Hi Peng, I usually only look closely for coding issues in core files, but I noticed in Eclipse there was a yellow warning flag on this file (does it show in your Eclipse?  Your warning preferences may be different).  In general, you should never do a pull request with warnings.  The overall code base should not have warnings.

It looks to me like this field is never used here (and similarly for other classes in this file), so I am going to remove it (please acknowledge this is ok).

You don't have to do another pull request.

-Chuck