Author: Shuo Xiang <shuoxiangpub@gmail.com>

Closes #6946 from coderxiang/en-java-code-example and squashes the following commits:

7a4bdf8 [Shuo Xiang] address comments
cddb02b [Shuo Xiang] add elastic net python example code
f4fa534 [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
6ad4865 [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
180b496 [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
aa0717d [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
5f109b4 [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
c5c5bfe [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
98804c9 [Shuo Xiang] fix bug in topBykey and update test