Abstract Classes and Interfaces in Java

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What is an Abstract Class?

A class must be declared abstract if any of the following condition is true

• The class has one or more abstract methods
• The class inherits one or more abstract methods for which it does not provide implementation
• The class declares that it implements an interface but does not provide implementation for every method of that interface

An abstract class cannot be instantiated. Abstract classes defer the implementation to subclasses. The subclass must provide the implementation of the abstract method or declare itself to be abstract in which case the implementation is deferred once again. You can declare a variable of an abstract class.  more...