question: return the key at the head of this rr . concode_field_sep RecordReader<K,U> rr concode_elem_sep ResetableIterator<U> vjoin concode_elem_sep WritableComparator cmp concode_elem_sep U vhead concode_elem_sep Configuration conf concode_elem_sep int id concode_elem_sep boolean empty concode_elem_sep K khead concode_field_sep boolean next concode_elem_sep boolean next concode_elem_sep Configuration getConf concode_elem_sep K createKey concode_elem_sep boolean hasNext concode_elem_sep void skip concode_elem_sep int compareTo concode_elem_sep float getProgress concode_elem_sep void setConf concode_elem_sep void accept concode_elem_sep long getPos concode_elem_sep int hashCode concode_elem_sep boolean equals concode_elem_sep U createValue concode_elem_sep int id concode_elem_sep void close
target: K function ( ) { return khead ; }