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

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 0.21.0
• Fix Version/s: 0.23.0
• Component/s: io
• Hadoop Flags: Incompatible change, Reviewed
• Release Note: MapWritable now implements equals() and hashCode() based on the map contents rather than object identity in order to correctly implement the Map interface.
• Assignee: Nicholas Telford
• Reporter: Nicholas Telford
• Created: 23/Feb/11 10:59
• Updated: 23/Jan/13 12:20
• Resolved: 24/Feb/11 17:59

Description

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.

Issue Links

• relates to: HADOOP-9124 SortedMapWritable violates contract of Map interface for equals() and hashCode()