hascode and equals

[akash azal]

why if two objects are equals their hashcode must be equal??

[Punit Singh]

akash azal wrote:why if two objects are equals their hashcode must be equal??

Did you read hashing and bucket mechanism in chapter 7 of K&B?
Do you know where hashCode() and equals() are used?

[akash azal]

i read k&b chapter 7 .
euals() method is used to check objects are meaningfully equivalent or not.
but my doubt is y reverse is not true

[Jari Timonen]

See general contracts:

HashCode

Equals

Also see my post: Overriding hashcode

[Henry Wong]

Hashing is more of an algorithm than the Java implementation. So if you want to know "why" something is, it may be better to pick up a book on algorithms, than look at the JavaDocs... All the JavaDocs will do is give you the contracts, not the explanations.

but my doubt is y reverse is not true

Reverse of what is not true? Please clarify the question.

Henry

[Himalay Majumdar]

Akash..in collections..let say in Sets. Duplicates are not allowed. For better performance, set first uses hashcode to determine if a similar object is already in the Set.
If hashcode differs it will directly put it inside the Set...so even if objects are meaningfully equal its putting them..and creating a duplicate element.

But the reverse is not true i.e two different objects can have the same hashcode. If a Set gets the same hashcode(hashcode is just the bucket-id)..it wont directly discard the object as duplicate. It will go to the appropriate bucket depending on hashcode and compare the objects using equals() method.

If you practice with them for a while..u will surely get it.