This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
There are lots of Java byte code engineering libraries. You can use one of them. That would be suggested approach instead of you parsing or invoking a separate native process to reverse engineer the .class file.