Inheritance - Java Software Solutions: Foundations of Program Design - page 467

Throughout this chapter, several design issues have been addressed in the discussion
of the nuts and bolts of inheritance in Java. The following list summarizes some of
the inheritance issues that you should keep in mind during the program design stage:

Every derivation should be an is-a relationship. The child should be a more
specific version of the parent.

■

Design a class hierarchy to capitalize on reuse, and potential reuse in the
future.

■

As classes and objects are identified in the problem domain, find their
commonality. Push common features as high in the class hierarchy as
appropriate for consistency and ease of maintenance.

■

Override methods as appropriate to tailor or change the functionality of a
child.

■

Add new variables to the child class as needed, but don't shadow (redefine)
any inherited variables.

■

Allow each class to manage its own data. Therefore use the super refer-
ence to invoke a parent's constructor and to call overridden versions of
methods if appropriate.

■

Use interfaces to create a class that serves multiple roles (simulating mul-
tiple inheritance).

■

Design a class hierarchy to fit the needs of the application, with attention
to how it may be useful in the future.

■

Even if there are no current uses for them, override general methods such
as toString and equals appropriately in child classes so that the inherited
versions don't cause unintentional problems later.

■

Use abstract classes to specify a common class interface for the concrete
classes lower in the hierarchy.

■

Use visibility modifiers carefully to provide the needed access in derived
classes without violating encapsulation.

■

Restricting Inheritance
We've seen the final modifier used in declarations to create constants many
times. The other uses of the final modifier involve inheritance and can have a
significant influence on software design. Specifically, the final modifier can be
used to curtail the abilities related to inheritance.
Earlier in this chapter we mentioned that a method can be declared
as final , which means it cannot be overridden in any classes that
extend the one it is in. A final method is often used to insist that
particular functionality be used in all child classes.

KEY CONCEPT
The final modifier can be used to
restrict inheritance.