Sometimes, it can be useful to read the Java bytecode, i.e. have a readable version of the class files generated during the compilation.
For example, you want to check if a class file in a jar contains a specific line of code or not. Who said this could never happen? Unfortunately, unless you are “the One”, it will be difficult to read such a file.
Java Decompiler (JD) is a small utility to be able to decode these files, in fact it reverses the process by decompiling the bytecode.
After decompiling, you will not see any comments, they have been voluntarily removed by the Java compiler for obvious optimization reasons.
JD is available as standalone application for most operating systems (Windows, Linux, Mac) or as a plugin for Eclipse or IntelliJ. With the plugin, the decompiler is automatically invoked when you open a class file from the IDE, very convenient!
Note that since version 14, IntelliJ includes a build-in decompiler, so there is no need to install any plugin.