JAVA SWING GUI TUTORIAL

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6   6th Step: Basic Event Handling

This is shown in ButtonFrame.java demo.

We consider the class Ëvent", which are all derived from java.util.EventObject. Examples of events are "button pushed", "key pushed", "mouse moved". Some subclasses of Event are ActionEvents and WindowEvents.

To understand events, you need to consider two types of interfaces, and their relationship: Event Detectors and Event Listeners. The former is set up to detect the occurence of certain types of event, and it sends notices to the listeners. It is the listeners that take the appropriate action.

Examples of Event Detectors are windows or buttons. In this first event demo, we use buttons, as they generate the simplest kind of events. Buttons detects only one type of event - called ActionEvents. In contrast, there are seven kinds of WindowEvents. For buttons, the ÄctionListener" is appropriate. But in order for the event objects to know which listener object to send the events to, we need to do three things:

(1) Implement the listener interface using ANY reasonable class. In our example, the class will be an extension of JPanel. To implement the ActionListener, you need to supply the method actionPerformed(ActionEvent) (the only method of this interface). The class for implementing Actionlistener here is "MyPanel":

		public class MyPanel extends JPanel
			implements ActionListener {
		  public void actionPerformed(ActionEvent e){
		    // reaction to button click goes here
		    ...
		  } // actionPerformed
		} // class MyPanel
	

What is the action "..." above? This is explained below.

(2) Create a listener object:

		Listener lis = new MyPanel();
	

(3) register this object with the event detector.

		button.addActionListener(lis); // button is the event detector;
	

The general form for registering listener objects is:

		<eventDetector>.add<EventType>Listener(<listenerObject>);
	

In our present demo, we will have two buttons (redButton and blueButton) in a panel. When redButton is pressed, the background of the panel changes to Red, and similarly when the blueButton is pressed. Thus, these buttons serve as event detectors. To use buttons, we need to create them:

		private JButton redButton;
		redButton = new JButton("RED"); // "RED" is label on button
	

In addition to (or instead of) "RED", we can supply an image file:

		redButton = new JButton(new ImageIcon("RED.gif");
	

Next, you add the buttons to a panel (called ButtonPanel here). We also register the listener object with the buttons - but the listener object will be "this" (i.e., current object)!

	class ButtonPanel extends JPanel {
	  // members:
	  private JButton redButton;
	  private JButton blueButton;
	  // constructors:
	  public ButtonPanel() {

	    // create buttons
	    redButton = new JButton("RED");
	    blueButton = new JButton("BLUE");

	    // add buttons to current panel
	    add(redButton);  // add button to current panel
	    add(blueButton); // add button to current panel

	    // register the current panel as listener for the buttons
	    redButton.addActionListener(this);  
	    blueButton.addActionListener(this); 

	  } // ButtonPanel constructor
	} // ButtonPanel class
	

We now return the details needed in the äctionPerformed(ActionEvent)" method from the ActionListener interface. First, you need to find out which Button caused this event. There are 2 ways to find out. First, the getSource() method from EventObject can be used:

	Color color = getBackground();  // color will be set
	Object source = e.getSource();
	if (source == redButton) color = Color.red
	else if (source == blueButton) color = Color.blue
	setBackground(color);
	repaint();  // when do we need this??
	

The second method, specific to ActionEvents, is to use the getActionCommand() method, which returns a "command string", which defaults to the button label. Thus,

	String com = e.getActionCommand();
	if (com.equals("RED")) ...; // "RED" is the label of redButton
	else if (com.equals("BLUE")) ...;
	

But the command string need not be the label of the button. That can be independently set:

	redButton.setActionCommand("RED ACTION");
	

The ActionListener interface is also used when (a) when an item is selected from a list box with a double click, (b) when a menu item is selected (c) when an enter key is clicked in the text field (d) when a specific time has elapsed for a "Timer" component.

In our present example of the red and blue buttons, the ActionListener interface is implemented by MyPanel for a good reason: the action of changing the background of the panel to red or blue ought to reside with the panel!

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7   7th Step: Window Events

(NO DEMO PROGRAM HERE! FIX) We now consider the JFrame as an event detector. A JFrame is basically synonymous with a "window" and it detect Window Events. We need to implement the "WindowListener" interface to be registered with the JFrame. This interface has 7 methods:

	public void windowClosed(WindowEvent e)
	public void windowIconified(WindowEvent e)
	public void windowOpened(WindowEvent e)
	public void windowClosing(WindowEvent e)
	public void windowDeiconified(WindowEvent e)
	public void windowActivated(WindowEvent e)
	public void windowDeactivated(WindowEvent e)
	

Recall that we already had a brief introduction to this interface, when we extended the "WindowAdapter" class to provide default empty implementations for all but the windowClosing method, which we implement by a call to "System.exit(0)". In Step 2 above, the extension of WindowAdapter was called Terminator. In general, all the AWT listener interfaces with more than one method comes with such an adapter class. Finally, we we create an instance of the Terminator class and register it with the JFrame using the äddWindowListener(WindowListener wl)" method:

	class MyFrame extends JFrame {
	  // Constructor:
	  public MyFrame() {
	    addWindowListener(new Terminator());
	    ...
	  } // MyFrame Constructor
	  ...
	} // MyFrame Class
	

We can even make the "Terminator" anonymous, as an inner class, as follows:

	class MyFrame extends JFrame {
	  // Constructor:
	  public MyFrame() {
	    addWindowListener(new WindowAdapter() {
	      public void windowClosing(WindowEvent e) {
	        System.exit(0);
	      } // windowClosing
	    }); // WindowAdapter
	    ...
	  } // MyFrame Constructor
	  ...
	} // MyFrame Class
	

In general, you can create an anonymous class by the construct

	new XXXclass() {... override methods, etc... }
	

Inner classes are also extremely useful, because the inner class can automatically get access to all the methods and fields of its parent class. In Component design, you often want a new class to subclass two different classes. But Java does not allow this. You get around this restriction by subclassing a subclass - an inner class is one way of doing this.

An example where you want to program the "windowDeactivated", "windowActivated", "windowIconified" and "windowDeiconified" is when your window displays an animation. You would want to stop or start the animations when these events occur.

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8   8th Step: Event Classes and Listener Interfaces

Now that we have seen two types of events (ActionEvents and WindowEvents), let us overview the general picture. Here is the event hierarchy:

				 | ActionEvent*	    | ContainerEv.*
				 | AdjustmentEv.*   | FocusEvent*   | KeyEvent*
    EventObject <-- AWT Event <--| ComponentEv.* <--| InputEvent <--|
				 | ItemEvent*	    | PaintEvent    | MouseEv.*
				 | TextEvent*	    | WindowEvent*
	

In this hierarchy, only 10 event classes, those indicated with asterisks (*) are actually passed to listeners. The 10 event classes are classified into 4 ßemantic" events and 6 "low-level" events. Intuitively, semantic events correspond to what the user intends (e.g., button click), while low-level events correspond to physical events.

Semantic Events
1) ActionEvent: button click, menu selection, selecting item in list, typing ENTER in text field
2) AdjustmentEvent: the user adjusted a scroll bar.
3) ItemEvent: the use made a selection from a set of checkbox or list items
4) TextEvent: the contents of a text field or area were changed.

Low-Level Events
1) ComponentEvent: component is resized, moved, shown, hidden. It is the base class for all low-level events.
2) KeyEvent: a key was pressed or released.
3) MouseEvent: the mouse button was depressed, released, moved, dragged.
4) FocusEvent: a component got focus, lost focus.
5) WindowEvent: window was (de)activated, (de)iconified, or closed.
6) ContainerEvent: a component has been added or removed. Usually, you don't have to worry about this class of event, as these events are (usually) not generated dynamically, but in your program.

All low-level events is derived from ComponentEvent. They all have a method "getComponent" (it is similar to the "getSource" method but the result is already cast as a component).

There are 11 listener interfaces in java.awt.event:

	ActionListener, 	AdjustmentListener,	ComponentListener*
	ContainerListener*,	KeyListener*,		MouseListener*
	MouseMotionListener*,	TextListener,		FocusListener*
	ItemListener,		WindowListener*.

The listeners with asterisks (*) have a corresponding adaptor class implementing it because they each have more than one method. There is a 1-1 correspondence between listeners and event types, with one exception: MouseEvents are sent to both MouseListeners and MouseMotionListener. The split into two types of listeners for MouseEvents is done for efficiency - so we can ignore an entire class of mouse events (such as mouse motion which can generate frequent events).

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9   9th Step: Focus Event

(NO DEMO PROGRAM HERE) (A) In Java, a component has the "focus" if it can receive key strokes. E.g., a text field has the focus when the cursor mark becomes visible, ready to receive key strokes. When a button has "focus", you can click it by pressing the space bar. (B) Only one component can have the focus at any moment. The user can choose the component to have focus by a mouse click in the component (in some system, just having the mouse cursor over a component is sufficient). The TAB key also moves the focus to the "next" component, and thus allows you to cycle over all "focusable" components. Some components like labels or panels are not "focusable" by default. You can make any component "focusable" or not by overriding the ïsFocusTraversable" method to return TRUE or FALSE. You can use the "requestFocus" method to move the focus to any specific component at run time, or you can use "transferFocus" method to move to the next component. The notion of "next" component can be changed. (C) The FocusListener interface has 2 methods: focusGained and focusLost. Each takes the "FocusEvent" object as parameter. Two useful methods for implementing this interface are "getComponent" and ïsTemporary". The latter returns TRUE if the focus lost is temporary and will automatically be restored.

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10   10th Step: KeyBoard Events and Sketch Demo

The "keyPressed" and "keyReleased" methods of the KeyListener interface handles raw keystrokes. However, another method "keyTyped" combines the response to these two types of events, and returns the characters actually typed. Java distinguished between "characters" and "virtual key codes". The latter are indicated by the prefix of "VK_" such as "VK_A" and "VK_SHIFT". These 3 methods are best illustrated with an example:

Suppose a user types an lower case ä". There are only 3 events:
(a) Pressed A key (keyPressed called for VK_A)
(b) Typed ä" (keyTyped called for character ä")
(c) Released A key (keyReleased called for VK_A)

Now, suppose the user types an upper case Ä". There are 5 events:
(a) Pressed the SHIFT key (keyPressed called for VK_SHIFT)
(b) Pressed A key (keyPressed called for VK_A)
(c) Typed Ä" (keyTyped called for character Ä")
(d) Released A key (keyReleased called for VK_A)
(e) Released SHIFT key (keyReleased called for VK_SHIFT)

To work with keyPressed and keyReleased methods, you need to check the "key code".

	public void keyPressed(KeyEvent e) {
	  int keyCode = e.getKeyCode();
	  ...
	}
	

	VK_A VK_B ... VK_Z
	VK_0 ... VK_9
	VK_COMMA VK_PERIOD ... etc
	

To work with keyTyped method, you can call the "getKeyChar" method.

The following demo is a Ëtch-A-Sketch" toy where you move a pen up, down, left, right with cursor keys or h, j, k, l. Holding down the SHIFT key at the same time will move the pen by larger increments.

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11   11th Step: Mouse Events and Mouse Demo

Some mouse events such as clicking on buttons and menus are handled internally by the various components and translated automatically into the appropriate semantic event (e.g, handled by the actionPerformed or itemStateChanged methods). But to draw with a mouse, we need to trap mouse events.

When the user clicks a mouse button, three kinds od MouseEvents are generated. The corresponding three MouseListener methods are: "mousePressed", "mouseReleased" and "mouseClicked". The last method generates only one event for each pressed-released combination. To obtain the position of the mouse when the events occur, use the methods MouseEvent.getX() and MouseEvent.getY(). To distinguish between single and double (and even triple) clicks, use the MouseEvent.getClickCount() method.

SIMPLIFY the example in the book (how about drawing a line to the current mouse click?) But I already have this under the "rubber line" example.

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12   12th Step: Action Interface

(I) WHAT ARE ACTIONS? Java even model allows us to choose any class as listener for its events. So far, we have let each class (i.e., "this") be its own listener. For bigger examples, we want to separate the responsibilites of detecting from listening/responding. We need an independent notion of äction" (="responding to action"). This is logically sensible, since the same action may be needed for different events. For example, suppose the action is to ßet background color" (to red/blue, etc). This action may be initiated in 3 ways:
(a) click on a color button (as in example above)
(b) selection of a color in a set-background menu
(c) press of a key (B=blue, R=red, etc).

(II) METHODS OF ACTION INTERFACE: Java provides the Äction" interface with these methods:
(1) void actionPerformed(ActionEvent e) - performs the action corresponding to event e
(2) void setEnabled(boolean b) - this turns the action on or off
(3) boolean isEnabled() - checks if the action is on
(4) void putValue(String key, Object val) - store a value under a key (String type). There are 2 standard keys: Action.NAME (name of action) and Action.SMALL_ICON (icons for action). E.g., Action.putValue(Action.SMALL_ICON, new ImageIcon("red.gif"));
(5) Object getValue(String key) - retrieve the value stored under key
(6) void addPropertyChangeListener(PropertyChangeListener listener) - Add a "Changelistener" object to our current list. Menus and toolbars are examples of "ChangeListeners", as these components must be notified when a property of an action that they are responsible for changes.
(7) void removePropertyChangeListener(PropertyChangeListener listener) - Similar to method in (6), but this one is to remove a "ChangeListener" from current list.
Actually, Äction" extends ÄctionListener" and method (1) above was the ONLY method in the ActionListener interface (cf. the ButtonFrame.java demo above). This method Thus, an Action object can be used whenever an ActionListener object is expected.

(III) MenuAction.java: IMPLEMENTING and USING AN ACTION INTERFACE. There are three steps:
STEP 1: Define a class implementing the Action interface
STEP 2: Instance the Action class
STEP 3: Associate the action instance with components
STEP 4: Add the components to the windowing system

STEP 1: As usual, there is a default implementation of the Action interface, called ÄbstractAction". You can adapt from this class, and only äctionPerformed" method needs to be explicitly programed by you. Usually, you also want to provide a constructor to set the values stored under various keys, and your class will want a member variable "target" to remember the component where the action is to be performed (recall that the Action object need not be the component itself, after our decoupling of ëvent generator" from ëvent listener"). Here is an implementation of the action ßet background color":

	class BackgroundColorAction extends AbstractAction {
	  //members:
	    private Component target;  // where you want the action done!

	  //constructor:
	  public BackgroundColorAction(
		String name, Icon i, Color c, Component comp) {
	    putValue(Action.NAME, name);
	    putValue(Action.SMALL\_ICON, i);
	    putValue("Color", c);
	    target = comp;
	  } // constructor

	  //methods:
	  public void actionPerformed(ActionEvent e) {
	    Color c = (Color)getValue("Color");
	    target.setBackground(c);
	    target.repaint();
	  }// actionPerformed method
	}// BackgroundColorAction class
	

STEP 2: we need to instance the class:

	Action redAction = new BackgroundColorAction(
		"Red", new ImageIcon("red.gif"), Color.red, panel);
	

STEP 3: associate the action with components or their instances: The following associates "redAction" with a component instance. The component illustrated here is a JButton instance.

	JButton redButton = new JButton("Red");
	redButton.addActionListener(redAction);
	

Alternatively, we associate any given Äction" with an entire class of components. For JButtons, we create button class that comes with an action:

	class ActionButton extends JButton {
	  public ActionButton(Action a) {
	    setText((String)a.getValue(Action.NAME));
	    Icon icon = (Icon)a.getValue(Action.SMALL\_ICON);
	    if (icon != null) setIcon(icon)
	    addActionListener(a);
	  }// ActionButton constructor
	}// ActionButton class

	// instance it!
	redButton = new ActionButton(redAction);
	

NOTE: If we introduce ÄctionButtons" then STEPS 2 and 3 should be interchanged!

STEP 4: Now add ActionButtons to a menu, then to the menu bar:

 	// create the menu of ActionButtons:
	JMenu m = new JMenu("Color");
	m.add(redAction);
	m.add(blueAction);

	// add menu to menubar
	JMenuBar mbar = new JMenuBar();
	mbar.add(m);
	setJMenubar(mbar);
	

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