The BlackBerry® Java® Virtual Machine maintains Screen objects in a display stack, which is an ordered set of Screen objects. The screen at the top of the stack is the active screen that the BlackBerry device user sees. When a BlackBerry device application displays a screen, it pushes the screen to the top of the stack. When a BlackBerry device application closes a screen, it removes the screen off the top of the stack and displays the next screen on the stack, redrawing it as necessary. Each screen can appear only once in the display stack. The BlackBerry JVM throws a runtime exception if a Screen that the BlackBerry device application pushes to the stack already exists. A BlackBerry device application must remove screens from the display stack when the BlackBerry device user finishes interacting with them so that the BlackBerry device application uses memory efficiently. Use only a few modal screens at one time, because each screen uses a separate thread.