Unit-vi

1. Unit-vi

2. Event handling: Delegation Event model • Closing a frame, mouse and keyboard events • Adapter classes, • Concepts of Applets, differences between applets and Applications • Life cycle of an applet • Types of applets, creating applets, passing parameters to applets

3. Event handling • An event is generated whenever a user clicks a mouse, presses or releases a key • Event Delegation Model is used to handle events • This process allows the program to register handlers called ‘listeners’ with objects whose events need to be captured Handlers are automatically called when an event takes place. • Event listeners are implemented as interfaces in Java • Steps to be followed – • Associate the class with the appropriate listener Interface • Identify all components that generate events • Identify all events to be handled • Implement the methods of the listener and write the event handling code within the methods

4. The Delegation Event Model Event object Event handling methods Event source Event listener

5. The delegation model • Sources: • The mouse and keyboard and the GUI components (Buttons, lists, checkboxes etc.) • Events: • Objects that describe a state change in a source. • Listeners: • Objects notified when events occur.

6. Action Events on Buttons ActionEvent actionPerformed(..) Button ActionListener

7. Events All Events are objects of Event Classes. Events can be generated as a consequence of a person interacting with elements in a graphical user interface. Events may also occur that are not directly caused by interactions with a user interface For example, an event may be generated when a timer expires, a counter exceeds a value, a software or hardware failure occurs

8. Event sources Event source is a GUI component with which user interacts. Event sources may generate more than one type of event. A source must register listeners in order for the listeners to receive notifications about a specific type of event. Each type of event has its own registration method. Here is the general form: public voidaddTypeListener(TypeListenerel) Here, Type is the name of the event and el is a reference to the event listener. For example,the method that registers a keyboard event listener is called addKeyListener( ). The method that registers a mouse motion listener is called addMouseMotionListener( ).

9. Event Classes The classes that represent events . All event classes are defined in java.awt.event package. • Event Listeners A listener is a class that is notified when an event occurs. It has two major requirements: • It must have been registered with one or more sources to receive notifications about specific types of events • It must implement methods to receive and process these notifications. The methods that receive and process events are defined in a set of interfaces found in java.awt.event. The listener class must implement listener interface to handle events. • .

10. Sources-events-listeners

11. Sources-events-listeners

12. The ActionListener Interface This interface defines the actionPerformed( ) method that is invoked when an action event occurs. Its general form is shown here: void actionPerformed(ActionEvent ae) The AdjustmentListener Interface This interface defines the adjustmentValueChanged( ) method that is invoked when an adjustment event occurs. Its general form is shown here: void adjustmentValueChanged(AdjustmentEvent ae) The ComponentListener Interface This interface defines four methods that are invoked when a component is resized,moved, shown, or hidden. Their general forms are shown here: void componentResized(ComponentEvent ce) void componentMoved(ComponentEvent ce) void componentShown(ComponentEvent ce) void componentHidden(ComponentEvent ce)

13. The ContainerListener Interface This interface contains two methods. When a component is added to a container, componentAdded( ) is invoked. When a component is removed from a container,componentRemoved( ) is invoked. Their general forms are shown here: void componentAdded(ContainerEvent ce) void componentRemoved(ContainerEvent ce) The FocusListener Interface This interface defines two methods. When a component obtains keyboard focus,focusGained( ) is invoked. When a component loses keyboard focus.focusLost( )is called. Their general forms are shown here: void focusGained(FocusEvent fe) void focusLost(FocusEvent fe)

14. The ItemListener Interface void itemStateChanged(ItemEvent ie) The KeyListener Interface void keyPressed(KeyEvent ke) void keyReleased(KeyEvent ke) void keyTyped(KeyEvent ke) The MouseListener Interface void mouseClicked(MouseEvent me) void mouseEntered(MouseEvent me) void mouseExited(MouseEvent me) void mousePressed(MouseEvent me) void mouseReleased(MouseEvent me) The MouseMotionListener Interface void mouseDragged(MouseEvent me) void mouseMoved(MouseEvent me)

15. Event handling steps • To handle an event you need 3 steps: • Implement the appropriate interface that will produce your listener class. • Create a listener object. • Register the listener to the source of interest.

16. ActionEvent ActionListener public interface ActionListener { void actionPerformed(ActionEventme) } class MyActionListener implements ActionListener { public void actionPerformed(ActionEvent ae) { // Handler_ code } } Registration method: C_ref.addActionListener( Listener);

17. Example class MyActionListener implements ActionListener { public void actionPerformed(ActionEvent ae ) { System.out.println(“button clicked”); } } import java.awt.*; import java.awt.event.*; public class TestButtonAction extends Frame{ TestButtonAction() { Button b=new Button(“ok”); add(b); b.addActionListener(new MyActionListener( ) ); setSize(200,200); setVisible(true); } public static void main(String[] args){ new TestButtonAction(); } }

18. MouseEvent MouseListener public interface MousListener{ void mouseClicked(MouseEvent me) void mouseEntered(MouseEvent me) void mouseExited(MouseEvent me) void mousePressed(MouseEvent me) void mouseReleased(MouseEvent me) } class MyMouseListener implements MouseListener { public void mouseClicked(MouseEvent me) { // Handler_ code } public void mouseEntered(MouseEvent me) { // Handler_ code } public void mouseExited(MouseEvent me) { // Handler_ code } public void mousePressed(MouseEvent me) { // Handler_ code } public void mouseReleased(MouseEvent me) { // Handler_ code } } Registration methods: C_ref.addMouseListener( Listener);

19. MouseEvent MouseMotionListener public interface MousMotionListener{ void mouseDragged(MouseEvent me) void mouseMoved(MouseEvent me) } class MyMouseMotionListener implements MouseMotionListener { public void mouseDragged(MouseEvent me) { // Handler_ code } public void mouseMoved(MouseEvent me) { // Handler_ code } } Registration method: C_ref.addMouseMotion Listener( Listener);

20. // Demonstrate the mouse event handlers using Frame window. • import java.awt.*; • import java.awt.event.*; • public class MouseEvents extends Frame { • public MouseEvents(){ • addMouseListener(new ExMouse()); • addMouseMotionListener(new ExMouse()); • setSize(200,200); • setVisible(true); • } • public static void main(String args[]){ • new MouseEvents(); • } • }

21. class ExMouse implements MouseListener, MouseMotionListener{ • // Handle mouse clicked. • public void mouseClicked(MouseEvent me) { • System.out.println( "Mouse clicked."); • } • // Handle mouse entered. • public void mouseEntered(MouseEvent me) { • System.out.println( "Mouse entered."); • } • // Handle mouse exited. • public void mouseExited(MouseEvent me) { • System.out.println("Mouse exited."); • } • // Handle button pressed. • public void mousePressed(MouseEvent me) { • System.out.println( "Down"); • }

22. // Handle button released. • public void mouseReleased(MouseEvent me) { • System.out.println( "Up"); • } • // Handle mouse dragged. • public void mouseDragged(MouseEvent me) { • System.out.println("Dragging mouse "); • } • // Handle mouse moved. • public void mouseMoved(MouseEvent me) { • // show status • System.out.println("Moving mouse "); • } • }

23. KeyEvent KeyListener public interface KeyListener{ void keyPressed(KeyEvent ke) void keyReleased(KeyEvent ke) void keyTyped(KeyEvent ke) } class MyKeyListener implements KeyListener { public void keyPressed(KeyEvent me) { // Handler_ code } public void keyReleased(KeyEvent me) { // Handler_ code } public void keyTyped(KeyEvent me) { // Handler_ code } } Registration method: C_ref.addKeyListener( Listener);

24. // Demonstrate the key event handlers. • import java.awt.*; • import java.awt.event.*; • public class SimpleKey extends Frame { • public SimpleKey() { • addKeyListener(new ExKey()); • requestFocus(); // request input focus • setVisible(true); • setSize(300,200); • } • public static void main(String args[]){ • new SimpleKey(); • } } • class ExKey implements KeyListener{ • public void keyPressed(KeyEvent ke) { • System.out.println("Key Down"); • } • public void keyReleased(KeyEvent ke) { • System.out.println("Key Up"); • } • public void keyTyped(KeyEvent ke) { • msg += ke.getKeyChar(); • System.out.println(msg); • } }

25. Handling Mouse Events using Applets • // Demonstrate the mouse event handlers. • import java.awt.*; • import java.awt.event.*; • import java.applet.*; • /* <applet code="MouseEvents" width=300 height=100> • </applet>*/ • public class MouseEvents extends Applet implements MouseListener, MouseMotionListener { • String msg = ""; • int mouseX = 0, mouseY = 0; // coordinates of mouse • public void init() { • addMouseListener(this); • addMouseMotionListener(this); • }

26. // Handle mouse clicked. • public void mouseClicked(MouseEvent me) { • // save coordinates • mouseX = 0; • mouseY = 10; • msg = "Mouse clicked."; • repaint(); • } • // Handle mouse entered. • public void mouseEntered(MouseEvent me) { • // save coordinates • mouseX = 0; • mouseY = 10; • msg = "Mouse entered."; • repaint(); • } • // Handle mouse exited. • public void mouseExited(MouseEvent me) { • // save coordinates • mouseX = 0; • mouseY = 10; • msg = "Mouse exited."; • repaint(); • }

27. // Handle button pressed. • public void mousePressed(MouseEvent me) { • // save coordinates • mouseX = me.getX(); • mouseY = me.getY(); • msg = "Down"; • repaint(); • } • // Handle button released. • public void mouseReleased(MouseEvent me) { • // save coordinates • mouseX = me.getX(); • mouseY = me.getY(); • msg = "Up"; • repaint(); • } • // Handle mouse dragged. • public void mouseDragged(MouseEvent me) { • // save coordinates • mouseX = me.getX(); • mouseY = me.getY(); • msg = "*"; • showStatus("Dragging mouse at " + mouseX + ", " + mouseY); • repaint(); • }

28. // Handle mouse moved. • public void mouseMoved(MouseEvent me) { • // show status • showStatus("Moving mouse at " + me.getX() + ", " + me.getY()); • } • // Display msg in applet window at current X,Y location. • public void paint(Graphics g) { • g.drawString(msg, mouseX, mouseY); • } • }

29. Adapter classes • Adapter classes are fully abstract classes that correspond to listener interfaces. • They are extended (not implemented) and thus you can ignore methods that do not interest you. • You can use them instead of listeners in the case that only some of the methods of the interface are to be used. • You don’t use the abstract specifier and of course you cannot use multiple inheritence. • Some adapter classes: • KeyAdapter (instead of KeyListener) • MouseAdapter (MouseListener) • MouseMotionAdapter (MouseMotionListener) etc…

30. Cont.. Adapter Class Listener Interface MouseAdapter MouseListener KeyAdapter KeyListener MouseMotionAdapter MouseMotionListener WindowAdapter WindowListener ContainerAdapter ContainerListener FocusAdapter FocusListener

31. import java.awt.*; • import java.awt.event.*; • public class MouseEvents extends Frame { • public MouseEvents(){ • addMouseListener(new ExMouse()); • addMouseMotionListener(new ExMouse2()); • setSize(200,200); • setVisible(true); • } • public static void main(String args[]){ • new MouseEvents(); • } • } • class ExMouse extends MouseAdapter{ • // Handle mouse clicked. • public void mouseClicked(MouseEvent me) { • System.out.println( "Mouse clicked."); • } • }

32. class ExMouse2 extends MouseMotionAdapter{ • // Handle mouse dragged. • public void mouseDragged(MouseEvent me) { • System.out.println("Dragging mouse at " ); • } • }

33. Concepts of Applets, differences between applets and Applications Life cycle of an applet Types of applets, creating applets, passing parameters to applets

34. What is an applet? Applet: A small Java program that can be inserted into a web page and run by loading that page in a browser. An applet is a special kind of Java program that is designed to be transmitted over the Internet and automatically executed by a Java- compatible web browser. Applets are small applications that are accessed on an Internet server, transported over the Internet, automatically installed, and run as part of a web document.

36. Applet classes in Java java.lang.Object java.awt.Component java.awt.Container java.awt.Panel java.applet.Applet javax.swing.JApplet

37. How Applets Differ from Applications Although both the Applets and stand-alone applications are Java programs, there are certain restrictions are imposed on Applets due to security concerns: Applets don’t use the main() method, but when they are loaded, automatically call certain methods (init, start, paint, stop, destroy). They are embedded inside a web page and executed in browsers. Takes input through Graphical User Input ( GUI ). They cannot read from or write to the files on local computer. They cannot run any programs from the local computer. The above restrictions ensures that an Applet cannot do any damage to the local system.

38. Applets can be embedded in HTML pages and downloaded over the Internet whereas Applications have no special support in HTML for embedding or downloading. Applets can only be executed inside a java compatible web browser or appletviewer whereas Applications are executed at command line by java. After an applet arrives on the client, it has limited access to resources on local computer. Applications have no restriction to access resources. Applets don’t have the main() method as in applications. Instead they operate on an entirely different mechanism where they are initialized by init(),started by start(),stopped by stop() or destroyed by destroy(). A Java Applet is made up of at least one public class that has to be subclasses from java.applet.Applet. Whereas, A Java application is made up of a main() method declared as public static void that accepts a string array argument, along with any other classes that main() calls.

39. Life cycle of an Applet init() Born Begin stop() start() Running Idle destroy() paint() start() Dead End

40. Life cycle of an Applet It is important to understand the order in which these methods are called. When an applet is started , the following sequence of method calls takes place: 1. init( ) 2. start( ) 3. paint( ) When an applet is terminated, the following sequence of method calls takes place: 1. stop( ) 2. destroy( )

41. Applet States Initialisation  The init( ) method is the first method to be called.  This is where you should initialize variables. This method is called only once during the run time of your applet. Running – more than once The start( ) method is called after init( ). It is also called to restart an applet after it has been stopped. Itis called each time an applet’s HTML document is displayed on screen. So, if a user leaves a web page and comes back, the applet resumes execution at start( ). Display – more than once  paint() happens immediately after the applet enters into the running state. It is responsible for displaying output.  paint( ) is also called when the applet begins execution.  The paint( ) method is called each time your applet’s output must be redrawn.  The paint( ) method has one parameter of type Graphics.

42. Idle  The stop( ) method is called when a web browser leaves the HTML document containing the applet—when it goes to another page. Dead/Destroyed State – only once The destroy( ) method is called when the environment determines that your applet needs to be removed completely from memory.  At this point, you should free up any resources the applet may be using. The stop( ) method is always called before destroy( ).

43. Building Applet Code: An Example import java.awt.*; import java.applet.Applet; public class SimpleApplet extends Applet { public void paint(Graphics g) { g.drawString (“A Simple Applet",20, 20); } } Begins with two import classes. java.awt.* -- required for GUI java.applet.* -- every applet you create must be a subclass of Applet, which is in java.applet package. The class should start with public, because is accessed from outside.

44. Contd.. Applets do not begin execution at main(). An applet begins its execution when the name of its class is passed to an applet viewer or to a java compatible browser. Compile the applet in the same way that we have been compiling programs. Running an applet involves a different process.

45. Running an Applet There are two ways in which you can run an applet: Executing the applet within a Java-compatible browser. Using a tool called , appletviewer. An applet viewer executes your applet in a window. This is generally the fastest and easiest way to test your applet.

46. Executing in a web browser. To execute an applet in a web browser, you need to write a short HTML file that contains a tag ( Applet ) that loads the applet. HTML file that contains a SimpleApplet <APPLET code=“SimpleApplet“ width=400 height=300> </APPLET> Save the file with .html extension (Example: Simple.html) After you create this file, open your browser and then load this file, which causes SimpleApplet to be executed. width and height specify the dimensions of the display used by the applet.

47. Executing by using appletviewer There are two ways 1. Use earlier html page, which contains applet tag, then execute by using following command. C:\>appletviewer htmlfilename.html 2. Include a comment at the beginning of your source code file that contains the applet tag, then start applet viewer with your java source code file.C:\>appletviewer SimpleApplet.java import java.awt.*; import java.applet.Applet; /* <applet code=“SimpleApplet” width=200 height=60 ></applet> */ public class SimpleApplet extends Applet { public void paint(Graphics g) { g.drawString (“A Simple Applet",20, 20); } }

48. Four of these methods init(), start(), stop(), and destroy() are defined by Applet. Another, paint() is defined by the AWT Component class. Although the above program does not do anything, it can be compiled and run.

49. Structure of an applet // An Applet AppletStructure import java.awt.*; import java.applet.*; /* <applet code="AppletStructure" width=300 height=100> </applet> */ public class AppletStructure extends Applet { // Called first. public void init() { // initialization } /* Called second, after init(). Also called whenever the applet is restarted. */ public void start() { // start or resume execution } // Called when the applet is stopped. public void stop() { // suspends execution } /* Called when applet is terminated. This is the last method executed. */ public void destroy() { // perform shutdown activities } // Called whenever an applet's output must be redisplayed. public void paint(Graphics g) { // redisplay contents of window } }

50. Creating Applets /* A simple applet that sets the foreground and background colors and outputs a string. */ import java.awt.*; import java.applet.*; /* <applet code="Sample" width=300 height=50> </applet> */ public class Sample extends Applet{ String msg; public void init() { // set the foreground and background colors. setBackground(Color.cyan); setForeground(Color.red); msg = "Inside init( ) --"; } public void start() { // Initialize the string to be displayed. msg += " Inside start( ) --"; } public void paint(Graphics g) { // Display msg in applet window. msg += " Inside paint( )."; g.drawString(msg, 60, 40); } }