You must decompile the .class first then make a new class in eclipse and just copy and paste the code, eclipse does have a plugin i believe that can do this... (The plugin is JD-GUI lol..)
Once again just drag and drop into the JD-GUI menu and you will be able to see all of the code and reconstruct it from there.
Most people release in .jars so just extract the .jar either by the: jar -x (jarfile) command or just use something like 7Zip, Winrar, windows default extractor.