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Jon Skeet
Java : Type of Ref or new () decides which methods are visible
Say Person is base class.
Doctor class extends Person.
Now suppose diagnose() is method which belongs to Doctor class.
Person p = New Person();
Doctor q = (Doctor)p;
q.diagnose();
Why would this throw error?
q is object reference of kind Doctor. All objects of class share same copy of methods, so diagnose() should be available to q.
There are no fields of methods which would be available to p would not be available in Doctor.
Why would java not allow this?
Why would this throw error?
Because you're casting an object of type Person to Doctor. The value of p is a reference to an instance of Person, not to an instance of Doctor, therefore the cast fails.
If you had:
Person p = new Doctor();
Doctor d = (Doctor) p;
d.diagnose();
then it would be fine.
This isn't a matter of what's visible - d.diagnose() is visible whenever d is a variable of type Doctor, as it is here. It's a matter of execution-time casting in the line before the diagnose() call.
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