Inheritance - Java Software Solutions: Foundations of Program Design - page 445

characteristics: They are warmblooded, have hair, and produce milk to feed their
young. Now consider a subset of mammals, such as horses. All horses are mam-
mals and have all of the characteristics of mammals, but they also have unique
features that make them different from other mammals such as dogs.
If we translate this idea into software terms, an existing class called Mammal
would have certain variables and methods that describe the state and behavior of
mammals. A Horse class could be derived from the existing Mammal class, auto-
matically inheriting the variables and methods contained in Mammal . The Horse
class can refer to the inherited variables and methods as if they had been declared
locally in that class. New variables and methods can then be added to the derived
class to distinguish a horse from other mammals.
The original class that is used to derive a new one is called the par-
ent class, superclass, or base class. The derived class is called a child
class, or subclass. Java uses the reserved word extends to indicate
that a new class is being derived from an existing class.
The process of inheritance should establish an

KEY CONCEPT
Inheritance creates an is-a relation-
ship between the parent and child
classes.

is-a relationship
between two classes. That is, the child class should be a more specific version of
the parent. For example, a horse is a mammal. Not all mammals are horses, but
all horses are mammals. For any class X that is derived from class Y, you should
be able to say that “X is a Y.” If such a statement doesn't make sense, then that
relationship is probably not an appropriate use of inheritance.
Let's look at an example. The program shown in Listing 9.1 instantiates an
object of class Dictionary , which is derived from a class called Book . In the main
method, three methods are invoked through the Dictionary object: two that were
declared locally in the Dictionary class and one that was inherited from the Book
class.
The Book class (see Listing 9.2) is used to derive the Dictionary class (see
Listing 9.3) using the reserved word extends in the header of Dictionary .
The Dictionary class automatically inherits the definition of the setPages and
getPages methods, as well as the pages variable. It is as if those methods and
the pages variable were declared inside the Dictionary class. Note that, in the
Dictionary class, the computeRatio method explicitly references the pages vari-
able, even though the variable is declared in the Book class.
Also note that although the Book class is needed to create the definition of
Dictionary , no Book object is ever instantiated in the program. An instance of a
child class does not rely on an instance of the parent class.
Inheritance is a one-way street. The Book class cannot use variables or methods
that are declared explicitly in the Dictionary class. For instance, if we created
an object from the Book class, it could not be used to invoke the setDefinitions
method. This restriction makes sense, because a child class is a more specific