Question { 3264 }

Tell me the difference between an applet and a Japplet?

Answer

A JApplet is a modern Applet. It is also an Applet since it
extends Applet and could therefore do anything an Applet
can do. JApplet is part of the javax.swing-packet and is
therefor compatible with swing components, the new graphcs
component packet preferable before the old java.awt. For
new applications JApplet is recommended. Only if you are
designing an applet for an old java-version you should use
Applet.

Question { SparkTG, 8827 }

if a java file without any source code is valid java file?

Answer

(I suppose you mean IS not IF)

No, an empty file is not a valid java file since you have
to use the same class-name as file name. (A file with a
valid class declaration but an empty body is valid thou)