equals and hashCode

Question

Hi, I am running into a question about equals and hashCode contracts: here it is

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.)

Correct Answer 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 have a question about the last choice D, say if the two objects A: code.length=10, bal=10, rate = 100 B: code.length=10, bal=100, rate = 10 then using the equals() method in D, we get A.equals(B) evaluating to true right? but the they get different hashCode because they have a different balance? is it I misunderstood the concept somewhere ? can someone clarify this for me?

Thanks

Yang

Answer 1

You're right - D would be inappropriate because of this.

More generally, hashCode and equals should basically take the same fields into account, in the same way. This is a very strange equals implementation to start with, of course - you should normally be checking for equality between each of the fields involved. In a few cases fields may be inter-related in a way which would allow for multiplication etc, but I wouldn't expect that to involve a string length...

One important point which often confuses people is that it is valid for unequal objects to have the same hash code; it's the case you highlighted (equal objects having different hash codes) which is unacceptable.

Answer 2

You have to check at least all the fields used by .hashCode() so objects which are equal do have the same hash. But you can check more fields in equals, its totally fine to have different objects with the same hash. It seems your doing SCJP 1.6? This topic is well covered in the SCJP 1.6 book from Katherine Sierra and Bert Bates.

Note: thats why its legit to implement a useful .equals() while returning a constant value from .hashCode()