ProGuard is a free Java class file shrinker, optimizer, and obfuscator. It removes unused classes, fields, methods, and attributes. It then optimizes the bytecode. It then renames the remaining classes, fields, and methods using short meaningless names.

A Java decompiler.

The project is kinda abandoned, but still worth a look.

A professional java software contains nine Java tools: create EXE file, executable Jars, display class code and hierarchy, examine zip and jar files, ...

IzPack is a one-stop solution for packaging, distributing and deploying applications.

A java tool for socket testing. It can create both TCP and UDP client or server. It can be used to test any server or client that uses TCP or UDP protocol to communicate.

JCSC is a powerful tool to check source code against a highly definable coding standard and potential bad code.