About Abstract Methods and Classes

this piece of writing is reprinted from the following URL:

http://download.oracle.com/javase/tutorial/java/IandI/abstract.html

just for the use of self learning.

 

An abstract class is a class that is declared abstract—it may or may not include abstract methods.

An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon)

If a class includes abstract methods, the class itself must be declaredabstract

When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, the subclass must also be declaredabstract.

Note: All of the methods in an interface are implicitly abstract, so theabstract modifier is not used with interface methods (it could be—it's just not necessary).

Unlike interfaces, abstract classes can contain fields that are not static andfinal, and they can contain implemented methods. Such abstract classes are similar to interfaces, except that they provide a partial implementation, leaving
it to subclasses to complete the implementation. If an abstract class containsonly abstract method declarations, it should be declared as an interface instead.

By comparison, abstract classes are most commonly subclassed to share pieces of implementation. A single abstract class is subclassed by similar classes that have a lot in common (the implemented parts of the abstract class), but also have some differences
(the abstract methods).

An abstract class may have static fields and static methods. You can use these static members with a class reference—for example,
AbstractClass.staticMethod()—as you would with any other class.