Anonymous classes are a common way of creating implementations of an interface or abstract class whose functionality is really only needed once. Duplicating the definition of an anonymous class in several places is usually a sign that refactoring is necessary.

Code duplication in general is highly undesirable for a range of reasons. The artificially inflated amount of code is more difficult to understand, and sequences of similar but subtly different lines can mask the real purpose or intention behind them. Also, there is always a risk that only one of several copies of the code is updated to address a defect or add a feature.

Introduce a concrete class that contains the definition just once, and replace the anonymous classes with instances of this concrete class.

In the following example, the definition of the class addActionListener is duplicated for each button that needs to use it. A better solution is shown that defines just one class, MultiplexingListener, which is used by each button.

E. Juergens, F. Deissenboeck, B. Hummel, S. Wagner. Do code clones matter? Proceedings of the 31st International Conference on Software Engineering, 485-495, 2009.