urgent help on K&B chapter 7 Self Test Questions

[Niranjan Deshpande]

Q 3. Which two statements are true about comparing two instances of the same class, given that the
equals() and hashCode() methods have been properly overridden? (Choose two.)
A. If the equals() method returns true, the hashCode() comparison == must
return true.
B. If the equals() method returns false, the hashCode() comparison != must return
true.
C. If the hashCode() comparison == returns true, the equals() method must return
true.
D. If the hashCode() comparison == returns true, the equals() method might
return true.
E. If the hashCode() comparison != returns true, the equals() method might
return true.

As per table 418 of k&b book, options B and D are correct.But the book says only option b is correct
the explanation why D is not correct is nonsense

Q 10.Given the following,
1. public class X {
2. public static void main(String [] args) {
3. X x = new X();
4. X x2 = m1(x);
5. X x4 = new X();
6. x2 = x4;
7. doComplexStuff();
8. }
9. static X m1(X mx) {
10. mx = new X();
11. return mx;
12. }
13. }
After line 6 runs. how many objects are eligible for garbage collection?
A. 0
B. 1
C. 2
D. 3
E. 4

when i had solved this question previously i was quite convinced that the answer is B.
But as i solved it again today, i got confused and marked answer A. could anyone please
conveince me how come the answer is A.

[jiju ka]

For Q 3 refer https://coderanch.com/t/252120/java-programmer-SCJP/certification/equals-hashcode

Q 10.Given the following,
1. public class X {
2. public static void main(String [] args) {
3. X x = new X();
4. X x2 = m1(x);
5. X x4 = new X();
6. x2 = x4;
7. doComplexStuff();
8. }
9. static X m1(X mx) {
10. mx = new X();
11. return mx;
12. }
13. }
After line 6 runs. how many objects are eligible for garbage collection?
A. 0
B. 1
C. 2
D. 3
E. 4

Answer B is correct.
The statements which are creating objects are at line 3, 10, 5. After line 6 runs Object created at line 10 is not referenced any more; Object created at line 3 is still refrenced by variable x, Object created at line 5 is still referenced by variables x2 and x4; But there is no variable holding refrence to object created at line 10
[ December 29, 2005: Message edited by: jiju ka ]

[Bert Bates]

Hey Niranjan,

In response to Q3: I don't think that you copied it from the book correctly! Go back and make sure that what you wrote in your post matches what's in the book!

Thanks,

Bert

[Niranjan Deshpande]

thanls a lot bert bates...

here is the corrected question and relecant stuff...please explain

1. Given the following,
11. x = 0;
12. if (x1.hashCode() != x2.hashCode() ) x = x + 1;
13. if (x3.equals(x4) ) x = x + 10;
14. if (!x5.equals(x6) ) x = x + 100;
15. if (x7.hashCode() == x8.hashCode() ) x = x + 1000;
16. System.out.println("x = " + x);
and assuming that the equals () and hashCode() methods are property implemented, if the
output is �x = 1111�, which of the following statements will always be true?

A. x2.equals(x1)
B. x3.hashCode() == x4.hashCode()
C. x5.hashCode() != x6.hashCode()
D. x8.equals(x7)

As per table 418 of k&b book, options B and D are correct.But the book says only option b is correct
the explanation why D is not correct is nonsense

here is the explanation

1. B. By contract, if two objects are equivalent according to the equals() method, then
the hashCode() method must evaluate them to be ==.
A is incorrect because if the hashCode() values are not equal, the two objects must not
be equal. C is incorrect because if equals() is not true there is no guarantee of any result
from hashCode(). D is incorrect because hashCode() will often return == even if the
two objects do not evaluate to equals() being true. (this last line doesnt make sense to me)

[Dave Butterwick]

I've been studying for the SCJP cert myself and as I understand the hashcode section hashcodes can be legally computed so as to allow to not equal objects to have the same hashcode incidently. There was an example in the book that showed how this could happen.

Bottom line in my mind is that: if x.equals(y) is TRUE then x.hashcode() MUST EQUAL y.hashcode() BUT if x.hashcode() happens to EQUAL y.hashcode() THEN x.equals(y) might be true but isn't required to be.

[chandra chan]

...
15. if (x7.hashCode() == x8.hashCode() ) x = x + 1000;
16. System.out.println("x = " + x);
and assuming that the equals () and hashCode() methods are property implemented, if the
output is �x = 1111�, which of the following statements will always be true?

A. x2.equals(x1)
B. x3.hashCode() == x4.hashCode()
C. x5.hashCode() != x6.hashCode()
D. x8.equals(x7)
...

according to table 418 of k&b book, answer D, x8.equals(x7) is incorrect, cause although (x7.hashCode() == x8.hashCode()) is true, and it is allowed to have x7.equals(x8) == true, BUT it's not required..

chandra chan