|Bruce Eckel's Thinking in Java||Contents | Prev | Next|
Once a class has been created and tested, it should (ideally) represent a useful unit of code. It turns out that this reusability is not nearly so easy to achieve as many would hope; it takes experience and insight to achieve a good design. But once you have such a design, it begs to be reused. Code reuse is arguably the greatest leverage that object-oriented programming languages provide.
The simplest way to reuse a class is to just use an object of that class directly, but you can also place an object of that class inside a new class. We call this “creating a member object.” Your new class can be made up of any number and type of other objects, whatever is necessary to achieve the functionality desired in your new class. This concept is called composition, since you are composing a new class from existing classes. Sometimes composition is referred to as a “has-a” relationship, as in “a car has a trunk.”
Composition comes with a great deal of flexibility. The member objects of your new class are usually private, making them inaccessible to client programmers using the class. This allows you to change those members without disturbing existing client code. You can also change the member objects at run time , which provides great flexibility. Inheritance, which is described next, does not have this flexibility since the compiler must place restrictions on classes created with inheritance.
Because inheritance is so important in object-oriented programming it is often highly emphasized, and the new programmer can get the idea that inheritance should be used everywhere. This can result in awkward and overcomplicated designs. Instead, you should first look to composition when creating new classes, since it is simpler and more flexible. If you take this approach, your designs will stay cleaner. It will be reasonably obvious when you need inheritance.