I heard it from my senior that he can hack and reverse engineer my game easily if I send him JAR file.
I guess it is true, but how can I protect it?
Yes and no. Decompilers like http://java.decompiler.free.fr/?q=jdgui can convert byte code to readable Java code, minus comments and higher level features like generics. Of course without comments most code becomes useless for commercial considerations (unless you have highly valuable algorithms in there, which you most likely don't).
You can use an obfuscator like http://proguard.sourceforge.net/ to make your bytecode pretty much unreadable. Of course that also has disadvantages, like losing all debugging info.
To be honest, it's one of those "if you have to ask, your code likely won't be interesting enough" topics in my opinion ;)