Question

I’m reading Sybex Complete Java 2 Certification Study Guide April 2005 (ISBN0782144195). This book is for java developers who wants to pass java certification.

After a chapter about access modifiers (along with other modifiers) I found the following question (#17):

True or false: If class Y extends
class X, the two classes are in
different packages, and class X has a
protected method called abby(), then
any instance of Y may call the abby()
method of any other instance of Y.

This question confused me a little.

As far as I know you can call protected method on any variable of the same class (or subclasses). You cannot call it on variables, that higher in the hierarchy than you (e.g. interfaces that you implement).

For example, you cannot clone any object just because you inherit it.

But the questions says nothing about variable type, only about instance type.

I was confused a little and answered “true”.

The answer in the book is

False. An object that inherits a protected method from a superclass in a different package may call that method on itself but not on other instances of the same class.

There is nothing here about variable type, only about instance type.

This is very strange, I do not understand it.

Can anybody explain what is going on here?

Answer 1

True or false: If class Y extends class X, the two classes are in different packages, and class X has a protected method called abby(), then any instance of Y may call the abby() method of any other instance of Y.

"False. An object that inherits a protected method from a superclass in a different package may call that method on itself but not on other instances of the same class".

Let’s write that down, as BalusC did, and add to Y a method which calls the abby() of any other instance of Y:

package one;
public class X {
    protected void abby() {
    }
}

package other;
import one.X;
public class Y extends X {
    public void callAbbyOf(Y anyOther) {
        anyOther.abby();
    }
}

It is possible for Y to call the abby() method of any instance of Y to which it has a reference. So the answer in the book is blatantly wrong. Java does not have instance-specific scopes (unlike for example Scala which has an instance-private scope).

If we try to be merciful, maybe the question meant by saying "any other instance of Y" that can it access the method of any instance of Y which happens to be in memory – which is not possible, because Java does not have direct memory access. But in that case the question is so badly worded, that you could even answer: "False. You can not call methods on instances which are on a different JVM, or instances which have been garbage collected, or instances on a JVM which died one year ago etc."