code: protected synchronized void copy(Writable other) {
      if (other != null) {
        try {
          DataOutputBuffer out = new DataOutputBuffer();
          other.write(out);
          DataInputBuffer in = new DataInputBuffer();
          in.reset(out.getData(), out.getLength());
          readFields(in);
          
        } catch (IOException e) {
          throw new IllegalArgumentException("map cannot be copied: " +
              e.getMessage());
        }
        
      } else {
        throw new IllegalArgumentException("source map cannot be null");
      }
    }
docs: Used by child copy constructors.