Software Engineering with Inheritance

This section discusses customizing existing software with inheritance. When a new class extends an existing class, the new class inherits the non-private members of the existing class. We can customize the new class to meet our needs by including additional members and by overriding superclass members. Doing this does not require the subclass programmer to change the superclass's source code. Java simply requires access to the superclass's .class file so it can compile and execute any program that uses or extends the superclass. This powerful capability is attractive to independent software vendors (ISVs), who can develop proprietary classes for sale or license and make them available to users in bytecode format. Users then can derive new classes from these library classes rapidly and without accessing the ISVs' proprietary source code.

Software Engineering Observation 9.9

Despite the fact that inheriting from a class does not require access to the class's source code, developers often insist on seeing the source code to understand how the class is implemented. Developers in industry want to ensure that they are extending a solid classfor example, a class that performs well and is implemented securely.

Sometimes, students have difficulty appreciating the scope of the problems faced by designers who work on large-scale software projects in industry. People experienced with such projects say that effective software reuse improves the software development process. Object-oriented programming facilitates software reuse, potentially shortening development time.

The availability of substantial and useful class libraries delivers the maximum benefits of software reuse through inheritance. Application designers build their applications with these libraries, and library designers are rewarded by having their libraries included with the applications. The standard Java class libraries that are shipped with J2SE 5.0 tend to be rather general purpose. Many special-purpose class libraries exist and more are being created.

Software Engineering Observation 9.10

At the design stage in an object-oriented system, the designer often finds that certain classes are closely related. The designer should "factor out" common instance variables and methods and place them in a superclass. Then the designer should use inheritance to develop subclasses, specializing them with capabilities beyond those inherited from the superclass.

Software Engineering Observation 9.11

Declaring a subclass does not affect its superclass's source code. Inheritance preserves the integrity of the superclass.

Software Engineering Observation 9.12

Just as designers of non-object-oriented systems should avoid method proliferation, designers of object-oriented systems should avoid class proliferation. Such proliferation creates management problems and can hinder software reusability, because in a huge class library it becomes difficult for a client to locate the most appropriate classes. The alternative is to create fewer classes that provide more substantial functionality, but such classes might prove cumbersome.

Performance Tip 9.1

If subclasses are larger than they need to be (i.e., contain too much functionality), memory and processing resources might be wasted. Extend the superclass that contains the functionality that is closest to what is needed.

Reading subclass declarations can be confusing, because inherited members are not declared explicitly in the subclasses, but are nevertheless present in them. A similar problem exists in documenting subclass members.