Plymorphism Explaination in K&B Book

[udaya krishna]

Hi,I just started my preparation for OCPJP.While studying the polymorphism topic(chapter 2 Exam objectives 5.2 in Kathy Sierra & Bert Bates book) I read these two sentences which I couldnt quite understand-

1.A reference variable can be of only one type , and once declared,that type can never be changed.
2.A reference is a variable,so it can be reassigned to other objects.

Can anyone explain me in detail ? An example would be of real help
Thanks in advance

[Vijay keshava]

1.A reference variable can be of only one type , and once declared,that type can never be changed.

-- Dog d = new Dog(). Here d is the reference variable. d can be only of one type (Dog), it cannot be of type Dog and Cat and it cannot be changed from Dog to Cat any time. You can declare another reference variable say Cat d = new Cat(), in some other scope, but this is a different reference variable, even though the name is same.

2.A reference is a variable,so it can be reassigned to other objects.

Dog d1 = new Dog()
Dog d2 = new Dog();
d1 = d2 (A reference is a variable,so it can be reassigned to other objects.)

Remember the reassignment can be only to other objects of the same type (Same class or subclass)


Regards,
Vijay

[Atul Shrivastava]

The type of a reference variable is always kept intact, reassigning it a different value doesn't change its type.

[udaya krishna]

Thanks a lot vijay and atul I understood clearly now