Question

The _________ as just the value field append(value) and the key is a longwritable that contains the record number, count + 1.

Answer 1

Set File and Array File are based on Map File, and their implementation are just few lines of code.  

The Set File instead of append (key, value) as just the key field append(key) and the value is always the Null Writable instance.

The Array File as just the value field append(value) and the key is a Long Writable that contains the record number, count + 1.