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Java Overview

Java Overview. Java Overview Design goals & features platform independence, portable, secure, simple, object-oriented, … Programming models applications vs. applets vs. servlets intro to applets libraries, comments, classes, inheritance applet tag in HTML applet parameters. Java.

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Java Overview

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  1. Java Overview • Java Overview • Design goals & features • platform independence, portable, secure, simple, object-oriented, … • Programming models • applications vs. applets vs. servlets • intro to applets • libraries, comments, classes, inheritance • applet tag in HTML • applet parameters

  2. Java • Java was developed at Sun Microsystems, 1995 • originally designed for small, embedded systems in electronic appliances • initial attempts used C++, but frustration at limitations/pitfalls recall: C++ = C + OOP features the desire for backward compatibility led to the retention of many bad features • desired features (from the Java white paper): • simple object-oriented robust • platform independent architecture neutral portable • dynamic interpreted high-performance • distributed multi-threaded secure • note: these are desirable features for any modern language • thus, Java has become very popular, especially when Internet related • also, Sun distributes free compilers (JDK) and open source

  3. Language features • simple • syntax is based on C++ (familiarity  easier transition for programmers) • removed many confusing and/or rarely-used features • e.g., explicit pointers, operator overloading, automatic coercions • added memory management (reference count/garbage collection hybrid) • object-oriented • OOP facilities similar C++, all methods are dynamically bound • pure OOP – everything is a class, no independent functions* • robust • lack of pointers and memory management avoids many headaches/errors • libraries of useful, tested classes increases level of abstraction • arrays & strings are ADTs, well-defined interfaces

  4. Language features (cont.) • platform independence • want to be able to run Java code on multiple platforms • neutrality is achieved by mixing compilation & interpretation • Java programs are translated into byte code by a Java compiler • byte code is a generic machine code • byte code is then executed by an interpreter (Java Virtual Machine) • must have a byte code interpreter for each hardware platform • an Applet is a special form of Java application • byte code is downloaded with page, JVM is embedded in browser • architecture-neutral • no implementation dependent features • e.g., sizes of primitive types is set (unlike C++) • portable • byte code will run on any version of the Java Virtual Machine (JVM)

  5. Language features (cont.) • dynamic • JVM links classes at run-time as they are needed • if supporting class is recompiled, don’t have to recompile entire project • interpreted • needed for platform independence • interpreted  faster code-test-debug cycle, better run-time error checking • high-performance • faster than traditional interpretation since byte code is "close" to native code • still somewhat slower than a compiled language (e.g., C++)

  6. Language features (cont.) • secure • Java applications do not have direct access to memory locations • memory accesses are virtual, mapped by JVM to physical locations • downloaded applets cannot open, read, or write local files • JVM also verifies authenticity of classes as they are loaded • Sun claim: execution model enables virus-free*, tamper-free* systems • distributed • extensive libraries for coping with TCP/IP protocols like HTTP & FTP • Java applications can access remote URL's the same as local files • multi-threaded • a thread is like a separate program, executing concurrently • can write Java programs that deal with many tasks at once by defining multiple threads (same shared memory, but semi-independent execution) • threads are important for multi-media, Web applications

  7. Java programming models • Java applications are stand-alone programs • must be compiled into Java byte code by Java compiler, then distributed • executed by an interpreter (Java Virtual Machine) • Java applets provide for client-side programming • compiled into Java byte code, then downloaded as part of a Web page • executed by the JVM embedded within the Web browser • unlike JavaScript, Java is full-featured with extensive library support • Java and its APIs have become industry standards • the language definition is controlled by Sun, ensures compatibility • Applications Programming Interfaces standardize the behavior of useful classes and libraries of routines • Java servlets provide similar capabilities on the server-side • alternative to CGI programs, more fully integrated into Web server

  8. Java applets • important point: Java applets & applications look different! • if you want to define a stand-alone application, make an application • requires public static void main function, similar to C++ main • if you want to embed the code in a Web page, make an applet • requires public void paint, public void init, … • can define dual-purpose programs, but tricky • as with JavaScript, security is central • when a Java applet is downloaded, the bytecode verifier of the JVM verifies to see if it contains bytecodes that open, read, write to local disk • a Java applet can open a new window but they have Java logo to prevent them from being disguised as system window (for stealing password) • a Java applet is not allowed to connect back to other servers except the host • this secure execution environment is called sand box model

  9. First Java applet • import java.awt.*; • import java.applet.*; • /** • * This class displays "Hello world!" on the applet window. • */ • public class HelloWorld extends Applet • { • public void paint(Graphics g) • { • g.drawString("Hello world!", 10, 10); // writes starting 10 pixels over & down • } • } • libraries • Java provides extensive library support in the form of classes • libraries are loaded using import (similar to #include in C++) java.awt: contains Abstract Window Toolkit (for GUI classes & routines) java.applet: contains the applet class definition

  10. First Java applet • import java.awt.*; • import java.applet.*; • /** • * This class displays "Hello world!" on the applet window. • */ • public class HelloWorld extends Applet • { • public void paint(Graphics g) • { • g.drawString("Hello world!", 10, 10); // writes starting 10 pixels over & down • } • } • comments in Java • //and/* . . . */work the same as in C++ • /** . . . */designate documentation comments • can be used to automatically generate HTML documentation (javadoc)

  11. First Java applet • import java.awt.*; • import java.applet.*; • /** • * This class displays "Hello world!" on the applet window. • */ • public class HelloWorld extends Applet • { • public void paint(Graphics g) • { • g.drawString("Hello world!", 10, 10); // writes starting 10 pixels over & down • } • } • class definitions in Java • similar to C++ (but no semi-colon at end) can contain instance variables (data fields) & methods(member functions) precede class & method definitions with public to make available to all programs • there are no stand-alone functions in Java* • must be stored in a file of same name with .java extension e.g.,HelloWorld.java

  12. First Java applet • import java.awt.*; • import java.applet.*; • /** • * This class displays "Hello world!" on the applet window. • */ • public class HelloWorld extends Applet • { • public void paint(Graphics g) • { • g.drawString("Hello world!", 10, 10); // writes starting 10 pixels over & down • } • } • all applets inherit from the Applet class (in java.applet) • default methods include: • init():called when page is loaded to create/initialize variables by default, does nothing • paint(Graphics g):called to draw (after init) or redraw (after being obscured) here, the paint method is overridden to display text on the applet window

  13. Embedding an applet in HTML • <html> • <!-- Hello1.html --> • <head> • <title>Hello World Page</title> • </head> • <body> • <p> • <applet code="HelloWorld.class" height=100 width=100> • You must use a Java-enabled browser to view this applet. • </applet> • </p> • </body> • </html> • to include an applet in a Web page, use either • APPLET tag (deprecated) • CODE specifies applet name, HEIGHT and WIDTH specify window size • text between the APPLET tags is displayed if unable to execute (e.g., Java not enabled) • OBJECT tag • preferred for HTML 4, but not universally supported view page in browser

  14. HTML & applets • <html> • <!-- Hello2.html --> • <head> • <title>Hello World Page</title> • </head> • <body> • <p> • <div align="center"> • <table border=1> • <tr><td> • <applet code="HelloWorld.class" height=200 width=200> • You must use a Java-enabled browser to view this applet. • </applet> • </td></tr> • </table> • </div> • </p> • </body> • </html> an applet can be embedded within HTML elements just like any other element useful for formatting and layout view page in browser

  15. Parameters in HTML • <html> • <!-- Hello3.html --> • <head> • <title>Hello World Page</title> • </head> • <body> • <p> • <div align="center"> • <table border=1> • <tr><td> • <applet code="HelloWorld1.class" height=35 width=300> • <param name="name" value="Chris"> • <param name="age" value=20> • You must use a Java-enabled browser to view this applet. • </applet> • </td></tr> • </table> • </div> • </p> • </body> • </html> • can specify parameters to the APPLET when it is embedded in HTML • each parameter must have its own PARAM tag inside the APPLET element • specifies parameter name and value view page in browser

  16. Applet parameters • import java.awt.*; • import java.applet.*; • /** • * This class displays a message based on parameters. • */ • public class HelloWorld1 extends Applet • { • public void paint(Graphics g) • { • String userName = getParameter("name"); • int userAge = Integer.parseInt(getParameter("age")); • String message1 = "Hello " + userName + "."; • String message2 = "On your next birthday, you will be " + • (userAge+1) + " years old."; • g.drawString(message1, 10, 10); • g.drawString(message2, 10, 30); • } • } • can access parameters passed in from the HTML document • getParameteraccesses the value of the parameter (must know its name) • if the parameter represents a number, mustparseInt orparseFloat

  17. Java vs. C++ • in Java, every variable & method belongs to a class • as in C++, by default each object has its own copies of data fields thus, known as instance variables • as in C++, a variables declared static are shared by all class objects thus, known as class variables • similarly, can have a static method (class method) can only operate on class variables, accessible from the class itself class Math { public static final double PI = 3.14159; // access as Math.PI public static double sqrt(double num) { . . . } // access as in Math.sqrt(9.0) . . . }

  18. Primitive vs. reference types • primitive types are handled exactly as in C++ • space for a primitive object is implicitly allocated • variable refers to the actual data (stored on the stack) reference types (classes) are handled differently • space for a reference object must be explicitly allocated using new • variable refers to a pointer to the data (which is stored in the heap) Note: unlike with C++, programmer is not responsible for deleting dynamic objects JVM performs automatic garbage collection to reclaim unused memory • Java only provides by-value parameter passing • but reference objects are implemented as pointers to dynamic memory • resulting behavior mimics by-reference public void Init(int[] nums) { for (int i = 0; i < nums.length; i++) { nums[i] = 0; } } _____________________________ _ int nums[] = new int[10]; Init(nums);

  19. Java libraries • String class (automatically loaded from java.lang) int length() char charAt(index) int indexOf(substring) String substring(start, end) String toUpperCase() boolean equals(Object) ... • Array class (automatically loaded from java.lang) int length instance variable Type [](index) operator String toString() ... • Java provides extensive libraries of data structures & algorithms java.util Vector Stack LinkedList Dictionary HashTable Random Calendar String str = "foo" String str = new String("foo"); int[] nums = {1,2,3,4,5}; int[] nums = new int[10];

  20. Java & inheritance • Java does NOT have templates* • generic functions/classes are obtained via inheritance (e.g, Applet) • with inheritance, can derive a new class from an existing class • automatically inherit attributes & methods of the parent class • object of derived class can be used wherever parent object is expected • every reference type is implicitly derived from the Object class • by defining data structures that contain Objects, any reference type can be stored (and even mixed) • Stack things = new Stack(); • String str = "foobar"; • things.push(str); // pushes String as an Object • Calendar today = Calendar.getDate(); • things.push(today); // pushes Calendar as an Object • … • Calendar value1 = (Calendar)things.pop(); // pops & coerces • String value2 = (String)things.pop(); // pops & coerces

  21. Hello again • import java.awt.*; • import java.applet.*; • import java.util.Random; • /** • * This class displays lots of "Hello world!"s on the applet window. • */ • public class HelloWorld1 extends Applet • { • private static final int NUM_WORDS=100; • private static Random randy; • private int randomInRange(int low, int high) • { • return (Math.abs(randy.nextInt()) % (high-low+1)) + low; • } • public void init() • { • randy = new Random(); • } • public void paint(Graphics g) • { • for (int i = 0; i < NUM_WORDS; i++) { • int x = randomInRange(1, 200); • int y = randomInRange(1, 200); • g.drawString("Hello world!", x, y); • } • } • } Random class provides methods for generating random values can override init method to allocate & initialize (similar to a constructor) <applet code="HelloWorld1.class" height=200 width=200> You must use a Java-enabled browser to view this applet. </applet>

  22. Applet behavior • recall • the init method is called when the applet is first loaded • useful for initializing variables & objects • the paint method is called immediately after init, and whenever the applet needs to be redrawn (e.g., after window resized or obscured) • when paint is called, it is given the default Graphics object • Graphics methods include void drawString(String msg, int x, int y) void setColor(Color color) • Color class is predefined, constants include: • Color.red, Color.blue, Color.black, . . .

  23. A colorful hello • import java.awt.*; • import java.applet.*; • import java.util.Random; • /** • * This class displays lots of "Hello world!"s on the applet window. • */ • public class HelloWorld2 extends Applet • { • private static final int NUM_WORDS=100; • private static final Color[] colors = • {Color.black,Color.red,Color.blue,Color.green,Color.yellow}; • private static Random randy; • private int RandomInRange(int low, int high) • { • return (Math.abs(randy.nextInt()) % (high-low+1)) + low; • } • public void init() • { • randy = new Random(); • } • public void paint(Graphics g) • { • for (int i = 0; i < NUM_WORDS; i++) { • int x = RandomInRange(1, 140); • int y = RandomInRange(10, 200); • g.setColor(colors[RandomInRange(0,4)]); • g.drawString("Hello world!", x, y); • } • } • } can store possible colors in an array change drawing color using thesetColormethod view page in browser <applet code="HelloWorld2.class" height=200 width=200> You must use a Java-enabled browser to view this applet. </applet>

  24. Parameters & applet dimensions • recall: • can specify parameters in the HTML document using <PARAM> tags • access the parameter values (based on name) using getParameter method • can also access the dimensions of an applet using a Dimension object Dimension dim = getSize(); // stores applet dimensions can then access applet height via dim.height can then access applet width via dim.width

  25. Adaptive hello • import java.awt.*; • import java.applet.*; • import java.util.Random; • /** • * This class displays lots of "Hello world!"s on the applet window. • */ • public class HelloWorld3 extends Applet • { • private static final Color[] colors = • {Color.black,Color.red,Color.blue,Color.green,Color.yellow}; • private static Random randy; • private Dimension dim; • private int numReps; • private int RandomInRange(int low, int high) • { • return (Math.abs(randy.nextInt()) % (high-low+1)) + low; • } • public void init() • { • randy = new Random(); • dim = getSize(); • numReps = Integer.parseInt(getParameter("reps")); • } • public void paint(Graphics g) • { • for (int i = 0; i < numReps; i++) { • int x = RandomInRange(1, dim.width-65); • int y = RandomInRange(10, dim.height); • g.setColor(colors[RandomInRange(0,4)]); • g.drawString("Hello world!", x, y); • } • } • } getParameteraccesses the values of the parameters here, specify number of reps in Web page uses getSize to get dimensions, pick random coords for text within the applet <applet code="HelloWorld3.class" height=300 width=400> <param name="reps" value=200> You must use a Java-enabled browser to view this applet. </applet> view page in browser

  26. Applet graphics • in addition to displaying text • can also draw figures on a Graphics object void drawLine(int x1, int y1, int x2, int y2) void drawRect(int x, int y, int width, int height) void fillRect(int x, int y, int width, int height) void drawOval(int x, int y, int width, int height) void fillOval(int x, int y, int width, int height) • EXAMPLE: draw a red circle inscribed in a square, then draw random dots (dart pricks) • by counting the number of dots inside vs. outside the circle, can estimate the value of p p = 4 * (area of circle/area of square)

  27. Graphical applet • public class Monte1 extends Applet • { • private static Random randy; • private int NUM_POINTS; • private int SIZE; • private int RandomInRange(int low, int high) { CODE OMITTED } • private double distance(int x1, int y1, int x2, int y2) { CODE OMITTED } • public void init() • { • randy = new Random(); • NUM_POINTS = Integer.parseInt(getParameter("points")); • Dimension dim = getSize(); • SIZE = Math.min(dim.width, dim.height); • } • public void paint(Graphics g) • { • g.setColor(Color.red); • g.fillOval(0, 0, SIZE, SIZE); • for (int i = 0; i < NUM_POINTS; i++) { • int x = RandomInRange(0, SIZE); • int y = RandomInRange(0, SIZE); • if (distance(x, y, SIZE/2, SIZE/2) < SIZE/2) { • g.setColor(Color.white); • } • else { • g.setColor(Color.black); • } • g.drawLine(x, y, x, y); • } • } • } initmethod creates random number generator & gets parameters paintmethod draws a circle and a bunch of random points <applet code="Monte1.class" height=300 width=300> <param name="points" value=20000> You must use a Java-enabled browser... </applet> view page in browser

  28. Double buffering • note: paint is called every time the page is brought to the front • in current version of Monte, this means new dots are drawn each time the page is obscured and then brought back to the front • wastes time redrawing • dots are different each time the applet is redrawn • the double buffering approach works by keeping an off-screen image • in the init method (which is called when the page loads): • draw the figures on a separate, off-screen Graphics object • in the paint method (which is called whenever the page is brought forward): • simply display the off-screen image on the screen

  29. Buffered applet • public class Monte2 extends Applet • { • . . . • private Image offScreenImage; • private Graphics offScreenGraphics; • . . . • public void init() • { • randy = new Random(); • NUM_POINTS = Integer.parseInt(getParameter("points")); • Dimension dim = getSize(); • SIZE = Math.min(dim.width, dim.height); • offScreenImage = createImage(SIZE, SIZE); • offScreenGraphics = offScreenImage.getGraphics(); • offScreenGraphics.setColor(Color.red); • offScreenGraphics.fillOval(0, 0, SIZE, SIZE); • for (int i = 0; i < NUM_POINTS; i++) { • int x = RandomInRange(0, SIZE); • int y = RandomInRange(0, SIZE); • if (distance(x, y, SIZE/2, SIZE/2) < SIZE/2) { • offScreenGraphics.setColor(Color.white); • } • else { • offScreenGraphics.setColor(Color.black); • } • offScreenGraphics.drawLine(x, y, x, y); • } • } • public void paint(Graphics g) • { • g.drawImage(offScreenImage, 0, 0, null); • } • } initmethod is called when page is loaded does drawing to a separate, off-screen Graphics object paintis called afterinitand whenever the applet is revisited Note: don’t see image in progress <applet code="Monte2.class" height=300 width=300> <param name="points" value=20000> You must use a Java-enabled browser... </applet> view page in browser

  30. Better buffering • public class Monte3 extends Applet • { • . . . • public void init() { • randy = new Random(); • NUM_POINTS = Integer.parseInt(getParameter("points")); • Dimension dim = getSize(); • SIZE = Math.min(dim.width, dim.height); • } • public void paint(Graphics g) { • if (offScreenImage == null) { • offScreenImage = createImage(SIZE, SIZE); • offScreenGraphics = offScreenImage.getGraphics(); • offScreenGraphics.setColor(Color.red); • g.setColor(Color.red); • offScreenGraphics.fillOval(0, 0, SIZE, SIZE); • g.fillOval(0, 0, SIZE, SIZE); • for (int i = 0; i < NUM_POINTS; i++) { • int x = randomInRange(0, SIZE); • int y = randomInRange(0, SIZE); • if (distance(x, y, SIZE/2, SIZE/2) < SIZE/2) { • offScreenGraphics.setColor(Color.white); • g.setColor(Color.white); • } • else { • offScreenGraphics.setColor(Color.black); • g.setColor(Color.black); • } • offScreenGraphics.drawLine(x, y, x, y); • g.drawLine(x, y, x, y); • } • } • else { • g.drawImage(offScreenImage, 0, 0, null); • } • } • } if want to see image as it is drawn, must be done inpaint when first loaded, havepaintdraw on the graphics screen and also to an off-screen buffer on subsequent repaints, simply redraw the contents of the off-screen buffer <applet code="Monte3.class" height=300 width=300> <param name="points" value=20000> </applet> view page in browser

  31. GUI elements in applets • Java has extensive library support for GUIs (Graphical User Interfaces) • has elements corresponding to HTML buttons, text boxes, text areas, … • each element must be created and explicitly added to the applet nameLabel = new Label("User's name"); add(nameLabel); nameField = new TextField(20); nameField.setValue("Dave Reed"); add(nameField); • by default, GUI elements are placed in the order they were added, with elements moved to the next line as needed to fit

  32. Text boxes • public class Monte4 extends Applet • { • . . . • private Label insideLabel; • private TextField insideField; • private Label outsideLabel; • private TextField outsideField; • . . . • public void init() • { • randy = new Random(); • NUM_POINTS = • Integer.parseInt(getParameter("points")); • Dimension dim = getSize(); • SIZE = Math.min(dim.width, dim.height); • insideLabel = new Label("Inside:"); • add(insideLabel); • insideField = new TextField(6); • add(insideField); • outsideLabel = new Label("Outside:"); • add(outsideLabel); • outsideField = new TextField(6); • add(outsideField); • } public void paint(Graphics g) { . . . insideField.setText("0"); outsideField.setText("0"); . . . if (distance(x, y, SIZE/2, SIZE/2) < SIZE/2) { g.setColor(Color.white); int value = Integer.parseInt(insideField.getText())+1; insideField.setText(""+value); } else { g.setColor(Color.black); int value = Integer.parseInt(outsideField.getText())+1; outsideField.setText(""+value); } . . . } } <applet code="Monte4.class" height=300 width=300> <param name="points" value=20000> </applet> view page in browser

  33. GUI layout • public class Monte5 extends Applet • { • . . . • public void init() • { • randy = new Random(); • NUM_POINTS = Integer.parseInt(getParameter("points")); • Dimension dim = getSize(); • SIZE = Math.min(dim.width, dim.height); • setLayout(new BorderLayout()); • Panel p = new Panel(); • insideLabel = new Label("Inside:"); • p.add(insideLabel); • insideField = new TextField(5); • p.add(insideField); • outsideLabel = new Label("Outside:"); • p.add(outsideLabel); • outsideField = new TextField(5); • p.add(outsideField); • add(p, BorderLayout.SOUTH); • } • . . . • } Java provides several classes for controlling layout FlowLayout is default BorderLayoutallows placement of elements around the borders of the applet screen aPanelcan contain numerous elements <applet code="Monte5.class" height=300 width=300> <param name="points" value=20000> </applet> view page in browser

  34. Event handling • in order to handle events (e.g., text changes, button clicks), can use the event delegation model • must specify that the class implements the ActionListener interface • public class Monte6 extends Applet implements ActionListener • each source of events must be registered within the applet • dotButton = new Button("Click to generate dots"); • dotButton.addActionListener(); • must have an actionPerformed method to handle events • public void actionPerformed(ActionEvent e) • { • if (e.getSource() == dotButton) { • drawDots(); • } • }

  35. ActionListener public void drawCircle() { CODE FOR DRAWING CIRCLE } public void drawDots() { drawCircle(); Graphics g = getGraphics(); for (int i = 0; i < NUM_POINTS; i++) { CODE FOR DRAWING DOTS } } public void paint(Graphics g) { g.drawImage(offScreenImage, 0, 0, null); } public void actionPerformed(ActionEvent e) { if (e.getSource() == dotButton) { drawDots(); } } } • import java.awt.*; • import java.applet.*; • import java.awt.event.*; • import java.util.Random; • public class Monte6 extends Applet • implements ActionListener • { • . . . • private Button dotButton; • public void init() • { • randy = new Random(); • NUM_POINTS = • Integer.parseInt(getParameter("points")); • Dimension dim = getSize(); • SIZE = dim.width; • setLayout(new BorderLayout()); • dotButton = • new Button("Click to generate dots"); • dotButton.addActionListener(this); • add(dotButton, BorderLayout.SOUTH); • drawCircle(); • } <applet code="Monte6.class" height=300 width=300> <param name="points" value=20000> </applet> view page in browser

  36. Applet examples • The Java Boutique has lots of sample applets with source code • Graphing Calculator • Mandlebrot Set • Email front-end • Web search front-end • Java Tetris

  37. Combining Java & JavaScript • integrating Java with JavaScript • calling Java routines from JavaScript • controlling an applet from JavaScript • accessing JavaScript & HTML elements from an applet • related topics • JavaBeans, Java archives (JARs)

  38. JavaScript vs. Java • recall: JavaScript is very good for simple tasks, GUI layout • flexible data typing, primitive object types fine for quick development • integration with HTML makes layout & control of GUI elements easy • not much library support, only primitive data structuring capabilities • not well-suited to multi-file projects, OO approach • recall: Java is better at complex tasks, especially graphics • full-featured, more robust, extensive libraries of classes/routines • can support large projects, interacting objects • GUI layout is difficult, integration with HTML not obvious • IDEAL: make use of the the strengths of each language • include applets in a page when needed (e.g., graphics) • allow communication between applet & JavaScript code

  39. Calling Java routines from JavaScript • Netscape Communicator allows direct calls to Java routines • specify full package name of routine, then call as in Java useful for more esoteric routines that are not supported directly in JavaScript • this feature is NOT supported by Internet Explorer • <html> • <!-- Note: works in Netscape only. --> • <head> • <title>Java+JavaScript Demo</title> • </head> • <body> • <script language="JavaScript"> • document.write(java.lang.Math.random()); • </script> • </body> • </html>

  40. Calling applet methods • more commonly, want to include an applet in a page, • control via HTML events & JavaScript • want to draw dots inside a square (with an inscribed circle) • could build GUI interface into applet, but required tricky layout manager • instead, leave graphics up to the applet, controlled via JavaScript • to call a Java applet method from JavaScript • document.appletName.methodCall(…)

  41. MontePI revisited initcreates the random number generator & gets applet size drawDotsdraws the dots on the screen and to the off-screen buffer paintredraws the screen using the buffer • import java.awt.*; • import java.applet.*; • import java.util.Random; • public class Monte7 extends Applet • { • private static Random randy; • private int SIZE; • private Image offScreenImage; • private Graphics offScreenGraphics; • private int randomInRange(int low, int high) {…} • private double distance(int x1, int y1, int x2, int y2) {…} • public void init() • { • randy = new Random(); • Dimension dim = getSize(); • SIZE = dim.width; • drawCircle(); • } • public void drawCircle() { • // DRAWS CIRCLE ON BOTH getGraphics() AND • // offScreenGraphics • } • public void drawDots(int numPoints) • { • // DRAWS numPoints RANDOM DOTS ON BOTH getGraphics() • // AND offScreenGraphics • } • public void paint(Graphics g) • { • g.drawImage(offScreenImage, 0, 0, null); • } • }

  42. MontePI example (cont.) • <html> • <!-- Monte7.html --> • <head> • <title>Monte Carlo Darts Page</title> • </head> • <body bgcolor="gray"> • <div style="text-align:center"> • <applet code="Monte7.class" name="MonteApplet" • height=300 width=300> • You must use a Java-enabled browser to view this applet. • </applet> • <br /><br /> • <form name="MonteForm"> • <input type="button" value="Generate points" • onClick="document.MonteApplet.drawDots(1000);"> • </form> • </div> • </body> • </html> here, HTML button controls the applet (via JavaScript) view page in browser

  43. Example (cont.) better interface: allow user to specify number of dots in text box each click adds new dots, have separate button to clear • <html> • <!-- Monte7a.html --> • <head> • <title>Monte Carlo Darts Page</title> • </head> • <body bgcolor="gray"> • <div style="text-align:center"> • <applet code="Monte7.class" name="MonteApplet" • height=300 width=300> • You must use a Java-enabled browser to view this applet. • </applet> • <br /><br /> • <form name="MonteForm"><input type="button" value="Generate" onClick="numDots = parseFloat(document.MonteForm.numPoints.value); document.MonteApplet.drawDots(numDots);"><input type="text" name="numPoints" size=6 value=100> points <br /><br /> <input type="button" value="Clear the screen" onClick="document.MonteApplet.drawCircle();"> </form> • </div> • </body> • </html> view page in browser

  44. Dividing control • where the control lies affects the efficiency/usability of an applet • want the applet to be as self-contained as possible, • take advantage of speed advantage, more advanced features • but if GUI controls are in HTML, then JavaScript needs overall control • consider adding counters for number of dots inside & outside circle • have the applet keep track of the dots in instance variables • after drawing all dots, JavaScript could access counts & display • can’t see counts in progress (in Netscape) • could have applet update the HTML text boxes itself • tricky (example later) • could return more control to the page, applet draws one dot at a time repetition is handled by JavaScript, can update boxes after each dot  slower, but more flexible

  45. JavaScript in control • import java.awt.*; • import java.applet.*; • import java.util.Random; • public class Monte8 extends Applet • { • . . . • public int numInside, numOutside; • . . . • public void clear() • { • numInside = 0; numOutside = 0; • . . . • } • public void drawDot() • { • . . . • if (distance(x, y, SIZE/2, SIZE/2) < SIZE/2) { • offScreenGraphics.setColor(Color.white); • g.setColor(Color.white); • numInside++; • } • else { • offScreenGraphics.setColor(Color.black); • g.setColor(Color.black); • numOutside++; • } • . . . • } • . . . • } change applet so that method only draws a single dot (repetition to be controlled by JavaScript) have applet keep track of number inside & out (can access & display with JavaScript)

  46. Example (cont.) • <html> • <!-- Monte8.html --> • <head> • <title>Monte Carlo Darts Page</title> • <script type="text/javascript"> • function doAll() • { • var numDots = parseFloat(document.MonteForm.numPoints.value); • for (var i = 0; i < numDots; i++) { • document.MonteApplet.drawDot(); • } • document.MonteForm.numIn.value = document.MonteApplet.numInside; • document.MonteForm.numOut.value = document.MonteApplet.numOutside; • } • function clearAll() • { • document.MonteApplet.drawCircle(); • document.MonteForm.numIn.value = 0; • document.MonteForm.numOut.value = 0; • } • </script> • </head> • <body bgcolor="gray"> • <form name="MonteForm"> • <table align="center"> • <tr><td><applet code="Monte8.class" name="MonteApplet" height=300 width=300> • You must use a Java-enabled browser to view this applet. • </applet> • <td><input type="button" value="Generate" onClick="doAll();"> • <input type="text" name="numPoints" size=6 value=100> points • <p><hr> • <p><input type="text" name="numIn" size=6 value=0> points inside • <p><INPUT TYPE="text" name="numOut" size=6 value=0> points outside • <p><hr> • <p><input type="button" value="Clear the screen" onClick="clearAll()"> • </table> • </form> • </body> • </html> Note: can utilize HTML table to achieve desired layout of elements view page in browser

  47. Accessing HTML/JavaScript from the applet • it is possible for the applet to access elements in the page • requires the JSObject class from the netscape.javascript package import netscape.javascript.JSObject; • usegetWindowandgetMembermethods to access components JSObject jsWin = JSObject.getWindow(this); // GETS WINDOW JSObject jsDoc = (JSObject) jsWin.getMember("document"); // GETS DOCUMENT JSObject MonteForm = (JSObject) jsDoc.getMember("MonteForm"); // GETS FORM numInside = (JSObject) MonteForm.getMember("numIn"); // GETS TEXT BOX • usegetMemberandsetMembermethods to access component attributes int num = Integer.parseInt( (String)numInside.getMember("value") ); numInside.setMember("value", ""+(num+1));

  48. Related topics • JavaBeans • reusable components (e.g., buttons, menus) that can be packaged and reused • requires special tools for compiling and packaging (e.g., BDK) • downloaded with an applet using the ARCHIVES attribute • <applet code="javaApp.class" archives="jarfile.jar"> • JAR files • for applets that are comprised of multiple classes, can bundle all necessary files into a Java Archive (JAR) file • uses the popular ZIP file format • download using ARCHIVES attribute, automatically unzipped by browser

  49. End of Java Applets

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