Marker interface pattern
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The marker interface pattern is a design pattern in computer science.
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 Cloneable (http://java.sun.com/j2se/1.4.2/docs/api/) interface should be implemented by a class if it fully supports the Object.clone() (http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html#clone()) method. Every class in Java has the Object (http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html) class at the root of its inheritance hierarchy and so every object instantiated from any class has an associated clone() method. However, developers should only call clone() on objects of classes which implement the Clonable interface, as it indicates that the cloning functionality is actually supported in a proper manner.
Unfortunately there is a problem with this Java example, namely that in Java you cannot "unimplement" an interface. So if you subclass a class that implements Clonable, and then do something in your subclass that means it can't be cloned properly, your subclass will still be marked as a Clonable whether you want it or not. One way out is to throw an Exception (a CloneNotSupportedException (http://java.sun.com/j2se/1.4.2/docs/api/java/lang/CloneNotSupportedException.html) is a good idea) in the clone() 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.