Convert .class to .java

Invoking javap to read the bytecode

The javap command takes class-names without the .class extension. Try

javap -c ClassName

Converting .class files back to .java files

javap will however not give you the implementations of the methods in java-syntax. It will at most give it to you in JVM bytecode format.

To actually decompile (i.e., do the reverse of javac) you will have to use proper decompiler. See for instance the following related question:

• How do I "decompile" Java class files?

I'm guessing that either the class name is wrong - be sure to use the fully-resolved class name, with all packages - or it's not in the CLASSPATH so javap can't find it.