Object-Oriented Programming in Java - Java in a Nutshell - page 111

Extending a Class
In Example 3-3 , we show how we can implement PlaneCircle as a subclass of the
Circle class.

Example 3-3. Extending the Circle class

public class PlaneCircle extends Circle {
// We automatically inherit the fields and methods of Circle,
// so we only have to put the new stuff here.
// New instance fields that store the center point of the circle
private final double cx , cy ;

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// A new constructor to initialize the new fields
// It uses a special syntax to invoke the Circle() constructor
public PlaneCircle ( double r , double x , double y ) {
super ( r ); // Invoke the constructor of the superclass, Circle()
this . cx = x ; // Initialize the instance field cx
this . cy = y ; // Initialize the instance field cy
}

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public double getCentreX () {
return cx ;
}

public double getCentreY () {
return cy ;
}

// The area() and circumference() methods are inherited from Circle
// A new instance method that checks whether a point is inside the circle
// Note that it uses the inherited instance field r
public boolean isInside ( double x , double y ) {
double dx = x - cx , dy = y - cy ; // Distance from center
double distance = Math . sqrt ( dx * dx + dy * dy ); // Pythagorean theorem
return ( distance < r ); // Returns true or false
}
}

Note the use of the keyword extends in the first line of Example 3-3 . This keyword
tells Java that PlaneCircle extends, or subclasses, Circle , meaning that it inherits
the fields and methods of that class.

There are several different ways to express the idea that our
new object type has the characteristics of a Circle as well as
having a position. This is probably the simplest, but is not
always the most suitable, especially in larger systems.