Java Applets Road Map Introduction to Java Applets



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Java Applets


Road Map

  • Introduction to Java Applets

  • Review applets that ship with JDK

  • Make our own simple applets

    • Introduce inheritance
    • Introduce the applet environment
  • html needed for applets

  • Reading: The applet coverage in the book is different from the coverage for today. The best review for this material is this PowerPoint presentation.



Applet Viewer and browsers

  • An applet is a program that suns inside an applet viewer.

  • The applet viewer takes care of providing the environment and calling several of the applet’s methods.

  • Modern browsers come with applet capabilities.

  • The JDK also comes with an applet viewer.

    • The JDK applet viewer is really just a minimum browser. It only understands the applet tag.


3.3 Simple Java Applet: Drawing a String

  • Now, create applets of our own

  • Upcoming program

    • Create an applet to display
    • "Welcome to Java!!"
    • Show applet and HTML file, then discuss them line by line




3.3 Simple Java Applet: Drawing a String

    • Import predefined classes grouped into packages
      • When you create applets, import Applet class (in the package java.applet)
      • import the Graphics class (package java.awt) to draw graphics
      • import specifies directory structure


3.3 Simple Java Applet: Drawing a String

    • Applets have at least one class declaration (like applications)
      • Rarely create classes from scratch
        • Use pieces of existing classes
        • Inheritance - create new classes from old ones (this will be the next major topic)
    • Begins class declaration for class WelcomeApplet
      • Keyword class then class name
    • extends followed by class name
      • Indicates class to extend (Applet)
        • Applet : superclass (base class)
        • WelcomeApplet : subclass (derived class)
      • WelcomeApplet now has methods and data of Applet


3.3 Simple Java Applet: Drawing a String

    • Class Applet defined for us
      • Someone else defined "what it means to be an applet"
        • Applets require over 200 methods!
      • extends Applet
        • Inherit methods, do not have to declare them all
      • Do not need to know every detail of class Applet


3.3 Simple Java Applet: Drawing a String

    • Class WelcomeApplet is a blueprint
      • appletviewer or browser creates an object of class WelcomeApplet
        • Keyword public required
        • File can only have one public class
        • public class name must be file name


3.3 Simple Java Applet: Drawing a String

    • Our class inherits method paint from Applet
      • By default, paint has empty body
      • Override (redefine) paint in our class
    • Methods init, start and paint.
      • Guaranteed to be called automatically
      • Our applet gets "free" version of these by inheriting from Applet
        • Free versions have empty body (do nothing)
        • Every applet does not need all three methods
          • Override the ones you need
    • Applet container “draws itself” by calling method paint


3.3 Simple Java Applet: Drawing a String

    • Method paint
      • Draws graphics on screen
      • void indicates paint returns nothing when finishes task
      • Parenthesis define parameter list - where methods receive data to perform tasks
        • Normally, data passed by programmer, as in JOptionPane.showMessageDialog
      • paint gets parameters automatically
        • Graphics object used by paint
      • Mimic paint's first line


3.3 Simple Java Applet: Drawing a String

    • Body of paint
      • Method drawString (of class Graphics)
      • Called using Graphics object g and dot (.)
      • Method name, then parenthesis with arguments
        • First argument: String to draw
        • Second: x coordinate (in pixels) location
        • Third: y coordinate (in pixels) location
    • Java coordinate system
      • Measured in pixels (picture elements)
      • Upper left is (0,0)


3.3 Simple Java Applet: Drawing a String

  • Running the applet

    • Compile
      • As you would a regular file in JCreator
        • or
      • javac WelcomeApplet.java (from the command line)
      • If no errors, bytecodes stored in WelcomeApplet.class
    • Create an HTML file
      • Loads the applet into appletviewer or a browser
      • Ends in .htm or .html
    • To execute an applet


3.3 Simple Java Applet: Drawing a String

    • Simple HTML file (WelcomeApplet.html)
      • Usually in same directory as .class file
      • Remember, .class file created after compilation
    • HTML codes (tags)
      • Usually come in pairs
      • Begin with < and end with >
    • Lines 1 and 4 - begin and end the HTML tags
    • Line 2 - begins tag
      • Specifies code to use for applet
      • Specifies width and height of display area in pixels
    • Line 3 - ends tag

      3.3 Simple Java Applet: Drawing a String

        • appletviewer only understands tags
          • Ignores everything else
          • Minimal browser
        • Executing the applet
          • appletviewer WelcomeApplet.html
          • Perform in directory containing .class file


          3.3 Simple Java Applet: Drawing a String

          • Running the applet in a Web browser



          3.4 Drawing Strings and Lines

          • More applets

            • First example
              • Display two lines of text
              • Use drawString to simulate a new line with two drawString statements
            • Second example
              • Method g.drawLine(x1, y1, x2, y2 )
                • Draws a line from (x1, y1) to (x2, y2)
                • Remember that (0, 0) is upper left
              • Use drawLine to draw a line beneath and above a string


          • import java.awt.*;

          • import java.applet.*;

          • public class WelcomeApplet2 extends Applet {

          • public void init() {

          • }

          • public void paint(Graphics g) {

          • g.drawString( "Welcome to", 25, 25 );

          • g.drawString( "Java Programming!", 25, 40 ); }

          • }



          HTML file Program Output



          WelcomeLines.java 2. Class WelcomeLines (extends Applet) 3. paint 3.1 drawLine 3.2 drawLine 3.3 drawString Program Output

          • import java.awt.*;

          • import java.applet.*;

          • public class WelcomeApplet3 extends Applet {

          • public void init() {

          • }

          • public void paint(Graphics g) {

          • g.drawString("Welcome to Java Programming!", 25, 25 );

          • g.drawLine (15, 10, 210, 10);

          • g.drawLine (15, 30, 210, 30);

          • }

          • }



          HTML file



          3.4 Drawing Strings and Lines

          • Method drawLine of class Graphics

            • Takes as arguments Graphics object and line’s end points
            • X and y coordinate of first endpoint
            • X and y coordinate of second endpoint




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