Java

Java public class TestThread { public static void main(String args[]) { RunnableDemo R1 = new RunnableDemo( \"Thread-1\"); R1.start(); RunnableDemo R2 = new RunnableDemo( \"Thread-2\"); R2.start(); } } This will produce the following result: Creating Thread-1 Starting Thread-1 Creating Thread-2 Starting Thread-2 Running Thread-1 Thread: Thread-1, 4 Running Thread-2 Thread: Thread-2, 4 Thread: Thread-1, 3 Thread: Thread-2, 3 Thread: Thread-1, 2 Thread: Thread-2, 2 Thread: Thread-1, 1 Thread: Thread-2, 1 Thread Thread-1 exiting. Thread Thread-2 exiting. Create a Thread by Extending a Thread Class The second way to create a thread is to create a new class that extends Thread class using the following two simple steps. This approach provides more flexibility in handling multiple threads created using available methods in Thread class. 490

Java Step 1 You will need to override run( ) method available in Thread class. This method provides an entry point for the thread and you will put your complete business logic inside this method. Following is a simple syntax of run() method: public void run( ) Step 2 Once Thread object is created, you can start it by calling start( ) method, which executes a call to run( ) method. Following is a simple syntax of start() method: void start( ); Example Here is the preceding program rewritten to extend the Thread: class ThreadDemo extends Thread { private Thread t; private String threadName; ThreadDemo( String name){ threadName = name; System.out.println(\"Creating \" + threadName ); } public void run() { System.out.println(\"Running \" + threadName ); try { for(int i = 4; i > 0; i--) { System.out.println(\"Thread: \" + threadName + \", \" + i); // Let the thread sleep for a while. Thread.sleep(50); } } catch (InterruptedException e) { System.out.println(\"Thread \" + threadName + \" interrupted.\"); } System.out.println(\"Thread \" + threadName + \" exiting.\"); } 491

Java public void start () { System.out.println(\"Starting \" + threadName ); if (t == null) { t = new Thread (this, threadName); t.start (); } } } public class TestThread { public static void main(String args[]) { ThreadDemo T1 = new ThreadDemo( \"Thread-1\"); T1.start(); ThreadDemo T2 = new ThreadDemo( \"Thread-2\"); T2.start(); } } This will produce the following result: Creating Thread-1 Starting Thread-1 Creating Thread-2 Starting Thread-2 Running Thread-1 Thread: Thread-1, 4 Running Thread-2 Thread: Thread-2, 4 Thread: Thread-1, 3 Thread: Thread-2, 3 Thread: Thread-1, 2 492

Java Thread: Thread-2, 2 Thread: Thread-1, 1 Thread: Thread-2, 1 Thread Thread-1 exiting. Thread Thread-2 exiting. Thread Methods Following is the list of important methods available in the Thread class. Sr. No. Methods with Description public void start() 1 Starts the thread in a separate path of execution, then invokes the run() method on this Thread object. public void run() 2 If this Thread object was instantiated using a separate Runnable target, the run() method is invoked on that Runnable object. public final void setName(String name) 3 Changes the name of the Thread object. There is also a getName() method for retrieving the name. public final void setPriority(int priority) 4 Sets the priority of this Thread object. The possible values are between 1 and 10. public final void setDaemon(boolean on) 5 A parameter of true denotes this Thread as a daemon thread. public final void join(long millisec) The current thread invokes this method on a second thread, causing the current 6 thread to block until the second thread terminates or the specified number of milliseconds passes. 493

Java public void interrupt() 7 Interrupts this thread, causing it to continue execution if it was blocked for any reason. public final boolean isAlive() 8 Returns true if the thread is alive, which is any time after the thread has been started but before it runs to completion. The previous methods are invoked on a particular Thread object. The following methods in the Thread class are static. Invoking one of the static methods performs the operation on the currently running thread. Sr. No. Methods with Description public static void yield() 1 Causes the currently running thread to yield to any other threads of the same priority that are waiting to be scheduled. public static void sleep(long millisec) 2 Causes the currently running thread to block for at least the specified number of milliseconds. public static boolean holdsLock(Object x) 3 Returns true if the current thread holds the lock on the given Object. public static Thread currentThread() 4 Returns a reference to the currently running thread, which is the thread that invokes this method. public static void dumpStack() 5 Prints the stack trace for the currently running thread, which is useful when debugging a multithreaded application. 494

Java Example The following ThreadClassDemo program demonstrates some of these methods of the Thread class. Consider a class DisplayMessage which implements Runnable: // File Name : DisplayMessage.java // Create a thread to implement Runnable public class DisplayMessage implements Runnable { private String message; public DisplayMessage(String message) { this.message = message; } public void run() { while(true) { System.out.println(message); } } } Following is another class which extends the Thread class: // File Name : GuessANumber.java // Create a thread to extentd Thread public class GuessANumber extends Thread { private int number; public GuessANumber(int number) { this.number = number; } 495

Java public void run() { int counter = 0; int guess = 0; do { guess = (int) (Math.random() * 100 + 1); System.out.println(this.getName() + \" guesses \" + guess); counter++; }while(guess != number); System.out.println(\"** Correct! \" + this.getName() + \" in \" + counter + \" guesses.**\"); } } Following is the main program, which makes use of the above-defined classes: // File Name : ThreadClassDemo.java public class ThreadClassDemo { public static void main(String [] args) { Runnable hello = new DisplayMessage(\"Hello\"); Thread thread1 = new Thread(hello); thread1.setDaemon(true); thread1.setName(\"hello\"); System.out.println(\"Starting hello thread...\"); thread1.start(); Runnable bye = new DisplayMessage(\"Goodbye\"); Thread thread2 = new Thread(bye); thread2.setPriority(Thread.MIN_PRIORITY); thread2.setDaemon(true); System.out.println(\"Starting goodbye thread...\"); thread2.start(); 496

Java System.out.println(\"Starting thread3...\"); Thread thread3 = new GuessANumber(27); thread3.start(); try { thread3.join(); }catch(InterruptedException e) { System.out.println(\"Thread interrupted.\"); } System.out.println(\"Starting thread4...\"); Thread thread4 = new GuessANumber(75); thread4.start(); System.out.println(\"main() is ending...\"); } } This will produce the following result. You can try this example again and again and you will get a different result every time. Starting hello thread... Starting goodbye thread... Hello Hello Hello Hello Hello Hello Goodbye Goodbye Goodbye Goodbye Goodbye ....... 497

Java Major Java Multithreading Concepts While doing Multithreading programming in Java, you would need to have the following concepts very handy:  What is thread synchronization?  Handling interthread communication  Handling thread deadlock  Major thread operations Thread Synchronization When we start two or more threads within a program, there may be a situation when multiple threads try to access the same resource and finally they can produce unforeseen result due to concurrency issues. For example, if multiple threads try to write within a same file then they may corrupt the data because one of the threads can override data or while one thread is opening the same file at the same time another thread might be closing the same file. So there is a need to synchronize the action of multiple threads and make sure that only one thread can access the resource at a given point in time. This is implemented using a concept called monitors. Each object in Java is associated with a monitor, which a thread can lock or unlock. Only one thread at a time may hold a lock on a monitor. Java programming language provides a very handy way of creating threads and synchronizing their task by using synchronized blocks. You keep shared resources within this block. Following is the general form of the synchronized statement: synchronized(objectidentifier) { // Access shared variables and other shared resources } Here, the objectidentifier is a reference to an object whose lock associates with the monitor that the synchronized statement represents. Now we are going to see two examples, where we will print a counter using two different threads. When threads are not synchronized, they print counter value which is not in sequence, but when we print counter by putting inside synchronized() block, then it prints counter very much in sequence for both the threads. 498

Java Multithreading Example without Synchronization Here is a simple example which may or may not print counter value in sequence and every time we run it, it produces a different result based on CPU availability to a thread. class PrintDemo { public void printCount(){ try { for(int i = 5; i > 0; i--) { System.out.println(\"Counter --- \" + i ); } } catch (Exception e) { System.out.println(\"Thread interrupted.\"); } } } class ThreadDemo extends Thread { private Thread t; private String threadName; PrintDemo PD; ThreadDemo( String name, PrintDemo pd){ threadName = name; PD = pd; } public void run() { PD.printCount(); System.out.println(\"Thread \" + threadName + \" exiting.\"); } public void start () { System.out.println(\"Starting \" + threadName ); if (t == null) { t = new Thread (this, threadName); t.start (); } } 499

Java } public class TestThread { public static void main(String args[]) { PrintDemo PD = new PrintDemo(); ThreadDemo T1 = new ThreadDemo( \"Thread - 1 \", PD ); ThreadDemo T2 = new ThreadDemo( \"Thread - 2 \", PD ); T1.start(); T2.start(); // wait for threads to end try { T1.join(); T2.join(); } catch( Exception e) { System.out.println(\"Interrupted\"); } } } This produces a different result every time you run this program: Starting Thread - 1 Starting Thread - 2 Counter --- 5 Counter --- 4 Counter --- 3 Counter --- 5 Counter --- 2 Counter --- 1 Counter --- 4 Thread Thread - 1 exiting. 500

Java Counter --- 3 Counter --- 2 Counter --- 1 Thread Thread - 2 exiting. Multithreading Example with Synchronization Here is the same example which prints counter value in sequence and every time we run it, it produces the same result. class PrintDemo { public void printCount(){ try { for(int i = 5; i > 0; i--) { System.out.println(\"Counter --- \" + i ); } } catch (Exception e) { System.out.println(\"Thread interrupted.\"); } } } class ThreadDemo extends Thread { private Thread t; private String threadName; PrintDemo PD; ThreadDemo( String name, PrintDemo pd){ threadName = name; PD = pd; } public void run() { synchronized(PD) { PD.printCount(); } 501

Java System.out.println(\"Thread \" + threadName + \" exiting.\"); } public void start () { System.out.println(\"Starting \" + threadName ); if (t == null) { t = new Thread (this, threadName); t.start (); } } } public class TestThread { public static void main(String args[]) { PrintDemo PD = new PrintDemo(); ThreadDemo T1 = new ThreadDemo( \"Thread - 1 \", PD ); ThreadDemo T2 = new ThreadDemo( \"Thread - 2 \", PD ); T1.start(); T2.start(); // wait for threads to end try { T1.join(); T2.join(); } catch( Exception e) { System.out.println(\"Interrupted\"); } } } 502

Java This produces the same result every time you run this program: Starting Thread - 1 Starting Thread - 2 Counter --- 5 Counter --- 4 Counter --- 3 Counter --- 2 Counter --- 1 Thread Thread - 1 exiting. Counter --- 5 Counter --- 4 Counter --- 3 Counter --- 2 Counter --- 1 Thread Thread - 2 exiting. Interthread Communication If you are aware of interprocess communication then it will be easy for you to understand interthread communication. Interthread communication is important when you develop an application where two or more threads exchange some information. There are three simple methods and a little trick which makes thread communication possible. All the three methods are listed below: Sr. No. Methods with Description 1 public void wait() Causes the current thread to wait until another thread invokes the notify(). public void notify() 2 Wakes up a single thread that is waiting on this object's monitor. public void notifyAll() 3 Wakes up all the threads that called wait( ) on the same object. These methods have been implemented as final methods in Object, so they are available in all the classes. All three methods can be called only from within a synchronized context. 503

Java Example This examples shows how two threads can communicate using wait() and notify() method. You can create a complex system using the same concept. class Chat { boolean flag = false; public synchronized void Question(String msg) { if (flag) { try { wait(); } catch (InterruptedException e) { e.printStackTrace(); } } System.out.println(msg); flag = true; notify(); } public synchronized void Answer(String msg) { if (!flag) { try { wait(); } catch (InterruptedException e) { e.printStackTrace(); } } System.out.println(msg); flag = false; notify(); } } class T1 implements Runnable { Chat m; String[] s1 = { \"Hi\", \"How are you ?\", \"I am also doing fine!\" }; 504

Java public T1(Chat m1) { this.m = m1; new Thread(this, \"Question\").start(); } public void run() { for (int i = 0; i < s1.length; i++) { m.Question(s1[i]); } } } class T2 implements Runnable { Chat m; String[] s2 = { \"Hi\", \"I am good, what about you?\", \"Great!\" }; public T2(Chat m2) { this.m = m2; new Thread(this, \"Answer\").start(); } public void run() { for (int i = 0; i < s2.length; i++) { m.Answer(s2[i]); } } } public class TestThread { public static void main(String[] args) { Chat m = new Chat(); new T1(m); new T2(m); } } 505

Java When the above program is complied and executed, it produces the following result: Hi Hi How are you ? I am good, what about you? I am also doing fine! Great! Above example has been taken and then modified from [http://stackoverflow.com/questions/2170520/inter-thread-communication-in-java] Thread Deadlock Deadlock describes a situation where two or more threads are blocked forever, waiting for each other. Deadlock occurs when multiple threads need the same locks but obtain them in different order. A Java multithreaded program may suffer from the deadlock condition because the synchronized keyword causes the executing thread to block while waiting for the lock, or monitor, associated with the specified object. Here is an example. Example public class TestThread { public static Object Lock1 = new Object(); public static Object Lock2 = new Object(); public static void main(String args[]) { ThreadDemo1 T1 = new ThreadDemo1(); ThreadDemo2 T2 = new ThreadDemo2(); T1.start(); T2.start(); } private static class ThreadDemo1 extends Thread { public void run() { synchronized (Lock1) { System.out.println(\"Thread 1: Holding lock 1...\"); try { Thread.sleep(10); } catch (InterruptedException e) {} System.out.println(\"Thread 1: Waiting for lock 2...\"); 506

Java synchronized (Lock2) { System.out.println(\"Thread 1: Holding lock 1 & 2...\"); } } } } private static class ThreadDemo2 extends Thread { public void run() { synchronized (Lock2) { System.out.println(\"Thread 2: Holding lock 2...\"); try { Thread.sleep(10); } catch (InterruptedException e) {} System.out.println(\"Thread 2: Waiting for lock 1...\"); synchronized (Lock1) { System.out.println(\"Thread 2: Holding lock 1 & 2...\"); } } } } } When you compile and execute the above program, you find a deadlock situation and following is the output produced by the program: Thread 1: Holding lock 1... Thread 2: Holding lock 2... Thread 1: Waiting for lock 2... Thread 2: Waiting for lock 1... The above program will hang forever because neither of the threads in position to proceed and waiting for each other to release the lock, so you can come out of the program by pressing CTRL+C. 507

Java Deadlock Solution Example Let's change the order of the lock and run of the same program to see if both the threads still wait for each other: public class TestThread { public static Object Lock1 = new Object(); public static Object Lock2 = new Object(); public static void main(String args[]) { ThreadDemo1 T1 = new ThreadDemo1(); ThreadDemo2 T2 = new ThreadDemo2(); T1.start(); T2.start(); } private static class ThreadDemo1 extends Thread { public void run() { synchronized (Lock1) { System.out.println(\"Thread 1: Holding lock 1...\"); try { Thread.sleep(10); } catch (InterruptedException e) {} System.out.println(\"Thread 1: Waiting for lock 2...\"); synchronized (Lock2) { System.out.println(\"Thread 1: Holding lock 1 & 2...\"); } } } } private static class ThreadDemo2 extends Thread { public void run() { synchronized (Lock1) { System.out.println(\"Thread 2: Holding lock 1...\"); try { Thread.sleep(10); } catch (InterruptedException e) {} System.out.println(\"Thread 2: Waiting for lock 2...\"); 508

Java synchronized (Lock2) { System.out.println(\"Thread 2: Holding lock 1 & 2...\"); } } } } } So just changing the order of the locks prevent the program in going into a deadlock situation and completes with the following result: Thread 1: Holding lock 1... Thread 1: Waiting for lock 2... Thread 1: Holding lock 1 & 2... Thread 2: Holding lock 1... Thread 2: Waiting for lock 2... Thread 2: Holding lock 1 & 2... The above example is to just make the concept clear, however, it is a complex concept and you should deep dive into it before you develop your applications to deal with deadlock situations. Thread Control Core Java provides complete control over multithreaded program. You can develop a multithreaded program which can be suspended, resumed, or stopped completely based on your requirements. There are various static methods which you can use on thread objects to control their behavior. Following table lists down those methods: Sr. No. Methods with Description public void suspend() 1 This method puts a thread in the suspended state and can be resumed using resume() method. public void stop() 2 This method stops a thread completely. public void resume() 3 This method resumes a thread, which was suspended using suspend() method. 509

Java public void wait() 4 Causes the current thread to wait until another thread invokes the notify(). public void notify() 5 Wakes up a single thread that is waiting on this object's monitor. Be aware that the latest versions of Java has deprecated the usage of suspend( ), resume( ), and stop( ) methods and so you need to use available alternatives. Example class RunnableDemo implements Runnable { public Thread t; private String threadName; boolean suspended = false; RunnableDemo( String name){ threadName = name; System.out.println(\"Creating \" + threadName ); } public void run() { System.out.println(\"Running \" + threadName ); try { for(int i = 10; i > 0; i--) { System.out.println(\"Thread: \" + threadName + \", \" + i); // Let the thread sleep for a while. Thread.sleep(300); synchronized(this) { while(suspended) { wait(); } } } 510

Java } catch (InterruptedException e) { System.out.println(\"Thread \" + threadName + \" interrupted.\"); } System.out.println(\"Thread \" + threadName + \" exiting.\"); } public void start () { System.out.println(\"Starting \" + threadName ); if (t == null) { t = new Thread (this, threadName); t.start (); } } void suspend() { suspended = true; } synchronized void resume() { suspended = false; notify(); } } public class TestThread { public static void main(String args[]) { RunnableDemo R1 = new RunnableDemo( \"Thread-1\"); R1.start(); RunnableDemo R2 = new RunnableDemo( \"Thread-2\"); R2.start(); try { Thread.sleep(1000); R1.suspend(); System.out.println(\"Suspending First Thread\"); Thread.sleep(1000); 511

Java R1.resume(); System.out.println(\"Resuming First Thread\"); R2.suspend(); System.out.println(\"Suspending thread Two\"); Thread.sleep(1000); R2.resume(); System.out.println(\"Resuming thread Two\"); } catch (InterruptedException e) { System.out.println(\"Main thread Interrupted\"); } try { System.out.println(\"Waiting for threads to finish.\"); R1.t.join(); R2.t.join(); } catch (InterruptedException e) { System.out.println(\"Main thread Interrupted\"); } System.out.println(\"Main thread exiting.\"); } } The above program produces the following output: Creating Thread-1 Starting Thread-1 Creating Thread-2 Starting Thread-2 Running Thread-1 Thread: Thread-1, 10 Running Thread-2 Thread: Thread-2, 10 Thread: Thread-1, 9 Thread: Thread-2, 9 Thread: Thread-1, 8 Thread: Thread-2, 8 Thread: Thread-1, 7 512

Java Thread: Thread-2, 7 Suspending First Thread Thread: Thread-2, 6 Thread: Thread-2, 5 Thread: Thread-2, 4 Resuming First Thread Suspending thread Two Thread: Thread-1, 6 Thread: Thread-1, 5 Thread: Thread-1, 4 Thread: Thread-1, 3 Resuming thread Two Thread: Thread-2, 3 Waiting for threads to finish. Thread: Thread-1, 2 Thread: Thread-2, 2 Thread: Thread-1, 1 Thread: Thread-2, 1 Thread Thread-1 exiting. Thread Thread-2 exiting. ain thread exiting. 513

35. Java – Applet Basics Java An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal. There are some important differences between an applet and a standalone Java application, including the following:  An applet is a Java class that extends the java.applet.Applet class.  A main() method is not invoked on an applet, and an applet class will not define main().  Applets are designed to be embedded within an HTML page.  When a user views an HTML page that contains an applet, the code for the applet is downloaded to the user's machine.  A JVM is required to view an applet. The JVM can be either a plug-in of the Web browser or a separate runtime environment.  The JVM on the user's machine creates an instance of the applet class and invokes various methods during the applet's lifetime.  Applets have strict security rules that are enforced by the Web browser. The security of an applet is often referred to as sandbox security, comparing the applet to a child playing in a sandbox with various rules that must be followed.  Other classes that the applet needs can be downloaded in a single Java Archive (JAR) file. Life Cycle of an Applet Four methods in the Applet class gives you the framework on which you build any serious applet:  init: This method is intended for whatever initialization is needed for your applet. It is called after the param tags inside the applet tag have been processed.  start: This method is automatically called after the browser calls the init method. It is also called whenever the user returns to the page containing the applet after having gone off to other pages.  stop: This method is automatically called when the user moves off the page on which the applet sits. It can, therefore, be called repeatedly in the same applet. 514

Java  destroy: This method is only called when the browser shuts down normally. Because applets are meant to live on an HTML page, you should not normally leave resources behind after a user leaves the page that contains the applet.  paint: Invoked immediately after the start() method, and also any time the applet needs to repaint itself in the browser. The paint() method is actually inherited from the java.awt. A \"Hello, World\" Applet Following is a simple applet named HelloWorldApplet.java: import java.applet.*; import java.awt.*; public class HelloWorldApplet extends Applet { public void paint (Graphics g) { g.drawString (\"Hello World\", 25, 50); } } These import statements bring the classes into the scope of our applet class:  java.applet.Applet  java.awt.Graphics Without those import statements, the Java compiler would not recognize the classes Applet and Graphics, which the applet class refers to. The Applet Cla s s Every applet is an extension of the java.applet.Applet class. The base Applet class provides methods that a derived Applet class may call to obtain information and services from the browser context. These include methods that do the following:  Get applet parameters  Get the network location of the HTML file that contains the applet  Get the network location of the applet class directory  Print a status message in the browser  Fetch an image  Fetch an audio clip  Play an audio clip 515

Java  Resize the applet Additionally, the Applet class provides an interface by which the viewer or browser obtains information about the applet and controls the applet's execution. The viewer may:  Request information about the author, version, and copyright of the applet  Request a description of the parameters the applet recognizes  Initialize the applet  Destroy the applet  Start the applet's execution  Stop the applet's execution The Applet class provides default implementations of each of these methods. Those implementations may be overridden as necessary. The \"Hello, World\" applet is complete as it stands. The only method overridden is the paint method. Invoking an Applet An applet may be invoked by embedding directives in an HTML file and viewing the file through an applet viewer or Java-enabled browser. The <applet> tag is the basis for embedding an applet in an HTML file. Following is an example that invokes the \"Hello, World\" applet: <html> <title>The Hello, World Applet</title> <hr> <applet code=\"HelloWorldApplet.class\" width=\"320\" height=\"120\"> If your browser was Java-enabled, a \"Hello, World\" message would appear here. </applet> <hr> </html> Note: You can refer to HTML Applet Tag to understand more about calling applet from HTML. HTML <applet> Tag Description: The HTML <applet> tag specifies an applet. It is used for embedding a Java applet within an HTML document. It is not supported in HTML5. 516

Java Example <!DOCTYPE html> <html> <head> <title>HTML applet Tag</title> </head> <body> <applet code=\"newClass.class\" width=\"300\" height=\"200\"> </applet> </body> </html> Here is the newClass.java file: import java.applet.*; import java.awt.*; public class newClass extends Applet { public void paint (Graphics gh) { g.drawString(\"Tutorialspoint.com\", 300, 150); } } This will produce the following result: Global Attributes This tag supports all the global attributes described in - HTML Attribute Reference 517

Java HTML Attribute Reference There are few HTML attributes which are standard and associated to all the HTML tags. These attributes are listed here with a brief description. Global Attributes: Not valid in base, head, html, meta, param, script, style, and title elements. Attribute HTML-5 Description Specifies a shortcut key for an element to be used in accesskey place of keyboard. class The class of the element. Boolean attribute to specify whether the element is contenteditable Yes editable or not. contextmenu Yes Specifies a context menu for an element. data-* Yes Used to store custom data associated with the element. Boolean attribute to specify whether the element can be draggable Yes dragged or not. Specifies whether the dragged data is copied, moved, dropzone Yes or linked, when dropped. hidden Yes Specifies whether the element should be visible or not. id A unique id for the element. Specifies if the element must have it's spelling or spellcheck Yes grammar checked. style An inline style definition. tabindex Specifies the tab order of an element. title A text to display in a tool tip. Boolean attribute specifies whether the content of an translate Yes element should be translated or not. Language Attributes The lang attribute indicates the language being used for the enclosed content. The language is identified using the ISO standard language abbreviations, such as fr for French, en for English, and so on. RFC 1766 (http://www.ietf.org/rfc/rfc1766.txt) describes these codes and their formats. 518

Java Not valid in base, br, frame, frameset, hr, iframe, param, and script elements. Attribute Value Description dir ltr | rtl Sets the text direction lang language_code Sets the language code Specific Attributes The HTML <> tag also supports following additional attributes: Attribute Value Description Deprecated - Defines the text alignment around the align URL applet. Alternate text to be displayed in case the browser alt URL does not support the applet. Applet path when it is stored in a Java Archive, i.e. archive URL jar file. code URL A URL that points to the class of the applet. Indicates the base URL of the applet if the code codebase URL attribute is relative. height pixels Height to display the applet. Deprecated - Defines the left and right spacing hspace pixels around the applet. name name Defines a unique name for the applet. Specifies the resource that contains a serialized object name representation of the applet's state. Additional information to be displayed in tool tip of title test the mouse. Deprecated - Amount of white space to be inserted vspace pixels above and below the object. width pixels Width to display the applet. 519

Java Event Attributes This tag supports all the event attributes described in - HTML Events Reference HTML Events Reference When users visit your website, they do things like click various links, hover mouse over text and images, etc. These are examples of what we call events in Javascript and VBScript terminologies. We can write our event handlers using Javascript or VBScript and can specify some actions to be taken against these events. Though these are the events, they will be specified as attributes for the HTML tags. The HTML 4.01 specification had defined 19 events but later HTML-5 has added many other events, which we have listed below: Window Events Attributes Following events have been introduced in older versions of HTML but all the tags marked with are part of HTML-5. Events HTML-5 Description onafterprint Triggers after a document is printed. onbeforeprint Triggers before a document is printed. onbeforeonload Triggers before a document loads. onerror Triggers when an error occurs. onhaschange Triggers when a document has changed. onload Triggers when a document loads. onmessage Triggers when a message is triggered. onoffline Triggers when a document goes offline. ononline Triggers when a document comes online. onpagehide Triggers when a window is hidden. onpageshow Triggers when a window becomes visible. onpopstate Triggers when a window's history changes. onredo Triggers when a document performs a redo. 520

Java onresize Triggers when a window is resized. onstorage Triggers when a document loads. onundo Triggers when a document performs an undo. onunload Triggers when a user leaves the document. Form Events Following tags have been introduced in older versions of HTML but all the tags marked with are part of HTML-5. Events HTML-5 Description onblur Triggers when a window loses focus. onchange Triggers when an element changes. oncontextmenu Triggers when a context menu is triggered. onfocus Triggers when a window gets focus. onformchange Triggers when a form changes. onforminput Triggers when a form gets user input. oninput Triggers when an element gets user input. oninvalid Triggers when an element is invalid. onreset Triggers when a form is reset. onselect Triggers when an element is selected. onsubmit Triggers when a form is submitted. Keyboard Events Events HTML-5 Description onkeydown Triggers when a key is pressed. onkeypress Triggers when a key is pressed and released. 521

Java onkeyup Triggers when a key is released. Mouse Events Following tags have been introduced in older versions of HTML but all the tags marked with are part of HTML-5. HTML- Events Description 5 onclick Triggers on a mouse click. ondblclick Triggers on a mouse double-click. ondrag Triggers when an element is dragged. ondragend Triggers at the end of a drag operation. Triggers when an element has been dragged to a valid drop ondragenter target. ondragleave Triggers when an element leaves a valid drop target. Triggers when an element is being dragged over a valid drop ondragover target. ondragstart Triggers at the start of a drag operation. ondrop Triggers when a dragged element is being dropped. onmousedown Triggers when a mouse button is pressed. onmousemove Triggers when the mouse pointer moves. onmouseout Triggers when the mouse pointer moves out of an element. onmouseover Triggers when the mouse pointer moves over an element. onmouseup Triggers when a mouse button is released. onmousewheel Triggers when the mouse wheel is being rotated. onscroll Triggers when an element's scrollbar is being scrolled. 522

Java Media Events Following tags have been introduced in older versions of HTML but all the tags marked with are part of HTML-5. Events HTML-5 Description onabort Triggers on an abort event. Triggers when a media can start play, but might have oncanplay to stop for buffering. Triggers when a media can be played to the end, oncanplaythrough without stopping for buffering. ondurationchange Triggers when the length of a media is changed. Triggers when a media resource element suddenly onemptied becomes empty. onended Triggers when a media has reached the end. onerror Triggers when an error occurs. onloadeddata Triggers when media data is loaded. Triggers when the duration and other media data of a onloadedmetadata media element is loaded. Triggers when the browser starts loading the media onloadstart data. onpause Triggers when media data is paused. onplay Triggers when media data is going to start playing. onplaying Triggers when media data has started playing. onprogress Triggers when the browser is fetching the media data. Triggers when the playing rate of media data has onratechange changed. onreadystatechange Triggers when the ready-state changes. Triggers when the seeking attribute of a media onseeked element is no longer true, and the seeking has ended. 523

Java Triggers when the seeking attribute of a media onseeking element is true, and the seeking has begun. onstalled Triggers when there is an error in fetching media data. Triggers when the browser has been fetching media onsuspend data, but stopped before the entire media file was fetched. ontimeupdate Triggers when media changes its playing position. Triggers when a media changes the volume, also when onvolumechange the volume is set to \"mute\". Triggers when media has stopped playing, but is onwaiting expected to resume. Browser Support Chrome Firefox IE Opera Safari Android No Yes Yes No Yes No The code attribute of the <applet> tag is required. It specifies the Applet class to run. Width and height are also required to specify the initial size of the panel in which an applet runs. The applet directive must be closed with an </applet> tag. If an applet takes parameters, values may be passed for the parameters by adding <param> tags between <applet> and </applet>. The browser ignores text and other tags between the applet tags. Non-Java-enabled browsers do not process <applet> and </applet>. Therefore, anything that appears between the tags, not related to the applet, is visible in non-Java-enabled browsers. The viewer or browser looks for the compiled Java code at the location of the document. To specify otherwise, use the codebase attribute of the <applet> tag as shown: <applet codebase=\"http://amrood.com/applets\" code=\"HelloWorldApplet.class\" width=\"320\" height=\"120\"> If an applet resides in a package other than the default, the holding package must be specified in the code attribute using the period character (.) to separate package/class components. For example: <applet code=\"mypackage.subpackage.TestApplet.class\" width=\"320\" height=\"120\"> 524

Java Getting Applet Parameters The following example demonstrates how to make an applet respond to setup parameters specified in the document. This applet displays a checkerboard pattern of black and a second color. The second color and the size of each square may be specified as parameters to the applet within the document. CheckerApplet gets its parameters in the init() method. It may also get its parameters in the paint() method. However, getting the values and saving the settings once at the start of the applet, instead of at every refresh, is convenient and efficient. The applet viewer or browser calls the init() method of each applet it runs. The viewer calls init() once, immediately after loading the applet. (Applet.init() is implemented to do nothing.) Override the default implementation to insert custom initialization code. The Applet.getParameter() method fetches a parameter given the parameter's name (the value of a parameter is always a string). If the value is numeric or other non-character data, the string must be parsed. The following is a skeleton of CheckerApplet.java: import java.applet.*; import java.awt.*; public class CheckerApplet extends Applet { int squareSize = 50;// initialized to default size public void init () {} private void parseSquareSize (String param) {} private Color parseColor (String param) {} public void paint (Graphics g) {} } Here are CheckerApplet's init() and private parseSquareSize() methods: public void init () { String squareSizeParam = getParameter (\"squareSize\"); parseSquareSize (squareSizeParam); String colorParam = getParameter (\"color\"); Color fg = parseColor (colorParam); setBackground (Color.black); setForeground (fg); } 525

Java private void parseSquareSize (String param) { if (param == null) return; try { squareSize = Integer.parseInt (param); } catch (Exception e) { // Let default value remain } } The applet calls parseSquareSize() to parse the squareSize parameter. parseSquareSize() calls the library method Integer.parseInt(), which parses a string and returns an integer. Integer.parseInt() throws an exception whenever its argument is invalid. Therefore, parseSquareSize() catches exceptions, rather than allowing the applet to fail on bad input. The applet calls parseColor() to parse the color parameter into a Color value. parseColor() does a series of string comparisons to match the parameter value to the name of a predefined color. You need to implement these methods to make this applet work. Specifying Applet Parameters The following is an example of an HTML file with a CheckerApplet embedded in it. The HTML file specifies both parameters to the applet by means of the <param> tag. <html> <title>Checkerboard Applet</title> <hr> <applet code=\"CheckerApplet.class\" width=\"480\" height=\"320\"> <param name=\"color\" value=\"blue\"> <param name=\"squaresize\" value=\"30\"> </applet> <hr> </html> Note: Parameter names are not case sensitive. A p plication Conversion to Applets It is easy to convert a graphical Java application (that is, an application that uses the AWT and that you can start with the Java program launcher) into an applet that you can embed in a web page. 526

Java Following are the specific steps for converting an application to an applet.  Make an HTML page with the appropriate tag to load the applet code.  Supply a subclass of the JApplet class. Make this class public. Otherwise, the applet cannot be loaded.  Eliminate the main method in the application. Do not construct a frame window for the application. Your application will be displayed inside the browser.  Move any initialization code from the frame window constructor to the init method of the applet. You don't need to explicitly construct the applet object. The browser instantiates it for you and calls the init method.  Remove the call to setSize; for applets, sizing is done with the width and height parameters in the HTML file.  Remove the call to setDefaultCloseOperation. An applet cannot be closed; it terminates when the browser exits.  If the application calls setTitle, eliminate the call to the method. Applets cannot have title bars. (You can, of course, title the web page itself, using the HTML title tag.)  Don't call setVisible(true). The applet is displayed automatically. Event Handling Applets inherit a group of event-handling methods from the Container class. The Container class defines several methods, such as processKeyEvent and processMouseEvent, for handling particular types of events, and then one catch-all method called processEvent. In order to react to an event, an applet must override the appropriate event-specific method. import java.awt.event.MouseListener; import java.awt.event.MouseEvent; import java.applet.Applet; import java.awt.Graphics; public class ExampleEventHandling extends Applet implements MouseListener { StringBuffer strBuffer; public void init() { addMouseListener(this); 527

Java strBuffer = new StringBuffer(); addItem(\"initializing the apple \"); } public void start() { addItem(\"starting the applet \"); } public void stop() { addItem(\"stopping the applet \"); } public void destroy() { addItem(\"unloading the applet\"); } void addItem(String word) { System.out.println(word); strBuffer.append(word); repaint(); } public void paint(Graphics g) { //Draw a Rectangle around the applet's display area. g.drawRect(0, 0, getWidth() - 1, getHeight() - 1); //display the string inside the rectangle. g.drawString(strBuffer.toString(), 10, 20); } public void mouseEntered(MouseEvent event) { } 528

Java public void mouseExited(MouseEvent event) { } public void mousePressed(MouseEvent event) { } public void mouseReleased(MouseEvent event) { } public void mouseClicked(MouseEvent event) { addItem(\"mouse clicked! \"); } } Now, let us call this applet as follows: <html> <title>Event Handling</title> <hr> <applet code=\"ExampleEventHandling.class\" width=\"300\" height=\"300\"> </applet> <hr> </html> Initially, the applet will display \"initializing the applet. Starting the applet.\" Then once you click inside the rectangle, \"mouse clicked\" will be displayed as well. Displaying Images An applet can display images of the format GIF, JPEG, BMP, and others. To display an image within the applet, you use the drawImage() method found in the java.awt.Graphics class. Following is an example illustrating all the steps to show images: import java.applet.*; import java.awt.*; import java.net.*; public class ImageDemo extends Applet { private Image image; private AppletContext context; public void init() 529

Java { context = this.getAppletContext(); String imageURL = this.getParameter(\"image\"); if(imageURL == null) { imageURL = \"java.jpg\"; } try { URL url = new URL(this.getDocumentBase(), imageURL); image = context.getImage(url); }catch(MalformedURLException e) { e.printStackTrace(); // Display in browser status bar context.showStatus(\"Could not load image!\"); } } public void paint(Graphics g) { context.showStatus(\"Displaying image\"); g.drawImage(image, 0, 0, 200, 84, null); g.drawString(\"www.javalicense.com\", 35, 100); } } Now, let us call this applet as follows: <html> <title>The ImageDemo applet</title> <hr> <applet code=\"ImageDemo.class\" width=\"300\" height=\"200\"> <param name=\"image\" value=\"java.jpg\"> </applet> <hr> </html> 530

Java Playing Audio An applet can play an audio file represented by the AudioClip interface in the java.applet package. The AudioClip interface has three methods, including:  public void play(): Plays the audio clip one time, from the beginning.  public void loop(): Causes the audio clip to replay continually.  public void stop(): Stops playing the audio clip. To obtain an AudioClip object, you must invoke the getAudioClip() method of the Applet class. The getAudioClip() method returns immediately, whether or not the URL resolves to an actual audio file. The audio file is not downloaded until an attempt is made to play the audio clip. Following is an example illustrating all the steps to play an audio: import java.applet.*; import java.awt.*; import java.net.*; public class AudioDemo extends Applet { private AudioClip clip; private AppletContext context; public void init() { context = this.getAppletContext(); String audioURL = this.getParameter(\"audio\"); if(audioURL == null) { audioURL = \"default.au\"; } try { URL url = new URL(this.getDocumentBase(), audioURL); clip = context.getAudioClip(url); }catch(MalformedURLException e) { e.printStackTrace(); context.showStatus(\"Could not load audio file!\"); } } public void start() 531

Java { if(clip != null) { clip.loop(); } } public void stop() { if(clip != null) { clip.stop(); } } } Now, let us call this applet as follows: <html> <title>The ImageDemo applet</title> <hr> <applet code=\"ImageDemo.class\" width=\"0\" height=\"0\"> <param name=\"audio\" value=\"test.wav\"> </applet> <hr> </html> You can use test.wav on your PC to test the above example. 532

36. Java – Documentation Comments Java The Java language supports three types of comments: Comment Description /* text */ The compiler ignores everything from /* to */. // text The compiler ignores everything from // to the end of the line. /** This is a documentation comment and in general its called doc documentation comment. The JDK javadoc tool uses doc comments when */ preparing automatically generated documentation. This chapter is all about explaining Javadoc. We will see how we can make use of Javadoc to generate useful documentation for Java code. What is Javadoc? Javadoc is a tool which comes with JDK and it is used for generating Java code documentation in HTML format from Java source code, which requires documentation in a predefined format. Following is a simple example where the lines inside /*….*/ are Java multi-line comments. Similarly, the line which preceeds // is Java single-line comment. /** * The HelloWorld program implements an application that * simply displays \"Hello World!\" to the standard output. * * @author Zara Ali * @version 1.0 * @since 2014-03-31 */ public class HelloWorld { public static void main(String[] args) { // Prints Hello, World! on standard output. System.out.println(\"Hello World!\"); } } 533

Java You can include required HTML tags inside the description part. For instance, the following example makes use of <h1>....</h1> for heading and <p> has been used for creating paragraph break: /** * <h1>Hello, World!</h1> * The HelloWorld program implements an application that * simply displays \"Hello World!\" to the standard output. * <p> * Giving proper comments in your program makes it more * user friendly and it is assumed as a high quality code. * * * @author Zara Ali * @version 1.0 * @since 2014-03-31 */ public class HelloWorld { public static void main(String[] args) { // Prints Hello, World! on standard output. System.out.println(\"Hello World!\"); } } The javadoc Tags The javadoc tool recognizes the following tags: Tag Description Syntax @author Adds the author of a class. @author name-text Displays text in code font without {@code} interpreting the text as HTML markup or {@code text} nested javadoc tags. Represents the relative path to the {@docRoot} generated document's root directory {@docRoot} from any generated page. Adds a comment indicating that this API @deprecated deprecated- @deprecated should no longer be used. text 534

Java Adds a Throws subheading to the @exception generated documentation, with the class- @exception class-name description name and description text. Inherits a comment from the nearest {@inheritDoc} inheritable class or implementable Inherits a comment from the immediate surperclass. interface. Inserts an in-line link with the visible text label that points to the documentation for {@link {@link} package.class#member the specified package, class, or member label} name of a referenced class. Identical to {@link}, except the link's {@linkplain {@linkplain} label is displayed in plain text than code package.class#member font. label} Adds a parameter with the specified parameter-name followed by the @param parameter-name @param specified description to the \"Parameters\" description section. Adds a \"Returns\" section with the @return @return description description text. Adds a \"See Also\" heading with a link or @see @see reference text entry that points to reference. Used in the doc comment for a default @serial field-description | @serial serializable field. include | exclude Documents the data written by the @serialData writeObject( ) or writeExternal( ) @serialData data- description methods. Documents an ObjectStreamField @serialField field-name @serialField component. field-type field-description Adds a \"Since\" heading with the specified @since since-text to the generated @since release documentation. The @throws and @exception tags are @throws class-name @throws synonyms. description When {@value} is used in the doc {@value} comment of a static field, it displays the {@value package.class#field} value of that constant. 535

Java Adds a \"Version\" subheading with the @version specified version-text to the generated @version version-text docs when the -version option is used. Example Following program uses few of the important tags available for documentation comments. You can make use of other tags based on your requirements. The documentation about the AddNum class will be produced in HTML file AddNum.html but at the same time a master file with a name index.html will also be created. import java.io.*; /** * <h1>Add Two Numbers!</h1> * The AddNum program implements an application that * simply adds two given integer numbers and Prints * the output on the screen. * <p> * <b>Note:</b> Giving proper comments in your program makes it more * user friendly and it is assumed as a high quality code. * * @author Zara Ali * @version 1.0 * @since 2014-03-31 */ public class AddNum { /** * This method is used to add two integers. This is * a the simplest form of a class method, just to * show the usage of various javadoc Tags. * @param numA This is the first paramter to addNum method * @param numB This is the second parameter to addNum method * @return int This returns sum of numA and numB. */ public int addNum(int numA, int numB) { return numA + numB; } 536

Java /** * This is the main method which makes use of addNum method. * @param args Unused. * @return Nothing. * @exception IOException On input error. * @see IOException */ public static void main(String args[]) throws IOException { AddNum obj = new AddNum(); int sum = obj.addNum(10, 20); System.out.println(\"Sum of 10 and 20 is :\" + sum); } } Now, process the above AddNum.java file using javadoc utility as follows: $ javadoc AddNum.java Loading source file AddNum.java... Constructing Javadoc information... Standard Doclet version 1.7.0_51 Building tree for all the packages and classes... Generating /AddNum.html... AddNum.java:36: warning - @return tag cannot be used in method with void return type. Generating /package-frame.html... Generating /package-summary.html... Generating /package-tree.html... Generating /constant-values.html... Building index for all the packages and classes... Generating /overview-tree.html... Generating /index-all.html... Generating /deprecated-list.html... 537

Java Building index for all classes... Generating /allclasses-frame.html... Generating /allclasses-noframe.html... Generating /index.html... Generating /help-doc.html... 1 warning $ You can check all the generated documentation here: AddNum. If you are using JDK 1.7 then javadoc does not generate a great stylesheet.css, so we suggest to download and use standard stylesheet from http://docs.oracle.com/javase/7/docs/api/stylesheet.css 538