Hashcode and equals contract - K&B Mock

[raghu dubey]

Hello,

There is a mock question that says:

if(x1.hashcode()!=x2.hashcode()) - if the above statement evaluates to true, ie if hascodes for x1 and x2 are not equal, then x2.equals(x1) will always be true.

How?

It is appropriate according to the contract that2 objects can equal even if their hashcodes are not equal, but is it not a mandatory situation. Infact the above violates the equals contratc.....2 equal Objects should have equal hashcodes.

PLease help.


Please help.

[dipayan chatterjee]

this may happen if we override the equals function of the Object class but not the hashcode function . so as a result the equals() may compare the two objects say with respect to there state where as the hashcode() will compare according to hashcodes . i am not very sure as overriding both the equals() and hashcode() if any one of them is overrided is mandatory or is suggested as better programming practice(contract) to keep the results of both the functions consistent as their origrnal implementation in the Object class .please do correct me if i am wrong

[raghu dubey]

Hi Dipayan,

you are correct with what you say. It "may happen". It would even be legal, thought not appropriate/correct according to the hashcode/equals contract.

But the point is whether it should always happen. I dont think so. But I may be wrong.

[Kelvin Chenhao Lim]

You'll have to provide us with a reference or link to the full mock question before we can say anything with certainty. Perhaps there's something else in the question that indicates or implies that the class in question does implement the hashCode() and equals() methods in a way that satisfies the contract.

[Burkhard Hassel]

Howdy!

Raghu wrote:

It is appropriate according to the contract that 2 objects can equal even if their hashcodes are not equal, but is it not a mandatory situation.

The question:

if(x1.hashcode()!=x2.hashcode()) - if the above statement evaluates to true, ie if hascodes for x1 and x2 are not equal, then x2.equals(x1) will always be true.

The statement - as it is - is false.

The contract says:

If two objects are equal they must have the same hash code.

So if the hash codes are different, the two objects can not be equal. Provided they are fulfilling the contract.

Rember the bucket analogy: The hash codes are used for a sort of pre-grouping objects before the equals test is performed.
If the hash codes are different the equals test will not be performed.

If you have

import java.util.*;
 
class Raghu {
public boolean equals(Object o) {
return true;
}
 
public static void main(String[] args){
Raghu one = new Raghu();
Raghu two = new Raghu();
Set<Raghu> set = new HashSet<Raghu>();
set.add(one);
set.add(two);
System.out.println(one.equals(two));
System.out.println(set);
}
}

Then the output is

true
[Raghu@ad3ba4, Raghu@1372a1a]


Equals is overridden to return true in any case, hashCode is not overridden, so it returns the hashCode of class Object which are always different.

Testing the equals method directly returns true due to the override, but when you really use the hashCode for hashing, both objects - despite the fact that they are equal - will be added to a set. Showing that the contract is violated.

I think the easiest way to understand this hashCode and equals contract is always to think of the bucket analogy.

Yours,
Bu.