Subclasses and Inheritance

The Circle defined earlier is a simple class that distinguishes circle objects only by their radii. Suppose, instead, that we want to represent circles that have both a size and a position. For example, a circle of radius 1.0 centered at point 0,0 in the Cartesian plane is different from the circle of radius 1.0 centered at point 1,2. To do this, we need a new class, which we’ll call PlaneCircle. We’d like to add the ability to represent the position of a circle without losing any of the existing functionality of the Circle class. This is done by defining PlaneCircle as a subclass of Circle so that PlaneCircle inherits the fields and methods of its superclass, Circle. The ability to add functionality to a class by subclassing, or extending, is central to the object-oriented programming paradigm.

Extending a Class

Example 3-3 shows how we can implement PlaneCircle as a subclass of the Circle class.

Example 3-3. Extending the Circle class

public class PlaneCircle extends Circle { // We automatically inherit the fields and methods of Circle, // so we only have to put the new stuff here. // New instance fields that store the center point of the circle public double cx, cy; // A new constructor method to initialize the new fields // It uses a special syntax to invoke the Circle() constructor public PlaneCircle(double r, double x, double y) { super(r); // Invoke the constructor of the superclass, Circle() this.cx = x; // Initialize the instance field cx this.cy ...

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