I first came across Java Decompiler many years ago now and I have always found it to work very well. There are times when the ability to decompile a Java class file has proved to be very valuable. Since I last looked it has arrived on GitHub at java-decompiler (Java Decompiler) which is great to see. I have used JD-GUI on Windows but you can get it for Linux and Mac as well, alternatively there are plug-ins for Eclipse and IntelliJ.