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hashcode contract

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hashcode contract, a theoretical question

bhandari	20 Feb, 2008 - 03:05 AM Post #1
D.I.C Addict Joined: 31 Jan, 2008 Posts: 747 Dream Kudos: 900 My Contributions	If equals() returns true then hashcode comparison should also return true. If equals() returns false then hashcode comparison may return true or false. If hashcode comparison returns true then equals() may or may not return true. If hashcode comparison returns false then equals() must return false. I am unable to understand why the fourth is must staement. Any ideas? This post has been edited by bhandari: 20 Feb, 2008 - 03:53 AM
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AmitTheInfinity	RE: Hashcode Contract 20 Feb, 2008 - 03:50 AM Post #2
C Surfing ∞ Joined: 25 Jan, 2007 Posts: 1,167 Thanked: 45 times Dream Kudos: 125 My Contributions	QUOTE(bhandari @ 20 Feb, 2008 - 04:35 PM) If equals() returns true then hashcode comparison should also return true. If equals() returns false then hashcode comparison may return true or false. If hashcode comparison returns true then equals() may or may not return true. If hashcode comparison returns false then equals() must return false. I am unable to understand why the fourth is must staement. Any ideas? can not help you till you post the code. Well I think it's bit obvious statement but it's there to show the vice versa situation. 2nd and 3rd statements shows that vice versa may or may not true, so 4th statement is there to tell that vice versa statements are always false. I mean this is the first thought I had on this. There might be some other logic also.
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bhandari	RE: Hashcode Contract 20 Feb, 2008 - 04:02 AM Post #3
D.I.C Addict Joined: 31 Jan, 2008 Posts: 747 Dream Kudos: 900 My Contributions	my dilema is that equals() can check for anything (any attribute) and that shouldn't be a point for associating equals() with hashcode(). how does hashcode associates to equals at all? This post has been edited by bhandari: 20 Feb, 2008 - 04:03 AM
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baavgai	RE: Hashcode Contract 20 Feb, 2008 - 04:51 AM Post #4
Dreaming Coder Joined: 16 Oct, 2007 Posts: 2,289 Thanked: 136 times Dream Kudos: 475 Expert In: C, C++, Java, C#, ASP.NET, PHP, Perl, Python, Oracle, SQL Server, MySql, HTML, JavaScript, Lua, Cheese My Contributions	QUOTE(bhandari @ 20 Feb, 2008 - 06:05 AM) If equals() returns true then hashcode comparison should also return true. If equals() returns false then hashcode comparison may return true or false. If hashcode comparison returns true then equals() may or may not return true. If hashcode comparison returns false then equals() must return false. I am unable to understand why the fourth is must staement. Any ideas? The last line is more an assertion than an implementation requirement. If you think of the first statement as "If ConditionA then ConditionB", the fourth statement is "If not ConditionB then not ConditionA". Implement the first and the fourth will follow. Hope this helps.
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bhandari	RE: Hashcode Contract 20 Feb, 2008 - 05:46 AM Post #5
D.I.C Addict Joined: 31 Jan, 2008 Posts: 747 Dream Kudos: 900 My Contributions	thanks @baagvi, I tried to modify the 4th statement as: If hashcode comparison returns false then equals() must return true. which contradicts 1st statement and 1st statement is a mandate. thanks again
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