hadoop should provide a common way to wrap instances with different types into one type

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.