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From "Nicolas Spiegelberg (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-3387) Pair does not deep check arrays for equality.
Date Mon, 17 Jan 2011 21:01:45 GMT

    [ https://issues.apache.org/jira/browse/HBASE-3387?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12982861#action_12982861
] 

Nicolas Spiegelberg commented on HBASE-3387:
--------------------------------------------

Although the hasCode contract was the original issue that I noticed, a deeper issue that this
is a fundamental violation of equivalence relationships (http://en.wikipedia.org/wiki/Equivalence_relation
).  In particular, overriding Java functionality for one special class is making Java's equality
operations non-symmetric.  For example:

byte[] a = {0,1,2}, b = {0,1,2};
System.out.println( a.equals(b) );  // false
Pair<byte[], byte[]> pa = Pair.newPair(a,a); pb = Pair.newPair(b,b);
System.out.println(pa.equals(pb); // true
System.out.println(pa.getFirst().equals(pb.getFirst()) && pa.getSecond().equals(pb.getSecond()));
// false

The current equals override makes it such that a == c && b == d is not symmetric with
(a,b) == (c,d). 

I understand your problem with needing a deep comparison.  However, I think if you want deep
comparisons, you need to take another action other than overriding a contracted Java API.
 Example ideas:

1. Use org.apache.hadoop.hbase.util.Bytes.ByteArrayComparator for your collection's comparator
or hand-roll your own.
2. Create a DeepEqual template class that utilizes java.util.Arrays.deepEquals & deepHashCode.
 Then create a Pair<DeepEqual<T>, DeepEqual<T>>

However, the Pair class must be refactored to restore its equivalence relations.  The Pair/Tuple
class is a well-known idiom and should act the same here as it does in C++, Python , and other
languages.

> Pair does not deep check arrays for equality. 
> ----------------------------------------------
>
>                 Key: HBASE-3387
>                 URL: https://issues.apache.org/jira/browse/HBASE-3387
>             Project: HBase
>          Issue Type: Bug
>          Components: util
>    Affects Versions: 0.90.1
>         Environment: Any (discovered in Ubuntu 10.10 using TRUNK). 
>            Reporter: Jesse Yates
>            Priority: Minor
>             Fix For: 0.90.1, 0.92.0
>
>         Attachments: HBASE-3387.patch
>
>   Original Estimate: 0h
>  Remaining Estimate: 0h
>
> Pair does not deep check arrays for equality. It merely does x.equals(y) for the sent
Object. However, with any type of array this is merely going to compare the array pointers,
rather than the underlying data structure.
> It requires a rewriting of the private equals method in Pair to check for elements being
an array, then checking the underlying elements.

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