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Implementing an Interface

To declare a class that implements an interface, you include animplements clause in the class declaration. Your class can implement more than one interface, so theimplements keyword is followed by a comma-separated list of the interfaces implemented by the class. By convention, theimplements clause follows theextends clause, if there is one.

A Sample Interface, Relatable

Consider an interface that defines how to compare the size of objects.

public interface Relatable {            // this (object calling isLargerThan)    // and other must be instances of     // the same class returns 1, 0, -1     // if this is greater than,     // equal to, or less than other    public int isLargerThan(Relatable other);}

If you want to be able to compare the size of similar objects, no matter what they are, the class that instantiates them should implementRelatable.

Any class can implementRelatable if there is some way to compare the relative "size" of objects instantiated from the class. For strings, it could be number of characters; for books, it could be number of pages; for students, it could be weight; and so forth. For planar geometric objects, area would be a good choice (see theRectanglePlus class that follows), while volume would work for three-dimensional geometric objects. All such classes can implement theisLargerThan() method.

If you know that a class implementsRelatable, then you know that you can compare the size of the objects instantiated from that class.

Implementing the Relatable Interface

Here is theRectangle class that was presented in theCreating Objects section, rewritten to implementRelatable.

public class RectanglePlus     implements Relatable {    public int width = 0;    public int height = 0;    public Point origin;    // four constructors    public RectanglePlus() {        origin = new Point(0, 0);    }    public RectanglePlus(Point p) {        origin = p;    }    public RectanglePlus(int w, int h) {        origin = new Point(0, 0);        width = w;        height = h;    }    public RectanglePlus(Point p, int w, int h) {        origin = p;        width = w;        height = h;    }    // a method for moving the rectangle    public void move(int x, int y) {        origin.x = x;        origin.y = y;    }    // a method for computing    // the area of the rectangle    public int getArea() {        return width * height;    }        // a method required to implement    // the Relatable interface    public int isLargerThan(Relatable other) {RectanglePlus otherRect             = (RectanglePlus)other;        if (this.getArea() < otherRect.getArea())            return -1;        else if (this.getArea() > otherRect.getArea())            return 1;        else            return 0;                   }}

BecauseRectanglePlus implementsRelatable, the size of any twoRectanglePlus objects can be compared.