protected ImmutableBytesWritable createGroupKey(byte[][] vals) {
    if (vals == null) {
      return null;
    }
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < vals.length; i++) {
      if (i > 0) {
        sb.append(" ");
      }
      sb.append(Bytes.toString(vals[i]));
    }
    return new ImmutableBytesWritable(Bytes.toBytesBinary(sb.toString()));
  }
Create a key by concatenating multiple column values.
<p>
Override this function in order to produce different types of keys.
@param vals The current key/values.
@return A key generated by concatenating multiple column values.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/GroupingTableMapper.java