Applets and Event Handling in Java

Applets and Event Handling Java Programming

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Applets ™Applet Basics ™The Applet Architecture ™Applet Initialization & Termination ™Applet Restrictions ™Writing a Simple Applet ™Visualizing How an Applet Works

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Applets … ™Simple Applet Display Methods ™Overriding update() ™Requesting Repainting - Threads in Applets ™Using the Status Window ™The HTML APPLET Tag ™Passing Parameters to Applets

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Applet Basics ™Applet Applets are small application that are accessed on an internet server, Transported over the Internet, Automatically installed and run as part of an web document.

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Applet Basics … Component Container Panel Applet All applets Session 3 - Exception-Handling TCS Confidential

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Applet Basics … ™Run Examples (From appletviewer (From Browser

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Applet Basics … ™Provide support for (Applet execution (start and stop) (Load and display images (load and play audio clips.

™Applets must import java.applet.* and java.awt.* ™Execution by Web browser or an applet viewer provided by JDK Session 3 - Exception-Handling TCS Confidential

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The Applet Architecture Execution of Normal Class Begin with main()

Applet

main()

All

Only Few

O/P

SOP

AWT Methods

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init()

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The Applet Architecture ... ™This is a crucial point. (An applet should not maintain control for an extended period. (An Additional thread must be started for repetitive task.

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The Applet Architecture ... ™Second (The user initiates interaction with an applet– not the other way round. (These interactions are sent to the applet as events to which the applet must respond.

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The Applet Architecture ... ™Control applet’s execution by overriding set of methods. (init (start (stop (destroy

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Applet Initialization and Termination ™Begins order (init() (start() (paint()

™Termination order (stop() (destroy() Session 3 - Exception-Handling TCS Confidential

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Applet Initialization and Termination … ™init(): (First method (Variables initialization (Called only once

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Applet Initialization and Termination … ™start(): (It is called after init(); (Called to restart an applet (User comes back to a web page

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Applet Initialization and Termination … ™paint(): (It is called each time (Restoring the applet window (Window overwritten (When applet begins execution; (paint() has one parameter of type Graphics.

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Applet Initialization and Termination … ™stop(): (When a web browser leaves the HTML document containing the applet and goes to another page

™destroy(): (When the environment determines that your applet needs to be removed completely from memory. Session 3 - Exception-Handling TCS Confidential

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Applet Restrictions ™Which are not enforced at compile time. ™Result in run-time exception.

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Applet Restrictions …

™Applets may not (Read, write or delete files (Create or list directories (Check for the existence of a file/properties

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Applet Restrictions ... (Applets may not invoke a local program (Create a network connection

™Applets can read the file system on the server it has been downloaded from.

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Applet Restrictions ... ™These restrictions can be overwhelming. ™Be lifted for applets from a trusted source. ™Signing of an applet (Information is encoded that proves that the applet can be trusted. Session 3 - Exception-Handling TCS Confidential

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Demo of SampleApplet.java // Writing a Simple Applet public class SampleApplet extends Applet { String msg; // Initial settings. public void init() {msg = "In init() -- "; setBackground(Color.cyan); setForeground(Color.red); } public void start(){msg += "In start() -- "; } public void stop() {msg = ""; } // Display msg public void paint(Graphics g) { msg += "In paint()."; g.drawString(msg, 50, 150); } Session 3 - Exception-Handling }TCS Confidential Java

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Compiling/Running an Applet ™The applet subclass must be declared as public, because it will be accessed by code that is outside the program. ™Compile the program in the same way used for other Java programs: javac SampleApplet.java ™There are three ways to run the applet. Session 3 - Exception-Handling TCS Confidential

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Compiling/Running an Applet 1.Executing within a Javacompatible Web browser: Create file apv.html

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Compiling/Running an Applet ... 2.Above html file can also be executed from command line as follows at the DOS prompt: D:\suryoday> appletviewer apv.html

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Compiling/Running an Applet ... 3. Include the contents of the html file at the head (after includes) of your Java source code file.

D:\suryoday> appletviewer MovingBanner.java Applet Viewer: SampleApplet.class Applet In init() -- In start() -- In paint() Applet started. Session 3 - Exception-Handling TCS Confidential

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Visualizing How an Applet Works Applet Code

Hello.java

byte byte code code .class file .class file

Javac compiler HTML envelope

Hello.html

Java Javaenabled enabled Browser Browser OUTPUT OUTPUT: Hello,World! Session 3 - Exception-Handling TCS Confidential

Hello.class

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Java JavaRun-time Run-time Environment Environment Classes.zip Classes.zip 26

Applet Display Methods ™void drawString(String msg, int x, int y) ™Member of Graphics class. ™update() or paint().

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Applet Display Methods … ™void setBackground(Color newColor) void setForeground(Color newColor) ™The colors can be changed during execution.

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Applet Display Methods … ™Current setting of background/foreground color can be obtained by getBackground()/getForeground().

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Overriding update() ™Change default color in the Update methods instead of Paint method. public void update(Graphics g) { // Redisplay the window, here. } public void paint(Graphics g) { update(g); }

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Requesting Repainting ™One of architectural constraints imposed on applet is that ™it must quickly return control to the AWT run-time system. ™If the applet is displaying a moving banner, how can it itself update the window for its changing contents? Session 3 - Exception-Handling TCS Confidential

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Requesting Repainting ™It cannot create a loop inside paint() ™Well, it can simply call repaint() whenever it needs to update the information to be displayed. ™In turn, update() is called which then calls paint() by default. ™Since the scrolling of message is a repetitive task, it should be performed by a separate thread created by applet when it is initialized. Session 3 - Exception-Handling TCS Confidential

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Demo of MovingBanner.java

™Threads in applets: This applet scrolls a message, from right to left, across the applet’s window with the help of a thread. import java.awt.*; import java.applet.*; public class MovingBanner extends Applet implements Runnable { String msg = “HELLO WORLD”; Thread t = null; boolean stopFlag; public void init() { // set colors setBackground(Color.cyan); setForeground(Color.red); } // end of init Session 3 - Exception-Handling TCS Confidential

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MovingBanner.java ... public void start( ) { // start thread t = new Thread(this); stopFlag = false; t.start(); } // end of start public void run() { char ch; for ( ; ; ) { try {repaint(); Thread.sleep(250); ch = msg.charAt(0); msg = msg.substring(1, msg.length()); msg += ch; if (stopFlag) break; } catch(InterruptedException e) { } } } Session 3 - Exception-Handling TCS Confidential

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MovingBanner.java ... public void stop() { stopFlag = true; t = null; } // end of stop public void paint(Graphics g) { g.drawString(msg,100,100); } // end of paint } // end of class MovingBanner

Sample Output:

Applet Viewer: MovingBanner.class Applet WORLD HELLO Applet started. Session 3 - Exception-Handling

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Using the Status Window ™Also output a message to status window of the browser or applet viewer. ™By calling showStatus(). ™Let’s write paint() again: public void paint(Graphics g) { g.drawString(msg,10,10); showStatus(“This is shown in status window.”); } Applet Viewer: MovingBanner.class Applet HELLO WORLD Session 3 - Exception-Handling This is shown in status36window. TCS Confidential Java Programming

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The HTML APPLET Tag ™The APPLET tag is used to start an applet. ™An applet viewer will execute each APPLET tag in a separate window ™Browsers allow many applets on a single page. ™Syntax of Applet tag Session 3 - Exception-Handling TCS Confidential

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The HTML APPLET Tag … [] [< PARAM NAME = pname VALUE = pvalue > … [HTML displayed in the absence of Java]

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Demo of ParamDemo.java // Passing Parameters to Applets import java.awt.*; import java.applet.*; /* */ public class ParamDemo extends Applet { String fontName; int fontSize; float leading; Session 3 - Exception-Handling TCS Confidential

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ParamDemo.java ... public void start( ) { String param; fontName = getParameter(“font”); if (fontName.equals("")) fontName=“Not Found”; param = getParameter(“size”); try { if (param.equals("")) fontSize = 0; else fontSize = Integer.parseInt(param); } catch(NumberFormatException e) {fontSize=-1;} param = getParameter(“leading”); try { if (param.equals("")) leading = 0; else leading=Float.parseFloat(); } catch(NumberFormatException e) {leading=-1;} } Session 3 - Exception-Handling TCS Confidential

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ParamDemo.java ... public void paint(Graphics g) { g.drawString(“Font name: “ + fontName, 40, 50); g.drawString(“Font size: “ + fontSize, 40, 150); g.drawString(“Leading: “ + leading, 40, 200); } }

Applet Viewer: ParamDemo Applet Font name: TimesNewRoman Font size: 24 Leading: 25.68 Applet started. Session 3 - Exception-Handling TCS Confidential

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Event Handling ™Event Classes ™Sources of Events ™Event Listeners and Interfaces ™Handling Mouse Events ™Handling Keyboard events

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Event Handling ™Applets are event-driven programs. The most-commonly handled events are clicking of a mouse, pressing keyboard keys and controls like push button. Events are supported by java.awt.event package. In old Java 1.0 approach, an event was propagated up the containment hierarchy until it was handled by a component, wasting valuable time. Session 3 - Exception-Handling TCS Confidential

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Event Handling … ™The modern approach to handling events is based on the delegation event model in which listeners must register with a source in order to receive an event notification. A Source generates an event and sends it to one or more listeners The listener processes the event and then returns. Session 3 - Exception-Handling TCS Confidential

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Event Handling ... listener

source

listener

™An event is an object that describes a state change in a source. It can be generated due to interaction with the elements of a graphical user interface pressing a button, keying a character, selecting an item in a list, and clicking the mouse. Other events may also occur like a timer expires, counter exceeds a value, soft/hardware failure occurs, etc. Session 3 - Exception-Handling TCS Confidential

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Event Handling ... ™Source is an object that generates an event. Sources may generate more than one type of events. ™A listener is an object that is notified when an event occurs.

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Event Classes ™The classes that represent events are the core of Java’s event handling mechanism. The superclass of all events is EventObject which is in java.util. AWTEvent, defined within java.awt, is a subclass of EventObject. It is a superclass of all AWT events that are handled by the delegation event model. Session 3 - Exception-Handling TCS Confidential

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Event Classes … ™Package java.awt.event defines several types of events. Below are enumerated the most important of the event classes of this package: ActionEvent, AdjustmentEvent, ComponentEvent, ContainerEvent, FocusEvent, InputEvent, ItemEvent, KeyEvent, MouseEvent, TextEvent, WindowEvent. Session 3 - Exception-Handling TCS Confidential

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Sources of Events Button CheckBox Option1

Option2

Text Components Session 3 - Exception-Handling TCS Confidential

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Sources of Events… ™Here are some of the user interface components that can generate events: Event Source & Description ™Button: causes action events when button is pressed. ™CheckBox: causes item events when checkbox is (de)selected. ™Choice : causes item events when choice is changed. ™List: causes action events when item is double-clicked; causes item events when item is (de)selected. Session 3 - Exception-Handling TCS Confidential

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Sources of Events … ™Menu Item: causes action events when menu item is selected; causes item events when a checkable one is (de)selected. ™Scrollbar: causes adjustment events when it is manipulated. ™Text Components: causes text events on a character entry. ™Window: causes window events when a window is activated, closed, deactivated, opened, quit, (de)iconified. Session 3 - Exception-Handling TCS Confidential

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Event Listeners and Interfaces ™Event listeners are objects that are notified when an event occurs. The methods that receive and process events are defined in a set of interfaces found in java.awt.event package:

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Event Listeners and Interfaces … ™ActionListener (actionPerformed(ActionEvent) ™AdjustmentListener

(adjustmentValueChanged(AdjustmentEv ent)

™ComponentListener

(componentResized(ComponentEvent e) (componentMoved(ComponentEvent e) (componentShown(ComponentEvent e) (componentHidden(ComponentEvent e) Session 3 - Exception-Handling

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Event Listeners and Interfaces … ™MouseListener (mouseClicked(MouseEvent e) (mousePressed(MouseEvent e) (mouseReleased(MouseEvent e) (mouseEntered(MouseEvent e) (mouseExited(MouseEvent e)

™MouseMotionListener

(mouseDragged(MouseEvent e) (mouseMoved(MouseEvent e) Session 3 - Exception-Handling

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Event Listeners and Interfaces … ™TextListener (textValueChanged(TextEvent e)

™WindowListener

(windowOpened(WindowEvent e) (windowClosing(WindowEvent e) (windowClosed(WindowEvent e) (windowIconified(WindowEvent e) (windowDeiconified(WindowEvent e) (windowActivated(WindowEvent e) (windowDeactivated(WindowEvent e) Session 3 - Exception-Handling

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Event Listeners and Interfaces … ™FocusListener (focusGained(FocusEvent e) (focusLost(FocusEvent e)

™ItemListener

(itemStateChanged(ItemEvent e)

™KeyListener

(keyTyped(KeyEvent e) (keyPressed(KeyEvent e) (keyReleased(KeyEvent e) Session 3 - Exception-Handling

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Demo of MouseEvents.java Handling Mouse Events: The MouseListener and the MouseMotionListener interfaces must be implemented to handle the mouse events. import java.awt.*; import java.awt.event.*; import java.applet.*; public class MouseEvents extends Applet implements MouseListener, MouseMotionListener { String msg = " "; int mouseX = 0, mouseY = 0; // coordinates of mouse public void init() {addMouseListener(this); Session 3 - Exception-Handling TCS Confidential Java Programming addMouseMotionListener(this); 57

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MouseEvents.java ... public void mouseClicked(MouseEvent me) { msg = “Mouse clicked.”; mouseX=0; mouseY=10; repaint(); } // Handling mouse clicked. public void mouseEntered(MouseEvent me) { msg = “Mouse entered.”; mouseX=0; mouseY=10; repaint(); } // Handling mouse entered. public boolean mouseExited(MouseEvent me) { msg = “Mouse exited.”; mouseX=0;Session mouseY=10; repaint(); 3 - Exception-Handling 58 TCS Confidential Java Programming } 58

MouseEvents.java ... public void mousePressed(MouseEvent me) { mouseX = me.getX(); mouseY = me.getY(); }

msg = ”Down"; repaint( ); // Handling mouse pressed.

public void mouseReleased(MouseEvent me) { mouseX = me.getX(); mouseY = me.getY(); }

msg = ”Up"; repaint( ); // Handling mouse released

public void mouseMoved(MouseEvent me) { showStatus(“Moving mouse at “ + me.getX() + ”, “ + me.getY()); // Handling mouse moved. } TCS Confidential

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MouseEvents.java ... public void mouseDragged(MouseEvent me) { mouseX = me.getX(); mouseY = me.getY(); msg = ”*"; showStatus(“Dragging mouse at “ + mouseX + ”, “ + mouseY); repaint( ); }

// Handling Mose dragged

public void paint(Graphics g) { g.drawString(msg, mouseX, mouseY); } // Display msg in applet window at current } // X,Y location. Session 3 - Exception-Handling TCS Confidential

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Handling Keyboard events ™On pressing a key, a KEY_PRESSED event is generated; this results in a call to keyPressed() event handler. When the key is released, a KEY_RELEASED event is generated and the keyReleased() handler is executed. If a character is generated by the keystroke, then KEY_TYPED event is sent and keyTyped() handler is invoked. Session 3 - Exception-Handling TCS Confidential

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Handling Keyboard events ™Thus, on each key-press at least 2 and often 3 events are generated. If we care about only the actual characters, the key press and release events can be ignored, but to handle special keys (arrow / function keys), the key press events must watch for them through keyPressed() handler. Next program demonstrates keyboard input. Session 3 - Exception-Handling TCS Confidential

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Demo of SimpleKey.java // Demo of Keyboard Events import java.awt.*; import java.awt.event.*; import java.applet.*; public class SimpleKey extends Applet implements KeyListener {String msg = “ “; int X = 10, Y = 20; // output coordinates public void init() { addKeyListener(this); requestFocus();} // request input focus public void keyPressed(KeyEvent ke) { showStatus(“Key Down”); } Session 3 - Exception-Handling TCS Confidential

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SimpleKey.java ... public void keyReleased(KeyEvent ke) { showStatus(“Key UP”); } public void keyTyped(KeyEvent ke) { msg += ke.getKeyChar( ); repaint( ); } public void paint(Graphics g) { g.drawString(msg, X, Y); } } Session 3 - Exception-Handling TCS Confidential

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Creating animation using thread ™Animation is an ideal use of threads ™The program given below creates an animation and plays a sound as well in the background import java.awt.*; import java.applet.AudioClip; public class ImageAnim extends java.applet.Applet implements Runnable { Image pics[]=new Image[10]; Image currimage; AudioClip c1; Thread timage; Session 3 - Exception-Handling TCS Confidential

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Creating animation using thread …

public void init() { String picsource[]={"T1.gif","T2.gif","T3.gif","T4.gif", "T5.gif","T6.gif","T7.gif","T8.gif","T9.gif", "T10.gif"};

for (int i=0;i=300) yMove*=-1; if (yPos