Inheritance - Java - page 447

TIP: “is a” Versus “has a”

Early in this chapter, we defined a derived class called HourlyEmployee using the class
Employee as the base class. In such a case, an object of the derived class HourlyEmployee is
also an instance of the class Employee , or, stated more simply, an HourlyEmployee is an
Employee . This is an example of the “is a” relationship between classes. It is one way to
make a more complex class out of a simpler class.
Another way to make a more complex class out of a simpler class is known as the
“has a” relationship. For example, the class Employee defined earlier has an instance vari-
able of the class type Date . We express this relationship by saying an Employee “has a”
Date . Using the “has a” relationship to build a class (such as building the class Employee
by using Date as an instance variable) is often called composition .
Because the class HourlyEmployee inherits the instance variable of type Date from
the class Employee , it is also correct to say an HourlyEmployee “has a” Date . Thus, an
HourlyEmployee is an Employee and has a Date .

“is a”
relationship

“has a”
relationship

composition

■

Access to a Redefined Base Method
Suppose you redefine a method so that it has a different definition in the derived class
from what it has in the base class. The definition that was given in the base class is not
completely lost to the derived class objects. However, if you want to invoke the version
of the method given in the base class with an object in the derived class, you need
some way to say, “use the definition of this method as given in the base class (even
though I am an object of the derived class).” The way you say this is to use the key-
word super as if it were a calling object.
For example, the method toString of the class HourlyEmployee (Display 7.3) was
defined as follows:

super
relationship

public String toString() //in the derived class HourlyEmployee
{
return (getName() + " " + getHireDate().toString()
+ "\n$" + wageRate + " per hour for " + hours + " hours");
}

This overrides the following definition of toString() that was given in the definition
of the base class Employee :

public String toString() //in the base class Employee
{
return (name + " " + hireDate.toString());
}