@Override
  public TMap readMapBegin() throws TException {
    assert (!inner);
    TMap map = new TMap();
    if (columns[index] == null || columns[index].equals(nullString)) {
      index++;
      if (returnNulls) {
        return null;
      }
    } else if (columns[index].isEmpty()) {
      index++;
    } else {
      fields = mapPattern.split(columns[index++]);
      map = new TMap(ORDERED_TYPE, ORDERED_TYPE, fields.length / 2);
    }
    innerIndex = 0;
    inner = true;
    isMap = true;
    return map;
  }
Skip past the current field Just increments the field index counter.
serde/src/java/org/apache/hadoop/hive/serde2/thrift/TCTLSeparatedProtocol.java