Java in a Nutshell, 5th Edition

Important Methods of java.lang.Object

As we’ve noted, all classes extend, directly or indirectly, java.lang.Object. This class defines several important methods that you should consider overriding in every class you write. Example 3-6 shows a class that overrides these methods. The sections that follow the example document the default implementation of each method and explain why you might want to override it. You may also find it helpful to look up Object in the reference section for an API listing.

Some of the syntax in Example 3-6 may be unfamiliar to you. The example uses two Java 5.0 features. First, it implements a parameterized, or generic, version of the Comparable interface. Second, the example uses the @Override annotation to emphasize (and have the compiler verify) that certain methods override Object. Parameterized types and annotations are covered in Chapter 4.

Example 3-6. A class that overrides important Object methods

// This class represents a circle with immutable position and radius. public class Circle implements Comparable<Circle> { // These fields hold the coordinates of the center and the radius. // They are private for data encapsulation and final for immutability private final int x, y, r; // The basic constructor: initialize the fields to specified values public Circle(int x, int y, int r) { if (r < 0) throw new IllegalArgumentException("negative radius"); this.x = x; this.y = y; this.r = r; } // This is a "copy constructor"--a useful alternative to clone() ...