http://proguard.sourceforge.net/

ProGuard is a free Java class file shrinker, optimizer, obfuscator, and preverifier. It detects and removes unused classes, fields, methods, and attributes. It optimizes bytecode and removes unused instructions. It renames the remaining classes, fields, and methods using short meaningless names. Finally, it preverifies the processed code for Java 6 or for Java Micro Edition.

Some uses of ProGuard are:

• Creating more compact code, for smaller code archives, faster transfer across networks, faster loading, and smaller memory footprints.
• Making programs and libraries harder to reverse-engineer.
• Listing dead code, so it can be removed from the source code.
• Retargeting and preverifying existing class files for Java 6, to take full advantage of Java 6's faster class loading.