R & H Bonus1 question#36

[Parimala Somasundaram]

You are given the class file for a class called Secret. However, you do not have the source code or any information about the internals of the Secret class. You do know that the class has a protected int variable called i. What does the following application print out?
1. class Mine extends Secret {
2. public static void main(String[] args) {
3. Mine m1 = new Mine(); m1.i = 5;
4. Mine m2 = new Mine(); m1.i = 5;
5. if (m1.equals(m2))
6. System.out.println("YES");
7. else
8. System.out.println("NO");
9. }
10. }
A. Yes
B. No
C. Impossible to know
Correct answer is C. The result of the call to equals() on line 5 depends on how the method was written in the Secret superclass.
Can someone please explain?

[Art Metzer]

Hi, Parimala.
First off, I assume in line #4 you meant to say Mine m2 = new Mine(); m2.i = 5;.
The point here is, whoever programmed Secret.java could have made the requirements for equals() just about anything: without access to that code, you just don't know:
For instance, this version of Secret.java returns "NO" in Mine.java:

<font size="2.0">
public class Secret {

    protected int i;
    protected int j;

    public boolean equals( Object o ) {

        if ( o instanceof Secret ) {

            return ( ( this.i == ( (Secret) o ).i ) &&
                     ( this.j == ( (Secret) o ).j ) &&
                     ( this.i > 0 ) &&
                     ( this.j > 0 ) );
        }
        return false;
    }
}

And this version of Secret.java returns "YES":

<font size="2.0">
public class Secret {
    protected int i;
    public boolean equals( Object o ) {

        if ( o instanceof Secret ) {
            return ( this.i == ( ( (Secret) o ).i ) );
        }
        return false;
    }
}

Therefore, the answer is indeterminate, and "C" is correct.
Art