Whilst most of my work is around writing software, there is another side around reverse engineering, de-compiling and analysis. There are multiple reasons why this is done, one of the most common is understanding what "potentially malicious" software is actually doing.
These require more work to reverse engineer than Java or .NET programs, but there are some tools to help.
All Java programs run on the Java Virtual Machine, or JVM and as the JVM just executes byte-code, it is easier to reverse engineer this, see Decompiling Java for more on this.