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What are the constraints to Key and Value classes in Mapreduce ?



Any data type that can be used for a Value field in a mapper or reducer must implement org.apache.hadoop.io.Writable Interface to enable the field to be serialized and deserialized.
By default Key fields should be comparable with each other.  So, these must implement hadoop’s org.apache.hadoop.io.WritableComparable Interface which in turn extends hadoop’s Writable interface and java.lang.Comparable interfaces.
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