scrollbar : Java Glossary

region on the screen the user can drag a thumbtab to scroll vertically or horizontally or both. In AWT (Advanced Windowing Toolkit) you get them by adding your Component to a ScrollPane then adding the ScrollPane to your Container. In Swing you use JScrollPane instead. JScrollPane works with a ScrollPaneLayout which as a side effect decides when the scrollbars are needed and which part of the image to display. To get the JScrollPane to notice that your Component has changed size, you must call the Component’s revalidate method. If scroll bars fail to appear, make sure your getPreferredSize, getMinimumSize and getMaximumSize methods on your extended JPanel inside the ScrollPane are producing the size of the full image, not just the visible part.

with JScrollPane, if you don’t specify the scrolling policy, this is the default:

// note vertical comes first
new JScrollPane ( component,
                  VERTICAL_SCROLLBAR_AS_NEEDED,
                  HORIZONAL_SCROLLBAR_AS_NEEDED );