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Trail: Learning the Java Language
Lesson: Interfaces and Inheritance
Section: Inheritance
Interfaces and Inheritance
Interfaces
Defining an Interface
Implementing an Interface
Using an Interface as a Type
Rewriting Interfaces
Summary of Interfaces
Questions and Exercises
Inheritance
Overriding and Hiding Methods
Hiding Fields
Using the Keyword super
Object as a Superclass
Writing Final Classes and Methods
Abstract Methods and Classes
Summary of Inheritance
Questions and Exercises
Home Page > Learning the Java Language > Interfaces and Inheritance
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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`.

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

> ### 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).

> ### An Abstract Class Example

> In an object-oriented drawing application, you can draw circles, rectangles, lines, Bezier curves, and many other graphic objects. These objects all have certain states (for example: position, orientation, line color, fill color) and behaviors (for example: moveTo, rotate, resize, draw) in common. Some of these states and behaviors are the same for all graphic objects—for example: position, fill color, and moveTo. Others require different implementations—for example, resize or draw. All `GraphicObject`s must know how to draw or resize themselves; they just differ in how they do it. This is a perfect situation for an abstract superclass. You can take advantage of the similarities and declare all the graphic objects to inherit from the same abstract parent object—for example, `GraphicObject`, as shown in the following figure.

> Classes Rectangle, Line, Bezier, and Circle inherit from GraphicObject

> First, you declare an abstract class, `GraphicObject`, to provide member variables and methods that are wholly shared by all subclasses, such as the current position and the `moveTo` method. `GraphicObject` also declares abstract methods for methods, such as `draw` or `resize`, that need to be implemented by all subclasses but must be implemented in different ways. The `GraphicObject` class can look something like this:

> >     abstract class GraphicObject {
> >         int x, y;
> >         ...
> >         void moveTo(int newX, int newY) {
> >             ...
> >         }
> >         abstract void draw();
> >         abstract void resize();
> >     }

> Each non-abstract subclass of `GraphicObject`, such as `Circle` and `Rectangle`, must provide implementations for the `draw` and `resize` methods:

> >     class Circle extends GraphicObject {
> >         void draw() {
> >             ...
> >         }
> >         void resize() {
> >             ...
> >         }
> >     }
> >     class Rectangle extends GraphicObject {
> >         void draw() {
> >             ...
> >         }
> >         void resize() {
> >             ...
> >         }
> >     }

> ### 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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