Using Java on Symbian does not enable any magic. "Native" features are accessible only through the defined APIs, and there are no Symbian-specific APIs.
You might be able to put an application into the background by:
However, this is not guaranteed to work. Nor is it guaranteed that the home screen will display, even if the application does go into the background.
There is no way to open the task list, other than by the usual user-initiated mechanism.
There is no access to the system clipboard.