Bug in LearnKey SCJP 5 exams

[Rob Spoor]

I bought the SCJP 5 Study Guide by KAthy Sierra and Bert Bates quite a while ago, and today I decided to go through the exams on the CD that came with it.

I noticed there is one huge error in the exam on there. It's question 12 I believe:

Given:

class SortOf {
    String name;
    int bal;
    String code;
    short rate;
 
    public int hashCode() {
        return (code.length() * bal);
    }
 
    public boolean equals(Object o) {
        // insert code here
    }
}

Which of the following will fulfill the equals() and hashCode() contracts for this class? (Choose all that apply.)
A) return ((SortOf)o).bal == this.bal;
B) return ((SortOf)o).code.length() == this.code.length();
C) return ((SortOf)o).code.length() * ((SortOf)o).bal == this.code.length() * this.bal;
D) return ((SortOf)o).code.length() * ((SortOf)o).bal * ((SortOf)o).rate == this.code.length() * this.bal * this.rate;

According to the software, C and D must be the answers.

I see why A and B are wrong, but I strongly believe D is wrong as well, and I can even show it with an example.

Consider two SortOf objects:
- a with code == "a", bal = 5 and rate = 2
- b with code == "b", bal = 2 and rate = 5

According to the equals implementation of D, these objects are equal: 1 * 5 * 2 == 1 * 2 * 5. However, these objects have different hash codes: the hash code of a is 1 * 5 == 5, whereas the hash code of b is 1 * 2 == 2. Therefore, answer D clearly violates the contract (equal objects must have equal hash codes), and cannot be correct.


Just thought I mentioned this.

[Irina Goble]

If you do the search, you'll find this thread.