Lesson 6 Programming Techniques Event Handling /EvH/ AUBG ICoSCIS Team Assoc. Prof. Stoyan Bonev March, , 2013 SWU, Blagoevgrad

Lesson contents §EvH – theory §EvH – practice §Demo Programs

EvH – Theory §Event-driven programming §Event §Source §Listener §Respond to user events within any class you create l Prepare your class to accept event messages l Tell your class to expect events to happen l Tell your class how to respond to events

Basic terminology §Event l Occurs when a user takes action on a component, such as clicking the mouse on a JButton object l User might initiate any number of events in any order §Source l Component on which an event is generated §Listener l Object that is interested in an event §Respond to user events within any class you create l Prepare your class to accept event messages l Tell your class to expect events to happen l Tell your class how to respond to events

6Java Programming: From Problem Analysis to Program Design, 4e Handling an Event §Clicking a JButton creates an event, known as action event which sends a message to another object known as §action listener: When the listener receives the message, it performs some action. §Sending a message or an event to a listener object means that a method in the listener object is invoked automatically with the event as argument. §Two things are must to be done: l For each GUI control, you must specify listener object. In Java, you must register the listener. l You must define methods that will invoke when the event is sent to the listener. Normally, you write these methods and you never write the code for their invocation.

7 More on EvH §s§s

8 Motivations Suppose you wish to write a GUI program that lets user enter the loan amount, annual interest rate, & number of years, and click the Compute Loan button to obtain the monthly payment and total payment. How do you accomplish the task? You have to use event-driven programming to write the code to respond to the button-clicking event.

9 Motivations Suppose you wish to write a program that animates a rising flag, as shown in Figures below. How do you accomplish the task? An effective way to solve it is to use a timer in event-driven programming, which is the subject of this lecture.

10 Procedural vs. Event-Driven Programming §Procedural programming is executed in procedural order.  In event-driven programming, code is executed upon activation of events.

11 Event-Driven Programming Practical Demo Introduction Example: the ActionListener Interface (open file ProgDemoEvH1.java)

12 To feel Event-Driven Programming §The case: §3 buttons (OK, Cancel, Exit) placed into a panel. §Panel placed into a frame. §The program displays 3 buttons in the frame. §A message is displayed on the console and into a message box when a button is clicked.

13 §To respond to a button click, you must write the code to process the button-clicking action §The button is a source object where the action originates. §You need to create an object capable of handling the action event on a button. This object is called a listener. §Button Event Listener ↑ ↑ ↑ Clicking An eventListener object Button firesis an objectprocesses the action event event

14 How to understand source & listener §Source is an object, like component button §To be a listener: The object must be instance of the ActionListener interface. You need a user class to implement the interface ( method actionPeformed() ) and to create object of your user defined class. The object created as a listener must be registered with (i.e. to bind it to) the source using method source.addListener(listener)

15 How to understand source & listener // Create a button with text OK JButton jbtOK1 = new JButton("OK"); // creating listener as object/instance OK1ListenerClass listenerOK = new OK1ListenerClass(); // registering listener, i.e. binding listener by component jbtOK1.addActionListener(listenerOK); // user specified class to implement interface class OK1ListenerClass implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("OK button clicked" ); JOptionPane.showMessageDialog(null,"OK button clicked"); } // end of method } // end of class

16 The ActionListener Interface and Handling GUI Events Source object (e.g., button) Listener object contains a method for processing the event.

17 Trace Execution public class HandleEvent extends JFrame { public HandleEvent() { … OKListenerClass listener1 = new OKListenerClass(); jbtOK.addActionListener(listener1); … } public static void main(String[] args) { … } class OKListenerClass implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("OK button clicked"); }

18 Event-Driven Programming Comprehensive Introduction.

19 Java uses a delegation-based model for event handling: a source object fires an event; an object interested in the event, handles it Button Event Listener ↑ ↑ ↑ Clicking An eventListener object Button firesis an objectprocesses the action event event

20 Event Source §The component that creates an event and fires it, is called l Source object or l Source component §E.g. a button is a source object for a button- clicking action event

21 Listener §Java uses delegation-based model for EvH: l A source object fires an event, and l An object, interested in the event, handles it. The object is called listener. §For an object to be a listener for an event on a source object, two requirements must meet: l The object must be instance of the corresponding event- listener interface. You need a user class to implement the interface and to create object of your user defined class. The listener object created must be registered by (i.e. to bind it to) the source using method like source.addListener(listener)

22 Events An event can be defined as a type of signal to the program that something has happened. The event is generated l by external user actions such as mouse movements, mouse clicks, and keystrokes, OR l by the operating system, such as a timer.  An event is an instance of EventObject class.

23 Event Classes

24 Event Information An event object contains whatever properties are pertinent to the event. You can identify the source object of the event using the getSource() instance method in the EventObject class. The subclasses of EventObject deal with special types of events, such as button actions, window events, component events, mouse movements, and keystrokes. Table on next page lists external user actions, source objects, and event types generated.

25 Selected User Actions SourceEvent Type User ActionObjectGenerated Click a button JButton ActionEvent Click a check box JCheckBox ItemEvent, ActionEvent Click a radio button JRadioButton ItemEvent, ActionEvent Press return on a text field JTextField ActionEvent Select a new item JComboBox ItemEvent, ActionEvent Window opened, closed, etc. Window WindowEvent Mouse pressed, released, etc. Component MouseEvent Key released, pressed, etc. Component KeyEvent

26 The Delegation Model: Example JButton jbt = new JButton("OK"); ActionListener listener = new OKListener(); jbt.addActionListener(listener); Comment: when you click the button, the JButton source object ( jbt ) fires an ActionEvent event and passes it to invoke the listener’s actionPerformed() method to handle the event

27 Selected Event Handlers Event ClassListener InterfaceListener Methods (Handlers) ActionEventActionListeneractionPerformed(ActionEvent) ItemEventItemListeneritemStateChanged(ItemEvent) WindowEventWindowListenerwindowClosing(WindowEvent) windowOpened(WindowEvent) windowIconified(WindowEvent) windowDeiconified(WindowEvent) windowClosed(WindowEvent) windowActivated(WindowEvent) windowDeactivated(WindowEvent) ContainerEventContainerListenercomponentAdded(ContainerEvent) componentRemoved(ContainerEvent) MouseEventMouseListenermousePressed(MouseEvent) mouseReleased(MouseEvent) mouseClicked(MouseEvent) mouseExited(MouseEvent) mouseEntered(MouseEvent) KeyEventKeyListenerkeyPressed(KeyEvent) keyReleased(KeyEvent) keyTypeed(KeyEvent)

28 java.awt.event.ActionEvent

29 Demo program  Open file ProgDemoEvH1.java l Examine the source text  Three buttons, three classes to implement interface ActionListener, three listeners objects registered to the buttons l Compile l Run §Output – mixture of console output and showMessageDialog

30 Demo program  Open file ProgDemoEvH1.java §Examine the source text §Modify the program on your choice §Call ActionEvent methods like l getSource() l getActionCommand() l getWhen()

31 Demo program  Open file ProgDemoMouseEvents.java l Examine the source text  Panel into a frame, one user defined class to implement interface MouseListener and all its methods, one listener object registered to the panel l Compile l Run §Output – console output

32 MouseEvent

33 Handling Mouse Events  Java provides two listener interfaces, MouseListener and MouseMotionListener, to handle mouse events.  The MouseListener listens for actions such as when the mouse is pressed, released, entered, exited, or clicked.  The MouseMotionListener listens for actions such as dragging or moving the mouse.

34 Handling Mouse Events

35 Demo program  Open file ProgDemoMouseEvents.java §Examine the source text §Modify the program on your choice §Call some MouseEvent methods (details before 3 slides)

36 Demo program  Open file ProgDemoKeyEvents.java l Examine the source text  Panel into a frame, one user defined class to implement interface KeyListener and all its methods, one listener object registered to the panel l Compile l Run §Output – console output and graphic output

37 Inner Class Listeners A listener class is designed specifically to create a listener object for a GUI component (e.g., a button). It will not be shared by other applications. So, it is appropriate to define the listener class inside the frame class as an inner class.

38 Inner Classes Inner class: A class is a member of another class. Advantages: In some applications, you can use an inner class to make programs simple. §An inner class can reference the data and methods defined in the outer class in which it nests, so you do not need to pass the reference of the outer class to the constructor of the inner class.

39 Inner Classes, cont.

40 Inner Classes (cont.) §Inner classes can make programs simple and concise. §An inner class supports the work of its containing outer class and is compiled into a class named OuterClassName$InnerClassName.class. For example, the inner class InnerClass in OuterClass is compiled into OuterClass$InnerClass.class.

41 Demo program  Open file ProgDemoKeyEvents.java §Examine the source text §Modify the program on your choice

42 Handling Keyboard Events  keyPressed(KeyEvent e) Called when a key is pressed.  keyReleased(KeyEvent e) Called when a key is released.  keyTyped(KeyEvent e) Called when a key is pressed and then released. To process a keyboard event, use the following handlers in the KeyListener interface:

43 The KeyEvent Class  Methods: getKeyChar() method getKeyCode() method  Keys: Home VK_HOME End VK_END Page Up VK_PGUP Page Down VK_PGDN etc...

44 The KeyEvent Class, cont.

45 Example: Keyboard Events Demo Objective: Display a user-input character. The user can also move the character up, down, left, and right using the arrow keys.

46 Demo program  Open file ProgDemoControlRectangle.java l Examine the source text  Three buttons, three classes to implement interface ActionListener and its EvHandler method ActionPerformed(), three listeners objects registered to the buttons l Compile l Run §Output –console output and graphics output

47 Ideas to modify this demo Circle instead of rectangle.

48 Ideas to modify this demo String instead of rectangle.

49 Demo program  Open file ProgDemoControlRectangle.jav a §Examine the source text §Modify the program on your choice

50 Demo program  Open file ProgDemoWindowEvents.java l Examine the source text  Panel into a frame, one user defined class to implement interface WindowListener and all its seven methods, one listener object registered to the entire frame or window l Compile l Run §Output – console output

51 Demonstrate: Handling Window Events F Objective: Any subclass of the Window class can generate the following window events: F window opened, F window closing,window closed, F window activated,window deactivated, F window iconified,window deiconified.  This program creates a frame, listens to the window events, and displays a message to indicate the occurring event.

52 Demo program  Open file ProgDemoWindowEvents.java §Examine the source text §Modify the program on your choice

53 Demo program  Open file ProgDemoWindowEventsVer2.java l Examine the source text  Panel into a frame, one user defined class to implement interface WindowListener and all its seven methods, one anonymous listener object registered to the entire frame or window l Compile l Run §Output –console output

54 Anonymous Inner Classes §An anonymous inner class must always extend a superclass or implement an interface, but it cannot have an explicit extends or implements clause. §An anonymous inner class must implement all the abstract methods in the superclass or in the interface. §An anonymous inner class always uses the no-arg constructor from its superclass to create an instance. If an anonymous inner class implements an interface, the constructor is Object(). §An anonymous inner class is compiled into a class named OuterClassName$n.class. For example, if the outer class Test has two anonymous inner classes, these two classes are compiled into Test$1.class and Test$2.class.

55 Demo program  Open file ProgDemoWindowEventsVer2.java §Examine the source text §Modify the program on your choice

56 Demo program  Open file ProgDemoWindowEventsAdapter.java l Examine the source text  Panel into a frame, one user defined class to implement interface WindowAdapter and not all but only necessary methods, one anonymous listener object registered to the entire frame or window l Compile l Run §Output – console output

57 Demo program  Open file ProgDemoWindowEventsAdapter.java §Examine the source text §Modify the program on your choice

58 Demo program  Open file ProgDemoTimerRectangle.java l Examine the source text l Compile l Run §Output – graphics output

59 The Timer Class Some non-GUI components can fire events. The javax.swing.Timer class is a source component that fires an ActionEvent at a predefined rate. The Timer class can be used to control animations. For example, you can use it to display a moving message.

60 Demo program  Open file ProgDemoTimerRectangle.java §Examine the source text §Modify the program on your choice

61 Thank You For Your Attention!