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Posted to common-dev@hadoop.apache.org by "Nicholas Telford (JIRA)" <ji...@apache.org> on 2011/02/23 12:00:38 UTC

[jira] Created: (HADOOP-7153) MapWritable violates contract of Map interface for equals() and hashCode()

MapWritable violates contract of Map interface for equals() and hashCode()
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                 Key: HADOOP-7153
                 URL: https://issues.apache.org/jira/browse/HADOOP-7153
             Project: Hadoop Common
          Issue Type: Improvement
          Components: io
    Affects Versions: 0.21.0
            Reporter: Nicholas Telford
            Priority: Minor


o.a.h.io.MapWritable implements the java.util.Map interface, however it does not define an implementation of the equals() or hashCode() methods; instead the default implementations in java.lang.Object are used.

This violates the contract of the Map interface which defines different behaviour for equals() and hashCode() than Object does. More information here: http://download.oracle.com/javase/6/docs/api/java/util/Map.html#equals(java.lang.Object)

The practical consequence is that MapWritables containing equal entries cannot be compared properly. We were bitten by this when trying to write an MRUnit test for a Mapper that outputs MapWritables; the MRUnit driver cannot test the equality of the expected and actual MapWritable objects.

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