Marker interface pattern: Difference between revisions

Revision as of 09:01, 19 January 2006

Revision as of 22:17, 27 January 2006

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This pattern allows a class to implement a '''marker interface''', which exposes some underlying semantic property of the class that cannot be determined solely by the class' methods. Whereas a typical interface specifies functionality (in the form of method declarations) that an implementing class must support, a marker interface need not do so. The mere presence of such an interface indicates specific behavior on the part of the implementing class. Hybrid interfaces, which both act as markers and specify required methods, are possible but may prove confusing if improperly used.

One good example of a marker interface comes from the [[Java programming language]]. The {{Javadoc:SE|java/lang|Cloneable}} interface should be implemented by a class if it fully supports the {{Javadoc:SE|java/lang|Object|clone()}} method. Every class in Java has the {{Javadoc:SE|java/lang|Object}} class at the root of its inheritance hierarchy and so every object instantiated from any class ''has'' an associated <code>clone()</code> method. However, developers should only call <code>clone()</code> on objects of classes which implement the <code>ClonableCloneable</code> interface, as it indicates that the cloning functionality is actually supported in a proper manner.

Unfortunately there is a problem with this example, namely that in Java you cannot "unimplement" an interface. So if you subclass a class that implements <code>ClonableCloneable</code>, and then do something in your subclass that means it can't be cloned properly, your subclass will still be marked as a <code>ClonableCloneable</code> whether you want it or not. One way out is to throw an Exception (a {{Javadoc:SE|java/lang|CloneNotSupportedException}} is a good idea) in the <code>clone()</code> method, but then you are missing the whole point of the marker interface.

Still, this is generally a good idea, as subclasses usually ''do'' inherit behaviors from their parents and should inherit the marker interfaces as well.