hashcode contract S&B question

[Martin Sergeant]

This question is taken from the Sierra and Bates mock exam that comes with the SCJP 1.6 book

Given:-

 
public 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 when inserted will fufill equals and hascode conrtacts for this class

(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

I think the answer d is incorrect if you consider the following scenario

object 1 has code.length =2, bal=3 and rate=4
object 2 has code.length =4, bal=2 and rate=3

in this case their hashcodes will be different 2*3 != 4*2
but equals will return true 2*3*4 == 4*2*3
which breaks the hashcode equals contract

Therefore c is the only correct answer

Is this right or am I missing something?

Martin

[Wouter Oet]

You are correct

[Jeevan Reddy]

Wouter Oet wrote:You are correct

Does it mean, only fields used in evaluating hashCode should be used in equals method?
Please clarify.

[Ankit Garg]

janardhan nanduri wrote:Does it mean, only fields used in evaluating hashCode should be used in equals method?

Not necessarily. You just need to make sure that the hashCode and equals contract is fulfilled that's all. Example

public class SortOf {
    String name;
    int bal;
    String code;
    short rate;
    public int hashCode(){
        return (code.length()*bal);
    }
    public boolean equals(Object o){
        if(! o instanceof SortOf) {return false;}
        SortOf so = (SortOf)o;
        return (this.name.equals(o.name) && this.bal == o.bal && this.rate == o.rate);
    }
}

[Simran Dass]

You are right Martin . Only option C is correct.
I had also faced problem in the same question . I had chosen only option C.