A closer look at bytecode

A file with the extension class, generated by the compiler (javac), isn’t just
 a jumble of random bytes. It’s a file with a strictly defined structure.
 Optimizing it can improve performance. The 
Byte Code Engineering Library (BCEL)
 can do quite a lot in that regard. In his article Analyze Your Classes
 Vikram Goyal covers both this tool and the broader subject.