Here is the answer.
An event-listener interface allows describing the methods which must be implemented by one of the event handler for a specific event.
An event-adapter allows default implementations of an event-listener interface of a specific event.
When you declare to implement an event listener, then you should write the definitions of all the methods that have been declared in the interface. This causes inconvenience to the programmers. They need to implement the methods that would never be used. One way to do away with this inconvenience is to use adapter classes. An event adapter class has empty implementations by default.