functionality to existing interfaces without breaking the implementations. Default methods enhance code reuse, promote backward compatibility, and make it easier to evolve existing APIs. However, care should be taken to handle conflicts that may arise due to multiple default method implementations.