When two sequence files, which have same Key type but different Value types, are mapped out to reduce, multiple Value types is not allowed. In this case, we need a way to wrap instances with different types into one class type to reduce.
In current code, ObjectWritable is a sole choice. but it costs too many space, because the class declaration will be appended into output file as a string for every Key-value pair.
- is cloned by
HADOOP-839 Improvements to GenericWritable