Java gui event

Java gui event

• 1.
  11 Events and Applet JAVA/ Adv. OOP
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  22 Contents  What isan Event?  Events and Delegation Event Model  Event Class, and Listener Interfaces  Discussion on Adapter Classes  What, Why, Purpose and benefit  GUI Discussion
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  33 Events & EventHandling  Components (AWT and Swing) generate events in response to user actions  Each time a user interacts with a component an event is generated, e.g.:  A button is pressed  A menu item is selected  A window is resized  A key is pressed  An event is an object that describes some state change in a source  An event informs the program about the action that must be performed.  The program must provide event handlers to catch and process events. Unprocessed events are passed up through the event hierarchy and handled by a default (do nothing) handler
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  16-16-44 Listener  Events arecaptured and processed by “listeners” — objects equipped to handle a particular type of events.  Different types of events are processed by different types of listeners.  Different types of “listeners” are described as interfaces: ActionListener ChangeListener ItemListener etc.
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  16-16-66 Listeners (cont’d)  Eventobjects have useful methods. For example, getSource returns the object that produced this event.  A MouseEvent has methods getX, getY.  When implementing an event listener, programmers often use a private inner class that has access to all the fields of the surrounding public class.  An inner class can have constructors and private fields, like any other class.  A private inner class is accessible only in its outer class.
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  88 The Delegation EventModel  The model provides a standard mechanism for a source to generate an event and send it to a set of listeners  A source generates events.  Three responsibilities of a source:  To provide methods that allow listeners to register and unregister for notifications about a specific type of event  To generate the event  To send the event to all registered listeners. Source Listener Listener Listener events container
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  99 The Delegation EventModel  A listener receives event notifications.  Three responsibilities of a listener:  To register to receive notifications about specific events by calling appropriate registration method of the source.  To implement an interface to receive events of that type.  To unregister if it no longer wants to receive those notifications.  The delegation event model:  A source multicasts an event to a set of listeners.  The listeners implement an interface to receive notifications about that type of event.  In effect, the source delegates the processing of the event to one or more listeners.
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  1010 Event Classes Hierarchy Object EventObject AWTEvent ActionEventComponentEvent ItemEvent TextEvent FocusEvent InputEvent WindowEvent KeyEvent MouseEvent
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  1212 AWT Event Classesand Listener Interfaces Event Class Listener Interface ActionEvent ActionListener AdjustmentEvent AdjustmentListener ComponentEvent ComponentListener ContainerEvent ContainerListener FocusEvent FocusListener ItemEvent ItemListener KeyEvent KeyListener MouseEvent MouseListener, MouseMotionListener TextEvent TextListener WindowEvent WindowListener
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  1313 Semantic Event Listener The semantic events relate to operations on the components in the GUI. There are 3semantic event classes.  An ActionEvent is generated when there was an action performed on a component such as clicking on a menu item or a button. ― Produced by Objects of Type: Buttons, Menus, Text  An ItemEvent occurs when a component is selected or deselected. ― Produced by Objects of Type: Buttons, Menus  An AdjustmentEvent is produced when an adjustable object, such as a scrollbar, is adjusted. ― Produced by Objects of Type: Scrollbar  Semantic Event Listeners  Listener Interface: ActionListener, Method: void actionPerformed(ActionEvent e)  Listener Interface: ItemListener, Method: void itemStateChanged (ItemEvent e)  Listener Interface: AdjustmentListener, Method: void adjustmentValueChanged (AdjustmentEvent e)
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  1414 Using the ActionListener Stages for Event Handling by ActionListener  First, import event class import java.awt.event.*;  Define an overriding class of event type (implements ActionListener)  Create an event listener object ButtonListener bt = new ButtonListener();  Register the event listener object b1 = new Button(“Show”); b1.addActionListener(bt); class ButtonListener implements ActionListener { public void actionPerformed(ActionEvent e) { / / Write what to be done. . . label.setText(“Hello World!”); } } addActionListener ButtonListener action Button Click Event ① ②
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  two ways todo the same thing.  Inner class method  Using an anonymous inner class, you will typically use an ActionListener class in the following manner: ActionListener listener = new ActionListener({ public void actionPerformed(ActionEvent actionEvent) { System.out.println("Event happened"); } 1616
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  OR  Interface Method Implement the Listener interface in a high-level class and use the actionPerformed method. public class MyClass extends JFrame implements ActionListener { ... public void actionPerformed(ActionEvent actionEvent) { System.out.println("Event happened"); }…. } 1717
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  1818 MouseListener (Low-Level Listener)  TheMouseListener interface defines the methods to receive mouse events.  mouseClicked(MouseEvent me)  mouseEntered(MouseEvent me)  mouseExited(MouseEvent me)  mousePressed(MouseEvent me)  A listener must register to receive mouse events import javax.swing.*; import java.awt.event.*; class ABC extends JFrame implements MouseListener { ABC() { super("Example: MouseListener"); addMouseListener(this); setSize(250,100); } public void mouseClicked(MouseEvent me) { } public void mouseEntered(MouseEvent me) { } public void mouseExited(MouseEvent me) { } public void mousePressed(MouseEvent me) { } public void mouseReleased(MouseEvent me) { System.out.println(me); } public static void main(String[] args) { new ABC().setVisible(true); } } MouseEvent Mouse click
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  Adapter Classes  JavaLanguage rule: implement all the methods of an interface  That way someone who wants to implement the interface but does not want or does not know how to implement all methods, can use the adapter class instead, and only override the methods he is interested in. i.e.  An adapter classes provide empty implementation of all methods declared in an EventListener interface. 1919
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  2020 Adapter Classes (Low-LevelEvent Listener)  The following classes show examples of listener and adapter pairs:  package java.awt.event  ComponentListener / ComponentAdapter  ContainerListener / ContainerAdapter  FocusListener / FocusAdapter  HierarchyBoundsListener /HierarchyBoundsAdapter  KeyListener /KeyAdapter  MouseListener /MouseAdapter  MouseMotionListener /MouseMotionAdapter  WindowListener /WindowAdapter  package java.awt.dnd  DragSourceListener /DragSourceAdapter  DragTargetListener /DragTargetAdapter  package javax.swing.event  InternalFrameListener /InternalFrameAdapter  MouseInputListener /MouseInputAdapter
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  Listener Interface Style 2121 publicclass GoodJButtonSubclass extends JButton implements MouseListener { ... public void mouseClicked(MouseEvent mouseEvent) { // Do nothing } public void mouseEntered(MouseEvent mouseEvent) { // Do nothing } public void mouseExited(MouseEvent mouseEvent) { // Do nothing } public void mousePressed(MouseEvent mouseEvent) { System.out.println("I'm pressed: " + mouseEvent); } public void mouseReleased(MouseEvent mouseEvent) { // Do nothing } ... addMouseListener(this); ... }
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  Listener Inner ClassStyle MouseListener mouseListener = new MouseListener() { public void mouseClicked(MouseEvent mouseEvent) { System.out.println("I'm clicked: " + mouseEvent); } public void mouseEntered(MouseEvent mouseEvent) { System.out.println("I'm entered: " + mouseEvent); } public void mouseExited(MouseEvent mouseEvent) { System.out.println("I'm exited: " + mouseEvent); } public void mousePressed(MouseEvent mouseEvent) { System.out.println("I'm pressed: " + mouseEvent); } public void mouseReleased(MouseEvent mouseEvent) { System.out.println("I'm released: " + mouseEvent); } }; 2222
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  Using Adapter InnerClass 2323 MouseListener mouseListener = new MouseAdapter() { public void mousePressed(MouseEvent mouseEvent) { System.out.println("I'm pressed: " + mouseEvent); } public void mouseReleased(MouseEvent mouseEvent) { System.out.println("I'm released: " + mouseEvent); } };
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  Extend inner class fromAdapter Class 2424 import java.awt.*; import java.awt.event.*; public class FocusAdapterExample { Label label; public FocusAdapterExample() { Frame frame = new Frame(); Button Okay = new Button("Okay"); Button Cancel = new Button("Cancel"); Okay.addFocusListener(new MyFocusListener()); Cancel.addFocusListener(new MyFocusListener()); frame.add(Okay, BorderLayout.NORTH); frame.add(Cancel, BorderLayout.SOUTH); label = new Label(); frame.add(label, BorderLayout.CENTER); frame.setSize(450, 400); frame.setVisible(true); frame.addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent we) { System.exit(0); } }); } public class MyFocusListener extends FocusAdapter { public void focusGained(FocusEvent fe) { Button button = (Button) fe.getSource(); label.setText(button.getLabel()); } } public static void main(String[] args) { FocusAdapterExample fc = new FocusAdapterExample(); } }