Purpose of Default or Defender methods in Java 8

Java 8 has included a new feature called Defender methods which allows creation of default method implementation in interface.

Now first of all this is a huge paradigm shift for all condensed programmers in Java. I viewed a JavaOne 13 presentation given by Brian Goetz where he was discussing about the new stream() and parallelStream() implementations in Collections library.

For adding new methods in Collection interface, they could not have just added a new method without breaking the previous versions. So he told that for catering this a new feature of Default methods was added.

public interface SimpleInterface {
  public void doSomeWork();
   
  //A default method in the interface created using "default" keyword
  default public void doSomeOtherWork(){
    System.out.println("DoSomeOtherWork implementation in the interface");
  }
}

Now my question is basically that are default methods just helpful when needed to add new methods to interface without breaking client code? Or are there some other uses to it too?

Solution 1:

Besides having the possibility of adding methods to the interface in future versions, there is the important point of allowing an interface to stay a functional interface even if it has more than one method.

A functional interface has only one non-default abstract method which can be implemented via a lambda expression. One example is the Predicate interface which has only one abstract method (test) while providing default methods for negating a Predicate or combining it with another Predicate. Without default methods these methods had to be provided in another utility class like the pre-Java 8 Collections class (as you don’t want to give up the possibility of lambda implementations for such an interface).

Solution 2:

As you said, the main motivation was allowing the evolution of existing interfaces.

However there are reasons why you'd want to use them in brand new interfaces as well:

One such reason is methods that can easily be implemented using the other (non-default) methods of the interface. Using default methods for this reduces the need for Foo-interface/AbstractFoo-base-implementation combinations (see AbstractList for example).

While this does not create an entirely new field, it means that you can have end-user-friendly interfaces (with lots of useful methods), still keeping it simple to implement.