How to Write a Mouse-Wheel Listener

Mouse-wheel events notify when the wheel on the mouse rotates. For information on listening to other mouse events, such as clicks, see How to Write a Mouse Listener. For information on listening to mouse-dragged events, see How to Write a Mouse-Motion Listener. Not all mice have wheels and, in that case, mouse-wheel events are never generated. There is no way to programmatically detect whether the mouse is equipped with a mouse wheel.

Alternatively, use the corresponding MouseAdapter AWT class, which implements the MouseWheelListener interface, to create a MouseWheelEvent and override the methods for the specific events.

Usually it is not necessary to implement a mouse-wheel listener because the mouse wheel is used primarily for scrolling. Scroll panes automatically register mouse-wheel listeners that react to the mouse wheel appropriately.

However, to create a custom component to be used inside a scroll pane you may need to customize its scrolling behavior — specifically you might need to set the unit and block increments. For a text area, for example, scrolling one unit means scrolling by one line of text. A block increment typically scrolls an entire "page", or the size of the viewport. For more information, see Implementing a Scrolling-Savvy Client in the How to Use Scroll Panes page.

To generate mouse-wheel events, the cursor must be over the component registered to listen for mouse-wheel events. The type of scrolling that occurs, either WHEEL_UNIT_SCROLL or WHEEL_BLOCK_SCROLL, is platform dependent. The amount that the mouse wheel scrolls is also platform dependent. Both the type and amount of scrolling can be set via the mouse control panel for the platform.

The following example demonstrates mouse-wheel events.

The output from MouseWheelEventDemo for a system that uses unit increments for its mouse wheel might look as follows:

javax.swing.JTextArea: Mouse wheel moved UP 1 notch(es)
    Scroll type: WHEEL_UNIT_SCROLL
    Scroll amount: 3 unit increments per notch
    Units to scroll: -3 unit increments
    Vertical unit increment: 16 pixels

The scroll amount, returned by the getScrollAmount method, indicates how many units will be scrolled and always presents a positive number. The units to scroll, returned by the getUnitsToScroll method, are positive when scrolling down and negative when scrolling up. The number of pixels for the vertical unit is obtained from the vertical scroll bar using the getUnitIncrement method. In the preceding example, rolling the mouse wheel one notch upward should result in the text area scrolling upward 48 pixels (3x16).

For a system that uses block increments for mouse-wheel scrolling, for the same movement of the mouse wheel the output might look as follows:

javax.swing.JTextArea: Mouse wheel moved UP 1 notch(es)
    Scroll type: WHEEL_BLOCK_SCROLL
    Vertical block increment: 307 pixels

The vertical block increment is obtained from the vertical scroll bar using the getBlockIncrement method. In this case, rolling the mouse wheel upward one notch means that the text area should scroll upward 307 pixels.

Find the demo's code in the MouseWheelEventDemo.java file. The following code snippet is related to the mouse-wheel event handling:

public class MouseWheelEventDemo ... implements MouseWheelListener ... {
    public MouseWheelEventDemo() {
        //where initialization occurs:
        //Register for mouse-wheel events on the text area.
        textArea.addMouseWheelListener(this);
        ...
    }

    public void mouseWheelMoved(MouseWheelEvent e) {
       String message;
       int notches = e.getWheelRotation();
       if (notches < 0) {
           message = "Mouse wheel moved UP "
                        + -notches + " notch(es)" + newline;
       } else {
           message = "Mouse wheel moved DOWN "
                        + notches + " notch(es)" + newline;
       }
       if (e.getScrollType() == MouseWheelEvent.WHEEL_UNIT_SCROLL) {
           message += "    Scroll type: WHEEL_UNIT_SCROLL" + newline;
           message += "    Scroll amount: " + e.getScrollAmount()
                   + " unit increments per notch" + newline;
           message += "    Units to scroll: " + e.getUnitsToScroll()
                   + " unit increments" + newline;
           message += "    Vertical unit increment: "
               + scrollPane.getVerticalScrollBar().getUnitIncrement(1)
               + " pixels" + newline;
       } else { //scroll type == MouseWheelEvent.WHEEL_BLOCK_SCROLL
           message += "    Scroll type: WHEEL_BLOCK_SCROLL" + newline;
           message += "    Vertical block increment: "
               + scrollPane.getVerticalScrollBar().getBlockIncrement(1)
               + " pixels" + newline;
       }
       saySomething(message, e);
    }
    ...
}

The Mouse Wheel Listener API

The MouseWheelListener Interface

Although MouseWheelListener has only one method, it has the corresponding adapter class — MouseAdapter. This capability enables an application to have only one adapter class instance for the component to manage all types of events from the mouse pointer.

The MouseWheelEvent Class

Examples That Use Mouse Wheel Listeners

The following table lists the examples that use mouse-wheel listeners.