Event Handling Department of Computer Science PowerPoint Presentation

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Applets Event Handling Threads and more in Java Sami Khuri Mathematics & Computer Science Department

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Creating the “First” Applet Create a Java source file: First.java Compile First.java. The Java compiler creates the Java bytecode file First.class in the same directory. Create an HTML file First.html, for example, that includes First.class. Run the applet in a Java-enabled browser or a Java applet viewing program.

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java.lang.Object | +----java.awt.Component | +----java.awt.Container | +----java.awt.Panel | +----java.applet.Applet AWT: Abstract Windowing Toolkit

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Applets Methods defined in the class Applet: init(): Applet is being loaded by browser. Used for one-time initialization. start(): Browser entered page containing applet stop(): Browser leaves page containing applet destroy(): Applet is discarded by browser. paint() is used for drawing on applets. It is a method in the class Component.

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Java-enabled Browser When a Java-enabled browser encounters an tag, it: reserves a display area of the specified width and height for the applet loads the bytecodes for the specified Applet subclass creates an instance of the subclass calls the instance's init() and start() methods.

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HyperText Markup Language An HTML documents contains tags that specify formatting instructions. An HTML tag is enclosed in angle brackets: example and The width and height (in pixels) of the area in which the applet is displayed is included in the html file.

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Inheriting from above classes Some methods in Component: add(), remove() and setLayout(): controls the positions & sizes of components. Applets inherit the drawing and event handling methods from AWT Component class to produce user interfaces. Drawing: images, control of color and font. UI components: buttons, etc.. Event handling: detecting & responding to mouse dragging, button pushing, key pressing,..

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Component, Container, and Panel From AWT Container, applets get methods to hold components & use layout managers. Panels and applets can only be displayed on other graphical surfaces. A panel must be added to another container in order to be displayed. A component is added to a container by using add() from the Container class.

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Security Restrictions Restrictions imposed on applets loaded over the network. The applet cannot: Dynamically load native code libraries. Read from or write to the local file system. Make network connections to any computer except to the one from which it obtained the code. Read certain system properties. Start a printing job.

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More Applet Capabilities Applets can make network connections to the host from which they came. Applets that are loaded from the local file system (from a directory in the user's CLASSPATH) have none of the restrictions that applets loaded over the network do. Applets can load data files (show images) and play sounds.

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Displaying Images The code base, returned by the Applet getCodeBase() method, is a URL that specifies the directory from which the applet's classes were loaded. The document base, returned by the Applet getDocumentBase() method, specifies the directory of the HTML page that contains the applet.

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Example on Displaying Images Images must be in GIF or JPEG format. Example: Image file yawn.gif is in directory “images”. To create an image object “nekopicture” that contains “yawn.gif”: Image nekopicture = new Image nekopicture = getImage(getCodeBase(), "images/yawn.gif");

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Playing Sounds The AudioClip interface in the Applet class provides basic support for playing sounds. Sound format: 8 bit, 8000 Hz, one-channel, Sun ".au" files. Methods in AudioClip that need to be implemented in the applet: loop(): starts playing the clip repeatedly play() & stop() to play & stop the clip.

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Inheritance tree of applets & frames Object Component Container Window Frame Panel Applet Label Canvas Scrollbar Choice List Button Checkbox TextComponent

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Event Handling With event-driven programming, events are detected by a program and handled appropriately Events: moving the mouse clicking the button pressing a key sliding the scrollbar thumb choosing an item from a menu

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Three Steps of Event Handling Prepare to accept events import package java.awt.event Start listening for events include appropriate methods Respond to events implement appropriate abstract method

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1. Prepare to accept events Import package java.awt.event Applet manifests its desire to accept events by promising to “implement” certain methods Example: “ActionListener” for Button events “AdjustmentListener” for Scrollbar events

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2. Start listening for events To make the applet “listen” to a particular event, include the appropriate “addxxxListener”. Examples: addActionListener(this) shows that the applet is interested in listening to events generated by the pushing of a certain button.

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2. Start listening for events (cont) Example addAdjustmentListener(this) shows that the applet is interested in listening to events generated by the sliding of a certain scroll bar thumb. “this” refers to the applet itself - “me” in English

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3. Respond to events The appropriate abstract methods are implemented. Example: actionPerformed() is automatically called whenever the user clicks the button. Thus, implement actionPerformed() to respond to the button event.

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3. Respond to events (cont) Example: adjustmentValueChanged() is automatically invoked whenever the user slides the scroll bar thumb. So adjustmentValueChanged() needs to be implemented. In actionPerformed(ActionEvent evt), ActionEvent is a class in java.awt.event.

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north center south leftMsg rightMsg centerValue ranger statement pan2 pan3 pan1 StatBar

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Threads A lightweight sequential flow of control that shares an address space and resources with other threads. A thread, unlike processes (heavyweight), has a low context switching time. Threads are indispensable for sockets, image holding, and animation.

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Threads in Java Traditionally, threads are implemented at the system level, separate from the programming language. Java is a language and a runtime system and threads are integrated in both. The keyword synchronize is used to make a block of code accessible to at most one thread at a time.

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Purpose of Threads Making a User Interface more responsive. When one method can use the partial output of another without waiting for the first one to finish. Example: image-loading and image-displaying methods. Any type of application that lends itself to concurrency.

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How to create threads There are two ways of creating threads in Java: 1) Extend the “Thread” class We can instantiate the class Thread as many times as desired to achieve multi-threading. 2) Implement the “Runnable” interface Since multiple inheritance is not allowed in Java, this method is used when the program already extends another class (Ex. Applets)

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1) Extend the Thread class Create a subclass of java.lang.Thread: public class MyThread extends Thread { public void run() { \\put code here } } Instantiate MyThread: MyThread myTrd; myTrd.start(); // calls run() in MyThread

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Methods in Class Thread Three primary methods to control a thread: public native synchronized void start() prepares a thread to run public void run() actually performs the work of the thread public final void stop() to terminate the thread. The thread also dies when run() terminates.

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start() & run() in Thread start() causes the thread to begin execution and the Java Virtual Machine calls run(). Thus, we never have to call run() explicitly. The result is that two threads are running concurrently: the current thread which returns from the call to start() and the thread that executes run().

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Other Methods in Thread Other important methods in Thread include: suspend() and resume() sleep(mls) which causes the thread to temporarily stop execution for mls milliseconds yield() which causes the executing thread object to temporarily pause and allow other threads to execute getName() and getPriority()

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Class Thread Priorities The class Thread has three fields: MAX_PRIORITY MIN_PRIORITY NORM_PRIORITY: the default priority assigned to a thread A new created thread has its priority initially set equal to the priority of the creating thread.

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2) Creating a thread by using Runnable Interface Instantiate Thread and pass it “this” (the applet) as a parameter. Use the method start() to start running the instantiated thread. Place all the important code for the instantiated thread in the run() method. Set the instantiated thread to “null” in the stop() method.

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Implement Runnable interface Create a class that implements Runnable: public class MyFoo extends Applet implements Runnable; Runnable is an interface in java.lang that contains only one method: run(). Multiple inheritance is not allowed in Java, thus this method of creating threads is used when MyFoo already extends another class.

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The Life Cycle of a Thread New Thread Not Runnable Dead Runnable running start() run() terminates

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In and Out of Runnable Out of Runnable sleep() is invoked. wait() is invoked (for a specified condition to be satisfied). Thread is blocked on I/O. Back to Runnable Specified number of milliseconds elapsed. An object notifies the waiting thread that the condition is satisfied. I/O event the thread is blocked on, is completed.

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new dead runnable blocked start stop sleep Done sleeping suspend resume wait notify Block on I/O I/O complete Thread States from Core Java. Drawn by Ping Wu

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Managing Threads Launching threads and letting them compete for computer resources, in an uncontrolled fashion, may lead to very unpleasant results. A typical application involves two or more threads that share a common resource: file or block of memory, where one thread tries to modify the resource while that resource is still being used by another thread.

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Simple Model of a Bank [Horton97] A very small bank consists of: a bank: a computer that performs operations on accounts clerk1: processes credits (deposits) clerk2: processes debits (withdrawals) Each clerk can communicate directly with the bank Initially, the bank has only one customer

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theAccount theBank Credit operations Debit operations Credits Debits clerk1 clerk2 Computer operations are overlapped

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Four Classes of the Bank Model public class Account field: balance getBalance() and setBalance(balance) class Bank where credit & debit operations are performed and balance is updated. public void credit(Account theAcc, int amt) public void debit(Account theAcc, int amt)

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Clerk Class public class Clerk implements Runnable Bank theBank // The employer // Types of transactions // Details of the current transaction public void doCredit(Account theAcc, int amt) public void doDebit(Account theAcc, int amt) public void run() public boolean isBusy() //done with transactions?

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Driver Class public class BankOperation public static void main(String[] args) // initialization (balance, transactionCount) // create account, bank and clerks // create clerk1Thread and clerk2Thread and start them off // generate transactions of each type and pass them to the appropriate clerk // wait until clerks are done & output results

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Running the Example Original balance : $ 500 Total credits : $1252 Total debits : $ 852 Final balance : $ 100 Should be : $ 900 The problem: One operation is retrieving the account balance while another operation is still in the process of amending it.

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Synchronization The objective of synchronization is to make sure that when several threads need access to a shared resource, only one thread can access it at any given time. Use synchronized at the method level: declare methods to be synchronized block of code level: declare some code to be synchronized

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Synchronizing Methods [Bank_SyncM] One solution: declare the operations in class Bank as being synchronized [see Bank_SyncM] synchronized public void credit(Account theAcc, int amt) { int balance = theAcc.getBalance(); … balance = balance + amt; // update balance theAcc.setBalance(balance); // put it back in the bank } synchronized public void debit(Account theAcc, int amt) // same as above except for: balance = balance - amt

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Synchronizing Methods Only one of the synchronized methods in a class object can be executing at any time. synchronized public void method1() { // code for the method } synchronized public void method2() { // code for the method } …… // can also have none synchronized methods which will operate the usual way (not “protected”) synchronized public void method3() { // code for the method } …..

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Synchronizing Blocks of Code Specify a block of statements in the program as synchronized: synchronized(myObject) { statement; statement; ... } // synchronized with respect to myObject Any other statements in the program that are synchronized with myObject cannot execute while the above statements are executing.

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Synchronizing Code [Bank_SyncB] Other solution: declare the code in the methods to be synchronized [see Bank_SyncB] public void credit(Account theAcc, int amt) { synchronized(the Acc) { int balance = theAcc.getBalance(); … } } public void debit(Account theAcc, int amt) { synchronized(the Acc) { int balance = theAcc.getBalance(); … } }

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Handling Multiple Accounts [Bank_SyncB] Bank_SyncB handles multiple accounts Had we left the synchronization of the methods (rather than the block of code), no debit operation of any kind would have been able to be carried out while a credit operation is in progress, and vice versa. Synchronization of a block of code prevents overlapping of operations on the same account, and that is what we want.

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Thread Organization [Tan95] The possible organization of threads: dispatcher/worker model: an idle worker thread is chosen by the dispatcher thread for the incoming job team model: each thread works on its own request pipeline model: threads cooperate with each other in a sequential fashion. [see Figure]

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Flickering Flickering invariably occurs with animation. The reason behind flickering is the way Java paints and repaints each single frame of the applet. Calling repaint() results in a call to update() which in turn results in a call to paint(). Flickering is caused by update().

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update() and Flickering update() performs two functions: It fills the screen with the current background color of the applet. This is what is normally termed as “clearing” the screen. It calls paint() which then draws the contents of the current frame onto the screen. Flickering is a result of the quick alternation between the above two functions.

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Quick Alternation of Functions The quick alternation of the parts of the frame that do not change between clearing the screen and drawing the current contents of the frame, will cause flickering. In essence, what we see in quick successions are the screen background color and the current contents of the frame (being displayed several times per seconds).

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Avoiding Flickering The major two ways of avoiding (reducing) flickering in Java applets are: Overriding update() so as not to clear the screen at all or to clear only the parts of the screen that have been changed since the last alternation. Double-buffering which is achieved by overriding both: update() and paint(). It consists in drawing the current contents on a graphics surface that is not on the applet screen and then copying the surface on the applet screen.

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Overriding update() update() is a method in the Component class Recall that update() clears the screen and calls paint(). To avoid clearing the screen, we simply override update() by adding update() to the “flickering program”, in which we only have a call to paint(). Thus the clearing task of the original update() is eliminated.

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Overriding update() (cont.) Thus instead of the update() method of Component in which we have: public void update(Graphics g) { // code for clearing the screen paint(g); } we’ll simply have: public void update(Graphics g) { paint(g); }

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Double-Buffering Double buffering consists: in creating a second surface (an offscreen buffer) on which we perform all the drawing (and clearing) and then the contents of the offscreen buffer are transferred (“blitted”: bit block transferred) onto the applet’s surface. It is called double buffering because we are switching between two drawing buffers.

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Five Steps of Double-Buffering The five steps of double buffering are: 1) Create an offscreen buffer by declaring an Image object (offScreenImage) to hold the image and a Graphics object (offScreenGraphics) to hold the graphics context. 2) Create an image and a graphics context for the offscreen buffer (usually done by overriding init()).

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Five Steps of Double-Buffering 3) Perform all the painting on the offscreen buffer instead of the applet’s surface (i.e., instead of the main graphics buffer). It is usually done by overriding paint(). 4) Copy the contents of the offscreen buffer to the applet’s surface.

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Five Steps of Double-Buffering 5) Use dispose() to clean up the graphics context that was created for the offscreen buffer. It is usually done by overriding destroy(): public void destroy() { offScreenGraphics.dispose(); }

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Exceptions An exception is an event that occurs during the execution of a program that prevents the continuation of the normal flow of instructions. The purpose of exception handling is to make it possible for the program to either attempt to recover from the problem, or at worst shut down the program in a graceful manner, whenever an exception occurs.

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Exceptions: An Example The program contains code which attempts to divide by zero or tries to access an array element by using an index that is too small or too large. These are runtime exceptions. Common terminology states that when this happens, the system throws an exception. If an exception is not caught, a runtime error may occur.

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Advantages of Exception Handling Separating error handling code from “regular” code. Logical flow of program gets lost due to the cluttering of the modified program. Propagating errors up the call stack. Propagate notification up to calling method. Grouping error types & error differentiation.

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Who throws exceptions? Java provides the capability to detect and handle exceptions. The exception can be thrown either by the system or by code created by the programmer. There is a long list of exceptions, called “checked” exceptions, which will be thrown automatically by the Java system.

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The Class Throwable When an exceptional condition causes an exception to be thrown, that exception is an object derived, either directly, or indirectly from the class Throwable. The interpreter and many different methods in many different classes throw exceptions and errors. Error and Exception are subclasses of Throwable.

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public  class  java.lang.Throwable extends  java.lang.Object { // Constructors     public Throwable();      public Throwable(String  message);       // Methods     public Throwable fillInStackTrace();             public String getLocalizedMessage(); public String getMessage();      public void printStackTrace();           public void printStackTrace(PrintStream  s);  public void printStackTrace(PrintWriter s);     public String toString();    }

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Error and Exception Classes Throwable Exception Error Exceptions we should not catch Exceptions we can catch RunTimeException ... ... Results from catastrophic events or conditions (JVM runs out of memory; or class definition not found at load time) Compiler checks to see if the code handles these exceptions (except for RunTimeException) Object

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Subclasses of Throwable An Error indicates that a non-recoverable error has occurred that should not be caught. They usually cause the Java interpreter to display a message and exit.  An Exception indicates an abnormal condition that must be properly handled to prevent program termination.

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Handling Exceptions Exception handling in Java is done through try / catch / finally statements. The catch and finally clauses are responsible for exception handling and for the clean-up operations. 1) Design code such that you try to execute a block of code. Enclose the statements that might cause an exception in a try block.

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Handling Exceptions (cont) 2) Provide code (after the try block) to catch & process the exception object. Each catch block handles the type of exception indicated by its argument - the name of a class that inherits from the Throwable class. The first catch clause that has an argument of the appropriate type in invoked. The block of code then handles the exception.

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Handling Exceptions (cont) 3) Use the finally block to clean up and release system resources (e.g. closing files). Java runtime system executes the finally block regardless of what happens within the try block. Remark: A try statement must have at least one catch block or one finally block.