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Mock question on HashCode

venkatesh pendharkar
Ranch Hand

Joined: Apr 29, 2006
Posts: 106
This is the question from Mock:-

Given the following class, which are correct implementations of the hashCode() method?

1 return 0;
2 return a;
3 return a+b;
4 return a-b;
5 return a^b;
6 return (a16)|b;

Can anyone explain me the meaning of this question & whats the output.


[HENRY: Added Code Tags... BTW, in the future, please also mention which Mock is this question from.]
[ November 24, 2006: Message edited by: Henry Wong ]
Deepak Bala
Bartender

Joined: Feb 24, 2006
Posts: 6662
    
    5

The question is about the hashCode and equals contract. You have to figure out which of the return statements are legal. For example...

return 0; is legal but not efficient. It returns the same hash every time
return a+b; is legal since it is a function of both a and b and the hashcode will be the same even when the numbers are negative.
return a-b; is illegal since the hashCode is different for negative numbers.

and so on...


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Sathishkumar Ethiraju
Greenhorn

Joined: Nov 23, 2006
Posts: 10
Hey John,
Could you be please clear on
return a-b; is illegal since the hashCode is different for negative numbers.
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18896
    
  40

Originally posted by Sathishkumar Ethiraju:
Hey John,
Could you be please clear on
return a-b; is illegal since the hashCode is different for negative numbers.


Basically, you are trying to follow the rule that, if the equals() methods returns true, the hashcode should be the same.

If you examine the equals() method, if (a=1, b=2) in one object, and (a=2, b=1) in another object, they should be equal -- according to the equals() method. However, if the hash code is "a-b", it will be positive in one case and negative in the other. So... using "a-b" as the formula for hashcode doesn't work.

Henry


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