The Java TM Tutorial

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Trail: Creating a GUI with JFC/Swing
Lesson: Writing Event Listeners

## How to Write a Mouse-Motion Listener

> Mouse-motion events tell you when the user uses the mouse (or a similar input device) to move the onscreen cursor. For information on listening for other kinds of mouse events, such as clicks, see How to Write a Mouse Listener. If your program needs to detect both mouse events and mouse-motion events, you can use Swing's convenient `MouseInputAdapter` class, which implements both `MouseListener` and `MouseMotionListener`.

> The following applet contains a mouse-motion listener. It's exactly like the applet in How to Write a Mouse Listener, except for substituting `MouseMotionListener` for `MouseListener`, implementing the `mouseDragged` and `mouseMoved` methods instead of the mouse listener methods, and displaying coordinates instead of numbers of clicks. You can find the applet's code in `MouseMotionEventDemo.java` and `BlankArea.java`.

> This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

> > Try this:
> > 1. Move the cursor into the yellow rectangle at the top of the applet.
> >    You'll see one or more mouse-moved events.
> > 2. Press and hold the mouse button, and then move the mouse so that the cursor is outside the yellow rectangle.
> >    You'll see mouse-dragged events.

> Here is the code that implements the mouse-motion event handling:

> >     public class MouseMotionEventDemo extends JApplet
> >                                       implements MouseMotionListener {
> >         //...in initialization code:
> >             //Register for mouse events on blankArea and applet (panel).
> >             blankArea.addMouseMotionListener(this);
> >             addMouseMotionListener(this);
> >             ...
> >         }

> >         public void mouseMoved(MouseEvent e) {
> >            saySomething("Mouse moved", e);
> >         }

> >         public void mouseDragged(MouseEvent e) {
> >            saySomething("Mouse dragged", e);
> >         }

> >         void saySomething(String eventDescription, MouseEvent e) {
> >             textArea.append(eventDescription
> >                             + " (" + e.getX() + "," + e.getY() + ")"
> >                             + " detected on "
> >                             + e.getComponent().getClass().getName()
> >                             + newline);
> >         }
> >     }

> A more interesting example is `SelectionDemo`, which is discussed in Arguments to the repaint Method. The program draws a rectangle illustrating the user's current dragging. To do this, it must implement an event handler for three kinds of mouse events: mouse presses, mouse drags, and mouse releases. To be informed of all these events, the handler must implement both the `MouseListener` and `MouseMotionListener` interfaces, and be registered as both a mouse listener and a mouse-motion listener. To avoid having to define empty methods, the handler doesn't implement either listener interface directly. Instead, it extends `MouseInputAdapter`, as the following code snippet shows.

> >     ...//where initialization occurs:
> >         MyListener myListener = new MyListener();
> >         addMouseListener(myListener);
> >         addMouseMotionListener(myListener);
> >     ...
> >     class MyListener extends MouseInputAdapter {
> >         public void mousePressed(MouseEvent e) {
> >             int x = e.getX();
> >             int y = e.getY();
> >             currentRect = new Rectangle(x, y, 0, 0);
> >             updateDrawableRect(getWidth(), getHeight());
> >             repaint();
> >         }

> >         public void mouseDragged(MouseEvent e) {
> >             updateSize(e);
> >         }

> >         public void mouseReleased(MouseEvent e) {
> >             updateSize(e);
> >         }

> >         void updateSize(MouseEvent e) {
> >             int x = e.getX();
> >             int y = e.getY();
> >             ...
> >             repaint(...);
> >         }
> >     }

### The Mouse-Motion Event API

> The `MouseMotionListener` interface and its corresponding adapter class, `MouseMotionAdapter`, contain these methods:
> `void mouseDragged(MouseEvent)`
>   Called in response to the user moving the mouse while holding a mouse button down. This event is fired by the component that fired the most recent mouse-pressed event, even if the cursor is no longer over that component.
> `void mouseMoved(MouseEvent)`
>   Called in response to the user moving the mouse with no mouse buttons pressed. This event is fired by the component that's currently under the cursor.
> Each mouse-motion event method has a single parameter -- and it's not called `MouseMotionEvent`! Instead, each mouse-motion event method has a `MouseEvent` argument. See The Mouse Event API for information about using `MouseEvent` objects.

### Examples that Use Mouse-Motion Listeners

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

> Example | Where Described | Notes
> --- | --- | ---
> `MouseMotionEventDemo` | This section | Reports all mouse motion events that occur within a blank panel to demonstrate the circumstances under which mouse motion events are fired.
> `LayeredPaneDemo` and `LayeredPaneDemo2` | How to Use Layered Panes | Moves an image of Duke around within a layered pane in response to mouse motion events.
> `SelectionDemo` | Arguments to the repaint Method | An applet that lets the user drag a rectangle to select a portion of an image. Uses a subclass of `MouseInputAdapter` to listen to both mouse events and mouse-motion events.
> `GlassPaneDemo` | How to Use Root Panes | Uses a subclass of `MouseInputAdapter` to listen to mouse events and mouse-motion events on the root pane's glass pane. Redispatches the events to underlying components.

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