hadoop mapreduce 输出不带行号（用于预处理）You can suppress the key or the value (or both, making thi
hadoop mapreduce 输出不带行号（用于预处理）
You can suppress the key or the value (or both, making this output format equivalent
to NullOutputFormat, which emits nothing) from the output using a NullWritable type.
This also causes no separator to be written, which makes the output suitable for reading
in using TextInputFormat.