Extending Classes and Inheritance - Beginning Java - page 301

You can cast a reference to an object of a given class type upwards through its direct and indirect su-
perclasses. For example, you could cast a reference to an object of type Spaniel directly to type Dog , type
Animal , or type Object . You could write:
Spaniel aPet = new Spaniel("Fang");
Animal theAnimal = (Animal)aPet; // Cast the Spaniel to Animal
When you are assigning an object reference to a variable of a superclass type, you do not have to include
the cast. You could write the assignment as:
Animal theAnimal = aPet; // Cast the Spaniel to Animal
This would work just as well. The compiler is always prepared to insert a cast to a superclass type when
necessary.
When you cast an object reference to a superclass type, Java retains full knowledge of the actual class to
which the object belongs. If this were not the case, polymorphism would not be possible. Because inform-
ation about the original type of an object is retained, you can cast down a hierarchy as well. However, you
must always write the cast explicitly because the compiler is not prepared to insert it. For the cast to work,
the object must be a legitimate instance of the class you are casting to — that is, the class you are casting to
must be the original class of the object, or must be a superclass of the object. For example, you could cast
a reference stored in the variable theAnimal shown in the preceding example to type Dog or type Spaniel
because the object was originally a Spaniel , but you could not cast it to Cat or Duck because an object of
type Spaniel does not have Cat or Duck as a superclass. To cast theAnimal to type Dog , you would write
the following:
Dog aDog = (Dog)theAnimal; // Cast from Animal to Dog
Now the variable aDog refers to an object of type Spaniel that also happens to be a Dog . Remember,
you can only use the variable aDog to call the polymorphic methods from the class Spaniel that override
methods that exist in Dog . You can't call methods that are not defined in the Dog class. If you want to call a
method that is in the class Spaniel and not in the class Dog , you must first cast aDog to type Spaniel .