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Event Handling In this class we will cover: Keyboard Events Mouse Events Focus Events Action Interface Multicasting Keyboard Events Table 8-1 on page 298 shows all AWT listener interfaces, events, and event sources. Keyboard Events

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Event Handling

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Event handling l.jpg

Event Handling


In this class we will cover l.jpg

In this class we will cover:

  • Keyboard Events

  • Mouse Events

  • Focus Events

  • Action Interface

  • Multicasting


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Keyboard Events

  • Table 8-1 on page 298 shows all AWT listener interfaces, events, and event sources.

  • Keyboard Events

    • When the user pushes a key, a key_Pressed KeyEvent is generated.

    • When the user releases a key, a key_Released KeyEvent is generated.

    • You trap these events in the keyPressed and the keyReleaesd methods of any class that implements the KeyListener interface.

    • A third method, keyTyped, combines the two and it reports on the characters that were generated by the user’s keystrokes.


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Keyboard Events

  • The difference between characters and virtual key codes:

    • Virtual key codes correspond to keys on the keyboard.

      • e.g. VK_A denotes the key marked A

      • There is no separate lowercase virtual key code - the keyboard does not have separate lowercase keys.

      • Virtual key codes are indicated with a prefix of VK_ such as VK_A or VK_SHIFT

    • Here are the five events that are generated when a user tyes an uppercase A

      • Pressed the shift key (keyPressed called for VK_SHIFT)

      • Pressed the A key (keyPressed called for VK_A)

      • Typed “A” (keyTyped called for an “A”)

      • Released the A key (keyReleased called for VK_A)

      • Released the shift key (keyReleased called for VK_SHIFT)


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Keyboard Events

  • So, how can you see the virtual key codes:public void keyPressed (KeyEvent event) { int keyCode = event.getKeyCode();}

  • You can use the isShiftDown, isControlDown, isAltDown, and isMetaDown instead of looking for the virtual key codespublic void keyPressed (KeyEvent event) { int keyCode = event.getKeyCode(); if (keyCode == KeyEvent.VK_RIGHT && event.isShiftDown()) {…}}


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Keyboard Events

  • To obtain the actual character that was typed use the keyTyped method and call the getKeyChar method.public void keyTyped (KeyEvent event) { char keyChar = event.getKeyChar();}

  • Note: not all keystrokes result in a call to keyTyped. Only those keystrokes that generate a Unicode character can be captured in the keyTyped method. You need the keyPressed method to check for cursor keys and other command keys.


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Mouse Events

  • When a user clicks a mouse button, 3 listener methods are called:

    • mousePressed, mouseReleased and mouseClicked

  • You can use the getX and getY methods of the MouseEvent argument to get the x and y- coordinates of the mouse pointer.

  • If you want to distinguish between single, double and triple clicks, use the getClickCount method.

  • You can also change the cursor moved by the mouse with the getPredefinedCursor method of the Cursor class. See pg. 307 for sample cursor shapes.

  • If the user presses a button while the mouse is in motion, mouseDragged calls are generated instead of mouseClicked calls.


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Mouse Events

  • How do we listen to mouse events?

  • Mouse clicks are reported through the mouseClicked procedure, which is part of the MouseListener interface.

  • Mouse move and drag events are defined in a separate interface called MouseMotionListener.

    • because mouse move events occur much more frequently

  • See www2.bc.edu/~bernier/MC697/LectureNotes/Sketch.java


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Focus Events

  • When you type a keystroke, you keystrokes must go to a specific screen object.

  • The window manager (e.g. Windows) directs all keystrokes to the active window.

  • When the active window is controlled by a Java program, the Java window receives the keystrokes and directs them towards a particular component.

  • This component is then said to have focus.

  • When a component has focus, it usually gives a visual cue

    • text fields have a blinking cursor

    • a button has a rectangle around it

  • When a text field has focus, you can type in it.

  • When a button has focus, you can click on it.


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Focus Events

  • Only one component at a time can have focus.

  • When a user selects a component, that component is said to gain focus.

  • Conversely, when a user selects a different component, the original component is said to lose focus.

  • Some components, such as labels and panels, do not get focus by default because it is assumed that they are just there for decorations or grouping.

    • To override this (for example, to implement a drawing program with panels that paint something in reaction to keystrokes), you need to call:panel.setFocusable(true);


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Actions

  • It is common to have multiple ways to activate the same command.

  • You can do this by linking all he events to the same listener.

    • For example, you can have a button that changes the background color and you can have an alternative way of changing the background color by using a set of keystrokes (e.g. alt-b).

  • The Swing package provides a useful mechanism to encapsulate commands and attach them to multiple event sources - the Action interface


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Action Interface

  • The Action object encapsulates the following:

    • a description of the command (as a text string and optional icon)

    • parameters that are necessary to carry out the command (e.g. color to set).

  • You can store and retrieve arbitrary name/value pairs in the aciton object using the putValue and getValue methods

    • action.putValue(Action.NAME, “Blue”);action.putValue(Action.SMALLICON, new ImageIcon(“blue-ball.gif”));


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Action Interface

  • The Action interface and the AbstractAction adapter class are very useful when using menus. We will discuss this more when we talk about Swing User Interfaces.

  • Read more about the Action interface on page 316.


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Putting it all together

  • See www2.bc.edu/~bernier/MC697/LectureNotes/Sketch.java


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Final Notes on Event Handling

  • Multicasting

    • All AWT event sources support can send the same event to multiple listeners.

    • There is no guarantee about the order in which the events are delivered to the set of registered listeners.

  • Customizing Events

    • You can manipulate the event queue directly to add or remove events. This can be useful sometimes.

    • You can also create a new custom event type and insert it into the event queue and then have it dispatched to a listener - just like regular AWT events.

    • Read more about customizing events on page 32.


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