The method or property can be overridden.

1. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.
2.  When at least one of the methods in the class is abstract.

A class that cannot be instantiated.  An abstract class is a class that must be inherited and have the methods overridden.  An abstract class is essentially a blueprint for a class without any implementation.

They check to see if the references both point to the same object.

In C++ terminology, all public methods in Javaare virtual. Therefore, all Java methods can be overwritten in subclasses except those that are declared final, static, and private.

By using Adapter class we can implement the interface's required methods.
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