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Trail: Learning the Java Language
Lesson: Interfaces and Inheritance
Section: Interfaces
Interfaces and Inheritance
Interfaces
Defining an Interface
Implementing an Interface
Using an Interface as a Type
Rewriting Interfaces
Summary of Interfaces
Questions and Exercises
Inheritance
Overriding and Hiding Methods
Hiding Fields
Using the Keyword super
Object as a Superclass
Writing Final Classes and Methods
Abstract Methods and Classes
Summary of Inheritance
Questions and Exercises
Home Page > Learning the Java Language > Interfaces and Inheritance
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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 Creating Objects 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 to implement Relatable
> >         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.

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