Java AWT Reference

11

Scrolling

In this chapter:

• Scrollbar
• Scrolling An Image
• The Adjustable Interface
• ScrollPane

This chapter describes how Java deals with scrolling. AWT provides two means for scrolling. The first is the fairly primitive Scrollbar object. It really provides only the means to read a value from a slider setting. Anything else is your responsibility: if you want to display the value of the setting (for example, if you're using the scrollbar as a volume control) or want to change the display (if you're using scrollbars to control an area that's too large to display), you have to do it yourself. The Scrollbar reports scrolling actions through the standard event mechanisms; it is up to the programmer to handle those events and perform the scrolling.

Unlike other components, which generate an ACTION_EVENT when something exciting happens, the Scrollbar generates five events: SCROLL_LINE_UP, SCROLL_LINE_DOWN, SCROLL_PAGE_UP, SCROLL_PAGE_DOWN, and SCROLL_ABSOLUTE. In Java 1.0, none of these events trigger a default event handler like the action() method. To work with them, you must override the handleEvent() method. With Java 1.1, you handle scrolling events by registering an AdjustmentListener with the Scrollbar.addAdjustmentListener() method; when adjustment events occur, the listener's adjustmentValueChanged() method is called.

Release 1.1 of AWT also includes a ScrollPane container object; it is a response to one of the limitations of AWT 1.0. A ScrollPane is like a Panel, but it ...