Why should we overide hashCode method when we overide equals ?

java.lang.Swapnil wrote:
Why should we overide hashCode method when we overide equals ?For the purpose of hashing, you need to override both hashcode and equal method. It is because different objects(in most cases) have different hashcode.
Edited by: Hunky322 on 28.?ub.2009 16:21

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Dear Members :
I have custom hashCode method in conjunction with overriden equals method, in my program and have following doubts/observations :
[1] In order to verify whether the hashCode method is executed, I put a println statement within method, but this doesn't output when the program is run.
[2] The program is a simple equality testing of two dates and doesn't make use of any map. I want to know when and who gives (implicit ?) call to this hashCode method, when equals method is overriden, I mean the program flow.
Since SUN suggests to override hashCode, whenever equals method is overriden, and my program is not making use of map, how and when the hashCode method gets called and if so why my test print is not executed ?
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Atanu

uj_ wrote:
jverd wrote:
uj_ wrote:
One way to graciously avoid the contract is to override hashCode and let it throw an exception (or assertion) stating "hashCode not implemented. This class is not intended for use in hash-based collections" or something.What would be the advantage of doing that over just taking the 5 minutes needed to implement it properly?Who are you to stipulate other people's design needs? Just because something takes just 5 minutes doesn't mean it's the right thing to do.
You may want a class to have an equals relation based on content but you still may not want the class to be in a hash-based collection. The most compelling reason may be that it's not immutable.
The equals/hashCode contract is mostly a Sun promise regarding "standard" classes. Everybody else design their classes the way they see fit.Calm down UJ. I have as much right to offer my opinion as you have to offer yours. I just asked a simple question.
The requirement that a class not be used in a hash-based collection would be silly. It's up to the user of the class how to store instances. If the fields that go into determining hashCode are mutable, then that should be a documentation issue. There are good reasons to keep hashCode and equals in sync. The only reason not to do so would be the time it takes to write the code. Since that time is trivial for any well-designed class, it's an extremely weak reason.

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Why should we overide hashCode method when we overide equals ?

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