Friday, January 30, 2009

Abstract Methods and Classes  


An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed. 

An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this: 
abstract void moveTo(double deltaX, double deltaY);
If a class includes abstract methods, the class itself must be declared abstract, as in: 
public abstract class GraphicObject {
  // declare fields
  // declare non-abstract methods
  abstract void draw();
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 declared abstract.

Abstract Classes versus Interfaces:

Unlike interfaces, abstract classes can contain fields that are not static and final, 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 contains only abstract method declarations, it should be declared as an interface instead. 

Multiple interfaces can be implemented by classes anywhere in the class hierarchy, whether or not they are related to one another in any way. Think of Comparable or Cloneable, for example. 

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).

When an Abstract Class Implements an Interface:

In the section on Interfaces , it was noted that a class that implements an interface must implement all of the interface's methods. It is possible, however, to define a class that does not implement all of the interface methods, provided that the class is declared to be abstract. For example, 
abstract class X implements Y {
  // implements all but one method of Y

class XX extends X {
  // implements the remaining method in Y
In this case, class X must be abstract because it does not fully implement Y, but class XX does, in fact, implement Y. 

Class Members:

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.

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