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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