Implementing an Interface in java

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.