When you are running an applet in the IDE's Applet Viewer, you can edit the applet's source code and then see the results of your changes immediately.
To make a change to a running applet:
The applet will start running at the beginning, with the new change incorporated.
Running an Applet from the IDE
Creating an Applet or Application
Creating a Quick, Basic Sample Applet or Application
Saving Changes to Code
Debugging During the Development Cycle with the Integrated Debugger
Experimenting with Code Fragments