StackOverflow2013

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PO
primarykey
Id
3630923
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2010-09-02T20:23:14.437
FavoriteCount
0
LastActivityDate
2010-09-02T20:31:12.677
LastEditDate
2010-09-02T20:31:12.677
LastEditorUserId
22656
OwnerUserId
22656
ParentId
3630887
PostTypeId
2
Score
9
ViewCount
0
LastEditorDisplayName
text
Body
Simply coincidence, I suspect... there are bound to be collisions, and in this case it looks like the relevant different bits in the first value are being lost, effectively. However, it shouldn't make any difference - anything using hash codes must cope with collisions. EDIT: It's just the way the hashes happen to be calculated. This code shows what's going on: <pre><code>import java.util.*; public class Test { @SuppressWarnings("unchecked") public static void main (String[] args) { AbstractMap.SimpleEntry[] entries = { new AbstractMap.SimpleEntry("campaignId", 4770L), new AbstractMap.SimpleEntry("campaignId", 4936L), new AbstractMap.SimpleEntry("lazy", true), new AbstractMap.SimpleEntry("lazy", false) }; for (AbstractMap.SimpleEntry entry : entries) { System.out.println(entry + ": " + entry.hashCode()); } } } </code></pre> Results: <pre><code>campaignId=4770: -1318251287 campaignId=4936: -1318251261 lazy=true: 3315643 lazy=false: 3315617 </code></pre> So in one pair the first map has a hash 26 less than the second map, and in another pair the first map has a hash 26 more than the second map. <code>AbstractMap</code> just sums hash values (one way of making sure that ordering is irrelevant) so the two end up with the same hash code. It's really down to <code>Boolean.hashCode()</code> which looks like this: <pre><code>return value ? 1231 : 1237; </code></pre> ... and <code>Long.hashCode()</code> which looks like this: <pre><code>return (int)(value ^ (value >>> 32)); </code></pre> Given the values they happened to pick in <code>Boolean.hashCode()</code>, if your <code>long</code> values are only 26 apart (or 26 * 2^32 apart) then you'll run into the same thing.
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1. This table or related slice is empty.
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1. POJava Map::hashCode() collision - why?
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1. PTAnswer
UserLastEditorUserId
1. USJon Skeet
UserOwnerUserId
1. USJon Skeet
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1. POJava Map::hashCode() collision - why?
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2. VO
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