I finally take the OCA8 exam on 29 November 2016, after twelve months of preparation. This lasted so long than I excepted because of busy working times.
I'm PHP developer and Java is not a part of my daily work, so no hands-on exprience. I had to give break so many times. Then start again from beginning. This was very boring.
My main study materials are Oracle Certified Associate Java SE 8 Programmer I Study Guide by Scott Selikoff, Jeanne Boyarsky and Enthuware prep kit. Both of them were very helpful.
Also I solved the questions almost all of the OCA book, thanks to the Safari Books Online.