How to Create Optional Methods in a Java Interface

How can I implement optional methods within a Java interface?

public interface MyInterface {
    void requiredMethod(); // required method
    default void optionalMethod() {
        System.out.println("This is an optional method.");
    }
}

In Java, interfaces can contain methods that are either abstract or default. Default methods allow you to add functionality to interfaces without breaking existing implementations, making them perfect for optional methods.

// Implementation of interface
public class MyClass implements MyInterface {
    @Override
    public void requiredMethod() {
        System.out.println("Implementation of required method.");
    }
    // Optional method can be overridden if needed
    @Override
    public void optionalMethod() {
        System.out.println("Custom implementation of optional method.");
    }
}

Causes

• Multiple implementations of the interface
• The need for backward compatibility
• Separation of concerns in design

Solutions

• Use default methods in interfaces for optional behavior.
• Override default methods in implementing classes when specific behavior is needed.

Mistake: Not providing default methods when necessary.

Solution: Always consider if a default method would improve interface usability.

Mistake: Assuming all implementing classes will override the optional method.

Solution: Provide documentation for interface consumers about optional method usage.