CSC 205 – Java Programming II

1. CSC 205 – Java Programming II Applet

2. Types of Java Programs • Applets • Applications • Console applications • Graphics applications • Applications are stand-alone programs • An application must have a main method

3. What Is Applet • An applet is • a program that adheres to a set of conventions • run within a Java-compatible Web browser • downloaded from a Web server • An applet must extend the Applet class

4. Download From Web Server

5. Advantages & Disadvantages • Advantages • Can be accessed where Internet is available • When GUIs need to be upgraded, just change them at one location: on the Web server • Disadvantages • Time to download may be long • Security could be an issue

6. Security Policies • Browsers impose the following restrictions on any applet that is loaded over the network: • An applet cannot load libraries or define native methods. • It cannot ordinarily read or write files, or start any program on the host that's executing it. • It cannot makenetwork connections except to the host that it came from. • It cannot read certain system properties. • Windows that an applet brings up look different than windows that an application brings up.

7. Inheritance Hierarchy • java.appletClass Applet • java.lang.Object • | • +--java.awt.Component • | • +--java.awt.Container • | • +--java.awt.Panel • | • +--java.applet.Applet

8. A Typical Applet import java.applet.Applet; import java.awt.Graphics; public class Simple extends Applet { //methods to be overridden public void init() {…} public void start() {…} public void stop() {…} public void destroy() {…} void addItem(String newWord) { …; repaint(); } public void paint(Graphics g) {…} //inherited from the Container class }

9. Life Cycle of an Applet • Loading the Applet • An instance of the applet's controlling class (an Applet subclass) is created • The applet initializes itself • The applet starts running • Leaving and Returning to the Applet's Page • The applet stops running when leaving the page • the applet can start itself again when returning

10. Methods for Milestones • An applet can override the following methods • init To initialize the applet each time it's loaded (or reloaded). • start To start the applet's execution, such as when the applet's loaded or when the user revisits a page that contains the applet. • stop To stop the applet's execution, such as when the user leaves the applet's page or quits the browser. • destroy To perform a final cleanup in preparation for unloading

11. Methods for Drawing • An applet can override the following two display methods (of the Container class) : • paint The basic display method. Many applets implement the paint method to draw the applet's representation within a browser page. • update A method you can use along with paint to improve drawing performance

12. Animation • To update applet changes • invoke the repaint method (inherited from the Component class) periodically • The paint method will be invoked when the applet is repainted

13. Event Handling • Applets inherit a group of event-handling methods from the Component class • To react to an event, an applet must override • either the appropriate event-specific method, or • the handleEvent method (from the Component class)

14. Event Handling – Example • Adding the following code to the Simple applet makes it respond to mouse clicks. import java.awt.Event; . . . public boolean mouseDown(Event event, int x, int y) { addItem("click!... "); return true; }

15. Deprecation • Unfortunately, many of the samples available online or in textbooks use deprecated methods • You will be warned when you compile code with deprecated methods • Use the –deprecation option to see detailed info javac –deprecation Simple.java • Replace deprecated methods with newer methods as recommended by latest version of Java API

16. Deprecation

17. Using UI Components • Because the Applet class inherits from the AWT Container class, it's easy to add components to applets and to use layout managers to control the components' onscreen positions. • add Adds the specified Component • remove Removes the specified Component • setLayout Sets the layout manager

18. Testing Applets • Two ways to run an applet • Use the applet viewer appletviewer simple.html • Embed applets into Web pages • Both need to use the applet tag in HTML files <APPLET CODE=AppletSubclass.class WIDTH=anInt HEIGHT=anInt> </APPLET>

19. HTML – An Overview • Markup language • use tags to represent the meaning and/or content of the enclosed data • Some features • Not case sensitive • Loose syntax • Predefined tags • Text is the only data type

20. More Examples • The following two tags are equivalent <APPLET CODE=Simple.class WIDTH=100 HEIGHT=100> </APPLET> <applet code=“Simple.class” width=“100” height=“100”> </applet>

21. A Simple HTML File <html> <body> <p>Click on the applet to start the animation.</p> <!– this line is comment --> <applet code="SelectionSortApplet.class" width="300" height="300"> </applet> </body> </html>

22. Thread • A thread is a program unit that is executed independently of other parts of the program • The JVM executes each thread for a short time and then switches to another thread • A programmer can concentrate on what tasks each thread need to perform • and possibly, the communication between threads

23. Write a Thread • Follow the steps • Write a class that extends the Thread class • Place the code for the task in the run method • Create an object of the your thread class • Call the start method to activate your thread

24. Sample Thread Class public class GreetingThread extends Thread{ public GreetingThread(String aGreeting){ greeting = aGreeting; } public void run() { try { for (int i = 1; i <= REPETITIONS; i++) { Date now = new Date(); System.out.println(now + " " + greeting); sleep(DELAY); } } catch (InterruptedException exception){ } }

25. Running A Thread – I public class GreetingThreadTest{ public static void main(String[] args) { GreetingThread t1 = new GreetingThread("Hello, World!"); GreetingThread t2 = new GreetingThread("Goodbye, World!"); t1.start(); t2.start(); } }

26. Sample Output

27. The Runnable Interface • Problems of writing a thread which extends another class, e.g. the Frame class • Since Java doesn’t allow multiple inheritance • Can implement the Runnable interface instead public class MyFrame extends Frame implements Runnable { public MyFrame() {…} public void run() {…} }

28. Running A Thread – II • Follow the steps • Construct a Runnable object • Create a thread using the Runnable object • Invoke the start method of the thread MyFrame frame = new MyFrame(); Thread myThread = new Thread(frame); myThread.start();

29. start interrupt