You can code your application on your desktop computer and then copy/save it in the IFS (integrated file system) of the AS/400.
Then, you use the CrtJvaPgm command on the i5 side or use the Java command on the QShell interpreter (StrQSh).
You can create individual .java files and even use a .jar with the sources to compile them. As the i5 has its own JVM, you can generate the byte code and save it to the IFS and then use the Java or RunJva commands to run your application (there are performance considerations for this last option).
If you want to enter your Java directly through green-screen, use the EDTF command. This is an editor for streamfiles like SEU is an editor for source file members.