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Java Applets

Java Applets. A First Program. Applet Example. /* The world’s simplest program, repeated once more in another format in an attempt to turn all of you into zombies */ import javax.swing.*; // swing package import java.awt.*; // graphics package public class HelloWorld extends JApplet {

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Java Applets

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  1. Java Applets A First Program

  2. Applet Example /* The world’s simplest program, repeated once more in another format in an attempt to turn all of you into zombies */ import javax.swing.*; // swing package import java.awt.*; // graphics package public class HelloWorld extends JApplet { public void paint( Graphics g ) { g.drawString( "Hello World!", 0, 12 ); } } To create applets, we inherit from the JApplet class so we don’t rewrite the code to make an applet work! “drawString” is a method from the Graphics object named g

  3. Applet Example • Change some things and see if it works, or if you get errors • Change the coordinates from “0,12” to “0,0” – where does it go? • Add a second line of text – how do you align it under “Hello World”? • Try to center the text /* The world’s simplest program, repeated once more in another format in an attempt to turn all of you into zombies */ import javax.swing.*; // swing package import java.awt.*; // graphics package public class HelloWorld extends JApplet { public void paint( Graphics g ) { g.drawString( "Hello World!", 0, 12 ); } }

  4. Header Comments • A bloc of comments (crosses several lines) needs an opening and closing comment marker. /* opens a comment; */ closes • An in-line comment (one line only) requires a double slash // at the beginning of the line • Comments will be ignored by the compiler

  5. import import packagename.*; // imports all classes in package OR import packagename.classname; //imports a specific class in a package Examples: import java.awt.*; import javax.swing.*; import javax.swing.JApplet; • Java provides classes with graphics abilities • To use these classes we need to IMPORT the packages (java.awt package) • This way, we can reuse code already written instead of writing it ourselves! • Must be at HEAD of file The * designates to include ALL classes within the package

  6. Class Declaration: extends JApplet • Applets need to extend (inherit) the JApplet class: public class xyz extends JApplet This way, we can reuse code already written instead of writing it ourselves! • This class inherits many capabilities, including what to do to make applets work

  7. Paint Method • public void paint(Graphics g) { g.drawString(“Hello World”, 30, 20); } • We’ve imported the Graphics class, which includes the “drawString()” method, which lets us print text. • To use the Graphics class, we have to declare a variable/object “g” to which we can then apply the drawString() method. • The drawString() method has three parameters: the text, the x-coordinate, and the y-coordinate.

  8. X-Y coordinates • X-coordinate: horizontal measure in pixels, beginning at upper left corner of screen • Y-coordinate: vertical measure in pixels, beginning at upper left corner of screen 0,0 30,20

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