The Java AWT provides a number of adapter classes for the
different EventListener interfaces. These adapter classes
implements the corresponding interfaces with a series of
do-nothing methods. 

By subclassing the respective Adapter rather than
implementing EventListener interfaces directly, one can
avoid having to write the methods that are not actually
needed. Only those methods can be overridden that are
planned to be actually implemented.

By using Adapter class we can implement the interface's 
required methods.for ex. in an interface there are 100's of 
methods,but we want to implement only two methods.By using 
Adapter class we can implement those two methods and rest 
are defined by null body.

Adapter classes are the null implementation for the methods 
od container class