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Message: <blockquote data-quote="sihina_lahiru" data-source="post: 7285050" data-attributes="member: 149577">3.Implementing an Interface               To declare a class that implements an interface, you  include an implements  clause in the class declaration. Your class can implement more  than one interface, so the implements keyword is followed   by a comma-separated list of the interfaces implemented by the  class.   By convention, the implements clause follows the extends 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 implement Relatable.   Any class can implement Relatable 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 the  RectanglePlus class that follows), while volume would work  for three-dimensional geometric objects.  All such classes can implement the isLargerThan() method. If you know that a class implements Relatable, then you  know that you can compare the size of the objects instantiated from that  class. Implementing the Relatable Interface    Here is the Rectangle class that was presented in the  <a href="http://java.sun.com/docs/books/tutorial/java/javaOO/objectcreation.html" target="_blank">Creating  Objects</a> section, rewritten to implement Relatable. 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;                } } Because RectanglePlus implements Relatable, the size of any two RectanglePlus objects can be compared. Note: The isLargerThan  method, as defined in the Relatable interface, takes an object of type Relatable. The line of code, shown in bold in the previous example, casts other to a RectanglePlus instance. Type casting tells the compiler what the object really is. Invoking getArea directly on the other instance (other.getArea()) would fail to compile because the compiler does not understand that other is actually an instance of RectanglePlus.          </blockquote>

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