Object-Oriented Concepts - SCJP Sun Certified Programmer for Java Platform SE6 - page 403

System.out.println(“Cat is breathing”);
}
public void sleep() {
System.out.println(“Cat is sleeping”);
}
}

The is-a relationship is helpful when trying to understand polymorphism. Because Cat
extends Pet , a Cat object is a Pet object. Because Cat implements Mammal , a Cat object is
also a Mammal object. Therefore, the following statements are valid:

Cat c = new Cat(“Garfield”, 3);
Pet p = c;
Mammal m = c;

The reference p can only refer to Pet objects, but because a Cat object is a Pet object,
assigning p to c is valid. Similarly, the reference m can only refer to Mammal objects, but
because a Cat object is a Mammal , assigning m to c is also valid. As Figure 6.5 shows, there
is only one Cat object in memory, but the object is taking on three different forms. The c
reference is treating the object as a Cat , the p reference is treating the object as a Pet , and
the m reference is treating the object as a Mammal .

FIGURE 6.5 The single Cat object takes on different forms.

Cat object

Cat reference

c

name

“Garfield”

age

3

Pet reference

p

sleep()
breathe()
eat()

Mammal
reference

m