Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 588

Is-a Versus Has-a Relationships
There are ways to connect related objects without using inheritance. Consider the
task of writing a Circle class, in which each Circle object is specified by a center
point and a radius. It might be tempting to have Circle extend Point and add the
radius field. However, this approach is a poor choice because a class is only sup-
posed to capture one abstraction, and a circle simply isn't a point.
A point does make up a fundamental part of the state of each Circle object,
though. To capture this relationship in the code, you can have each Circle object
hold a Point object in a field to represent its center. One object containing another as
state is called a has-a relationship.

Has-a Relationship
A connection between two objects where one has a field that refers to the
other. The contained object acts as part of the containing object's state.

Has-a relationships are preferred over is-a relationships in cases in which your
class cannot or should not substitute for the other class. As a nonprogramming anal-
ogy, people occasionally need legal services in their lives, but most of them will
choose to have a lawyer handle the situation rather than to be a lawyer themselves.
The following code presents a potential initial implementation of the Circle
class:

1 // Represents circular shapes.
2 public class Circle {
3 private Point center;

4 private double radius;

5
6 // constructs a new circle with the given radius
7 public Circle(Point center, double radius) {
8 this .center = center;
9 this .radius = radius;
10 }
11
12 // returns the area of this circle
13 public double getArea() {
14 return Math.PI * radius * radius;
15 }
16 }

This design presents a Circle object as a single clear abstraction and prevents
awkward commingling of Circle and Point objects.