从Java中的超类引用子类

Question

Suppose I have an abstract class Person . There is another class Student which extends Person . But the Student class has a member variable, say college of type String , which is not there in Person class.

We know that we can reference a subclass from a superclass, for example,

Person p = new Student();

Will the object p have the member college ?

Answer 1

You won't be able to do p.college . However, you can cast it to Student and in this case it will have:

((Student) p).college;

Answer 2

In your sample, Person object IS a Student and hence will have the college member.

Since you cast the Student to a Person , any public routines or data not present in Person will be hidden by the cast assignment though.