hashCode() and equals() exam question

[jean-gobert de coster]

hi there, on a master exam there is this code

Q52. 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.)


the exam correction says that this would be a good answer:

return ((SortOf)o).code.length() * ((SortOf)o).bal * ((SortOf)o).rate == this.code.length() * this.bal * this.rate;

but I don't agree since I could have two SortOf, one with bal = 2 and rate = 3 and the other one with bal = 3 and rate = 2.

The hashCode value would not be the same while they would be considered meaningfully equivalent by that implementation of the equals() method.

Am I right to disagree or am I missing something?

[victor kamat]

You are right; first of there s/b be a getter for code, bal and rate ( the access modifier is package). Assuming we have this then the code should be

public boolean equals(Object o)
{
if ( o instanceof SortOf )
{
SortOf t = (SortOf) o;
return ( t.getCode().length() == code.length() &&
t.getBal() == bal &&
t.getRate() == rate );
}
else
return false;
}

[jean-gobert de coster]

actually you don't need the getter / setter in your example, but I do agree that the test on instanceOf is prefferable (but again, not required for the contract).

[Lidija Sketa]

I would agree with the above answer:

    
return ((SortOf)o).code.length() * ((SortOf)o).bal * ((SortOf)o).rate == this.code.length() * this.bal * this.rate;

To fulfill the hashCode contract the hashCode() method should always produce the same result for the same object.
I don't think the contract says anything about effectiveness.

[jean-gobert de coster]

This is not about effectiveness,

the contract says that if A.equals(B) then A.hashCode()==B.hashCode

Given
A.code.length()==B.code.length() // let's assume that it's 1
A.rate == 2
A.bal == 3
B.rate == 3
B.bal == 2

then A.equals(B) == true since 1*2*3 == 1*3*2

But A.hashCode() == 1*3
and B.hashCode() == 1*2

so A.hashCode() != B.hashCode()

which does not respect the contract

[Ankit Garg]

This is an error in the master exam. It has been pointed out before too...