@Override
    public boolean isNext() {

      // If another non-matched key is found, the key bytes will be referenced by keyRef, and
      // our ReusableRowContainer's BytesBytesMultiHashMap.Result will reference the value rows.
      currentIndex =
          hashMap.findNextNonMatched(
              currentIndex, keyRef, currentValue.getHashMapResult(), matchTracker);
      return (currentIndex != -1);
    }
For FULL OUTER MapJoin: Iterates through the Small Table hash table and returns the key and
value rows for any non-matched keys.
ql/src/java/org/apache/hadoop/hive/ql/exec/persistence/MapJoinBytesTableContainer.java