Interface Inheritance

Some useful interfaces found in the Java API:

An abstract class in Java is similar to an interface in that it cannot be instantiated and provides a set of methods that an inheriting class can implement.  Methods in an abstract class declared as abstract must be implemented by an inheriting class, much like any method declared in an interface.  Because abstract methods contain no implementation, a class that contains an abstract method is considered incomplete, and can therefore not be instantiated.  In other words, any class containing an abstract method becomes abstract itself, and must be declared so.  Even if an abstract class contains no abstract methods, it still cannot be instantiated.  Similarly, any subclass that does not implement all abstract methods is also abstract.