BMD Blackberry Guides

Display stack

Screen objects are maintained in a display stack, an ordered set of Screen objects. The screen at the top of the stack is the active screen that appears to the user. When an application displays a screen, it pushes the screen to the top of the stack. When an application closes a screen, it pops the screen off the stack and displays the next screen on the stack, redrawing it as necessary.

ONote: Each screen can appear only once in the display stack. The VM throws a runtime exception if the same screen is pushed onto the stack more than once. Applications must pop screens off of the display stack when the user finishes interacting with them so that memory is not used unnecessarily. Do not use more than a few modal screens at one time, because each screen uses a separate thread.