Abstract Methods, Interfaces and Classes

[Source: java.sun.com]

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.

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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).

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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 GraphicObjects 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.

Summary of Inheritance

Except for the Object class, a class has exactly one direct superclass. A class inherits fields and methods from all its superclasses, whether direct or indirect. A subclass can override methods that it inherits, or it can hide fields or methods that it inherits. (Note that hiding fields is generally bad programming practice.)

The table in Overriding and Hiding Methods section shows the effect of declaring a method with the same signature as a method in the superclass.

The Object class is the top of the class hierarchy. All classes are descendants from this class and inherit methods from it. Useful methods inherited from Object include toString(), equals(), clone(), and getClass().

You can prevent a class from being subclassed by using the final keyword in the class's declaration. Similarly, you can prevent a method from being overridden by subclasses by declaring it as a final method.

An abstract class can only be subclassed; it cannot be instantiated. An abstract class can contain abstract methods—methods that are declared but not implemented. Subclasses then provide the implementations for the abstract methods.

What Is an Interface?

As you've already learned, objects define their interaction with the outside world through the methods that they expose. Methods form the object's interface with the outside world; the buttons on the front of your television set, for example, are the interface between you and the electrical wiring on the other side of its plastic casing. You press the "power" button to turn the television on and off.

In its most common form, an interface is a group of related methods with empty bodies. A bicycle's behavior, if specified as an interface, might appear as follows:

interface Bicycle {
 
 void changeCadence(int newValue);
 
 void changeGear(int newValue);
 
 void speedUp(int increment);
 
 void applyBrakes(int decrement);
}

To implement this interface, the name of your class would change (to ACMEBicycle, for example), and you'd use the implements keyword in the class declaration:

class ACMEBicycle implements Bicycle {
 
 // remainder of this class implemented as before
 
}

Implementing an interface allows a class to become more formal about the behavior it promises to provide. Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.

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Note:  To actually compile the ACMEBicycle class, you'll need to add the public keyword to the beginning of the implemented interface methods. You'll learn the reasons for this later in the lessons on Classes and Objects and Interfaces and Inheritance.