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Writing Abstract Classes and Methods

Abstract Classes

Sometimes, a class that you define represents an abstract concept and, as such, should not be instantiated. Take, for example, food. Have you ever seen an instance of food? Probably not. What you see instead are instances of carrot, apple, and chocolate chip cookies. Food represents the abstract concept of what we can eat. It doesn't make sense for an instance of food to exist.

Similarly, in object-oriented programming, you may want to model an abstract concept without being able to create an instance of it. For example, the Number class represents the abstract concept of numbers. It makes sense to model numbers, but it doesn't make sense to create a generic number object. Instead, the Number class makes sense only as a superclass to such classes as Integer and Float, both of which implement specific kinds of numbers. A class such as Number, which represents an abstract concept and should not be instantiated, is called an abstract class. An abstract class can only be subclassed.

To declare that your class is an abstract class, use the keyword abstract before the class keyword in your class declaration:

abstract class Number {
    ...
}

If you attempt to instantiate an abstract class, the compiler displays an error message.

Abstract Methods

An abstract class can contain abstract methods — methods with no implementation. In this way, an abstract class can define a complete programming interface for its subclasses but allows its subclasses to fill in the implementation details of those methods. In practice, abstract classes usually provide a complete or partial implementation of at least one method. If an abstract class contains only abstract method declarations, it should be implemented as an interface instead. Interfaces are covered in the section Interfaces and Packages.

Let's look at an example of when you might want to create an abstract class with an abstract method in it. In an object-oriented drawing application, you can draw circles, rectangles, lines, Bezier curves, and so on. These graphic objects all have certain states (position, bounding box) and behaviors (move, resize, draw) in common. You can take advantage of these similarities and declare them all to inherit from the same parent object — for example, GraphicObject, as shown in the next figure.

Graphic objects are substantially different in many ways: drawing a circle is quite different from drawing a rectangle. The graphic objects cannot share these types of states or behavior. On the other hand, all GraphicObjects must know how to draw themselves; they just differ in how they are drawn. This is a perfect situation for an abstract superclass.

First, you would declare an abstract class, GraphicObject, to provide member variables and methods that were wholly shared by all subclasses, such as the current position and the moveTo method. GraphicObject also declares abstract methods for methods, such as draw, that need to be implemented by all subclasses but that are implemented in entirely different ways (no default implementation in the superclass makes sense). The GraphicObject class would look something like this:

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

Each nonabstract subclass of GraphicObject, such as Circle and Rectangle, must provide an implementation for the draw method:

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

An abstract class is not required to have an abstract method in it. But any class that has an abstract method in it or that does not provide an implementation for any abstract methods declared in its superclasses or implemented interfaces must be declared as an abstract class.

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