JDeveloper does not decompile the class file automaticaly. When you drop a .class in the editor, jdev only provides the class description with methods...not the java code.
Jdev configuration file located in <jdev-home>/jdev/bin/jdev.conf has to be modified to allow jad to be taken into account for decompilation. The following line has to be added at the end of the jdev.conf file.
AddVMOption -Djcncmd=c:/jadnt158/jad.exe -p -b -ff -nl -pi99999 -space -t2 -noinner
Take care to modify the jad path !