A scroll pane uses a JViewport instance to manage the visible area of the client. The viewport is responsible for positioning and sizing the client, based on the positions of the scroll bars, and displaying it.
A scroll pane may use two separate instances of JScrollBar for the scroll bars. The scroll bars provide the interface for the user to manipulate the visible area. The following figure shows the three areas of a scroll bar: the knob (sometimes called the thumb), the (arrow) buttons, and the track.