Are you saying you need two different windows - one showing the Greenfoot world and one showing something else? If so, be aware that Greenfoot isn't really designed to be used with Swing/AWT. You can do it, but doing it properly is very difficult. You must obey both the Swing threading rules and the Greenfoot threading rules - i.e. only call Swing methods from the Swing event dispatch thread, and only call Greenfoot API methods from the Greenfoot simulation thread. If you don't need two different windows, then you don't need to import Applet or Frame.

I'm still not sure if you need a separate window. When you export a Greenfoot scenario, it already includes an applet window which displays the world. You don't need to write code to do that yourself. Sorry, but I don't understand what you mean by that.

well, i referred from this code http://www.phys.hawaii.edu/~teb/java/ntnujava/Lens/thinLens.java. It uses java applet and frame. But when I tried to make it on greenfoot, i got a dialog box saying that "methods in java.awt.Component has been deprecated". So, greenfoot can't uses applet from AWT right? I got confused in how to make such methods (like show() or hide()) and TextField in greenfoot.

Those methods have been deprecated - the code is old. What you're seeing are warnings, not errors. The box says "Warnings" at the top. It doesn't make sense why you would take that class and compile it (as is) in Greenfoot. The class is an applet. You can have an applet class in your Greenfoot project, but it's useless. You'd be better of using another IDE such as BlueJ. That's not correct. You can use the class, it's just that it is usually pointless. Your Greenfoot scenario is already an applet (when you export it), but you don't have access to the applet instance from within the Greenfoot scenario. The Greenfoot world is not a Swing/AWT component. You can't use things like TextField (from AWT) in it. But there are alternatives which you can find if you search the site for "gui" for example.