Design pattern for default implementation with empty methods

Question

Is there a specific design pattern that describes the scenario where a non-abstract default implementation is provided that implements all or some of the methods on the interface with empty, NO-OP implementations. This being done with the intent of alleviating subclasses with the burden of implementing methods that they themselves may not need/use:

public interface MyInterface {
    public void doThis();
    public void doThat();
    public void done();
}

public class MyClass implements MyInterface {
    public void doThis() {
        // NO-OP
    }
    public void doThat() {
        // NO-OP
    }
    public void done() {
        // Some standard implementation
    }
}

public class MuSubClass extends MyClass {
    public void doThat() {
        // Subclass only cares about doThat()
    }
}

I have seen this pattern used a number of times including Java's DefaultHandler in the SAX framework, and MouseAdapter. In somes cases such classes are named as Adaptors, but I was under the impression that the adapter pattern translates between two different interfaces.

Given that in these instances there is only one declared interface that is being translated to an undefined subset of that interface - I am not clear on how this is in the spirit of the adapter pattern.

Furthermore, I don't quite see how this adheres to the NullObject pattern either, given that some methods could have an implementation, and the NullObject is traditionally a singleton.

Answer 1

Are you asking about the Null Object Pattern?

Further to your edit, the MyClass object is nothing more than a default implemenation. I don't think there's any particular design pattern that describes it.

Answer 2

To me this seems closest to the Special Case or Null Object pattern.

Your updates suggest something similar to Template Method expect that you don't have a single method that calls each template method e.g.

public void doEverything()
{
  doThis();
  doThat();
  done();
}

Answer 3

I have seen this design used in spring where they have a class named FlowExecutionListenerAdapter which saves you implementing all the FlowExecutionListener operations.

However, it does sound like the Null Object Pattern too. However I feel it sits better in the Adapter world purely because it changing the behavour of the interface by allowing you only to implement the bit you want...but its a tough one.

I'm sure this question has been asked before?

This sounds similar no? might be worth a read.

Answer 4

There are no design patterns for default implementation.

I usually append DoNothing prefix to the name of class. Depending on it's intent I use also Base or Default (the latter is widely used). Probably MouseAdapter should be called DefaultMouseListener.

In the case you care, you can stub sistematically an interface with a simple DynamicProxy, you must return only a "nice" default value (null for Object, 0 for numberics, etc).

BTW this is a very good question.

EDIT

Furthermore this is neither a Stub or a Mock: maybe it can be confused with a Stub but the intent is different.

Answer 5

It's also used in Swing (WindowAdapter, which implements WindowListener). It's only a convenience adapter, you only have to define 1-2 methods in this way to have a useful windowlistener. This is indeed an instance of the Adapter pattern, also shows the power of the abstract classes. It's even an example to illustrate why multiple implementation inheritance is useful sometimes.

As for the regular Design Patterns, in the Temlate Method you can define hook operations, which may be overriden (unlike abstract methods, which must be), but the default behaviour (usually the NO-OP) is meaningful too.