Inheritance - Java Software Solutions: Foundations of Program Design - page 454

Car Truck

PickupTruck

FIGURE 9.2 A UML class diagram showing multiple inheritance

child class to have multiple parents. This approach is called multiple inheritance
and is occasionally useful for describing objects that are in between two categories
or classes. For example, suppose we had a class Car and a class Truck and we
wanted to create a new class called PickupTruck . A pickup truck is somewhat like
a car and somewhat like a truck. With single inheritance, we must decide whether
it is better to derive the new class from Car or Truck . With multiple inheritance,
it can be derived from both, as shown in Figure 9.2.
Multiple inheritance works well in some situations, but it comes with a price.
What if both Truck and Car have methods with the same name? Which method
would PickupTruck inherit? The answer to this question is complex, and it
depends on the rules of the language that supports multiple inheritance.
The designers of the Java language explicitly decided not to support multiple
inheritance. Instead, we can rely on interfaces to provide the best features of
multiple inheritance without the added complexity. Although a Java class can
be derived from only one parent class, it can implement multiple interfaces.
Therefore, we can interact with a particular class in specific ways while inheriting
the core information from one parent class.

SELF-REVIEW QUESTIONS (see answers in Appendix N)
SR 9.1 Describe the relationship between a parent class and a child class.

SR 9.2 How does inheritance support software reuse?

SR 9.3 What relationship should every class derivation represent?

SR 9.4 What does the protected modifier accomplish?

SR 9.5 Why is the super reference important to a child class?

SR 9.6 Define a class SchoolBook2 that extends Book2 to include an attribute

indicating the age (4 through 16) that a book targets. The construc-
tor accepts the age as a parameter. The class also provides a level