Purpose of Default or Defender methods in Java 8

问题

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 Brain 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?

回答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).

回答2:

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

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

One such reason are 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), while still keeping it simple to implement.

回答3:

There was a problem with interfaces that they were not open to extension, which means if there was a need to add new method to an interface it would have broken the existing implementation of these interfaces. Thus it was imperative that all the classes implementing that interface had to provide implementation for the newly added method, even if the method was not needed. Thus interfaces were not easy to evolve.

One example that comes to mind is Java MapReduce API for Hadoop, which was changed in 0.20.0 release to favour abstract classes over interfaces, since they are easier to evolve. Which means, a new method can be added to abstract class (with default implementation), with out breaking old implementations of the class.

With the release of Java 8, it is now possible to add default method in interfaces also, thus making them easier to evolve too. With the addition of default method to an interface, addition of new method, to even an interface will not break the pre-existing code.

回答4:

1. default methods made possible the functional programming concept. For functional programming type code we need only one abstract method .
2. Also adding an method in interface will not made it compulsory for all the classes implementing an interface. Simplified the coding practise

来源：https://stackoverflow.com/questions/19998309/purpose-of-default-or-defender-methods-in-java-8