Advanced Internet Programming CSY3020

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Advanced Internet Programming CSY3020 Java Applets The three Java Applet examples produce a very rudimentary drawing applet. An Applet is compiled Java which is normally run within a browser.

1 Advanced Internet Programming CSY3020 Java Applets The three Java Applet examples produce a very rudimentary drawing applet. An Applet is compiled Java which is normally run within a browser. Java applets can be tested with the java applet viewer. Example 1 Below is a screen dump of the first Applet which allows the user to sketch with the mouse The code for the applet is as follows import java.applet.*; import java.awt.*; /* <applet code="scribble.class" width=300 height=200> </applet> */ // Compile: javac -deprecation Scribble // Run: appletviewer Scribble.java

2 public class Scribble extends Applet private int last_x = 0; private int last_y = 0; public boolean mousedown (Event e, int x, int y) public boolean mousedrag (Event e, int x, int y) Graphics g = getgraphics (); g.drawline (last_x, last_y, x, y); To compile and run the code, enter from the command line: javac -deprecation Scribble appletviewer Scribble.java The code consists of two call back methods mousedown and mousedrag which are called when a mouse is moved or when it is dragged (With the mouse button held down). The examples use deprecated code and AWT instead of Swing and will generate warnings. Exercise: The HTML code within the java source informs the appletviewer application to launch the class file as an applet in a 300 by 200 pixel frame. Copy the HTML code to a file and save it with an html extension. This file can then be opened by a browser which will launch the class file. Ensure that the html file and the class file are in the same directory.

3 Example 2 The second applet allows the user to clear the screen and starts a new drawing by clicking on a button. Below is a screen dump of the Applet. The code for the Applet is as follows: import java.applet.*; import java.awt.*; /* <applet code="scribble2.class" width=300 height=200> </applet> */ // Compile: javac -deprecation Scribble2 // Run: appletviewer Scribble2.java public class Scribble2 extends Applet private int last_x = 0; private int last_y = 0; private Button clear_button; public void init () this.setbackground (Color.white); clear_button = new Button ("Clear"); this.add (clear_button);

4 public boolean action (Event e, Object arg) if (e.target == clear_button) Graphics g = this.getgraphics (); Rectangle r = this.getbounds (); g.setcolor (this.getbackground ()); g.fillrect (r.x, r.y, r.width, r.height); else return super.action (e, arg); public boolean mousedown (Event e, int x, int y) public boolean mousedrag (Event e, int x, int y) Graphics g = getgraphics (); g.drawline (last_x, last_y, x, y); To compile and run the Applet enter at the command line: javac -deprecation Scribble2 appletviewer Scribble2.java Two further methods have been added as well as a button variable. The methods are init which is invoked when the Applet is first called and action which is called when any event takes place. The init method sets up the clear button and the action method deals with when a user clicks the clear button.

Example 3 The final example lets you choose to draw in four different colours by selecting the colours from a pull-down menu. Below is a screen dump of the final Applet.

5 Example 3 The final example lets you choose to draw in four different colours by selecting the colours from a pull-down menu. Below is a screen dump of the final Applet. The code to draw in colours follows. import java.applet.*; import java.awt.*; /* <applet code="scribble3.class" width=300 height=200> </applet> */ // Compile: javac Scribble3 // Run: appletviewer Scribble3.java public class Scribble3 extends Applet private int last_x = 0; private int last_y = 0; private Button clear_button; private Choice colour_choices; private Color current_colour = Color.black; public void init () this.setbackground (Color.white); clear_button = new Button ("Clear"); this.add (clear_button); colour_choices = new Choice ();

6 colour_choices.additem ("black"); colour_choices.additem ("red"); colour_choices.additem ("yellow"); colour_choices.additem ("green"); this.add (new Label ("Colour: ")); this.add (colour_choices); public boolean action (Event e, Object arg) if (e.target == clear_button) Graphics g = this.getgraphics (); Rectangle r = this.bounds (); g.setcolor (this.getbackground ()); g.fillrect (r.x, r.y, r.width, r.height); else if (e.target == colour_choices) if (arg.equals ("black")) current_colour = Color.black; else if (arg.equals ("red")) current_colour = Color.red; else if (arg.equals ("yellow")) current_colour = Color.yellow; else if (arg.equals ("green")) current_colour = Color.green; else return super.action (e, arg); public boolean mousedown (Event e, int x, int y) public boolean mousedrag (Event e, int x, int y) Graphics g = getgraphics (); g.setcolor (current_colour); g.drawline (last_x, last_y, x, y); Exercise: Try adding to the Applets the ability to draw not only lines but also rectangles and ellipses. Hint: use g.drawrect (x, y, h, w) and g.drawoval (x, y, h, w) where x and y are the co-ordinates of the top left of the rectangle or ellipse and h and w are the height and width of the rectangle and ellipse

CSC System Development with Java Introduction to Java Applets Budditha Hettige

CSC System Development with Java Introduction to Java Applets Budditha Hettige CSC 308 2.0 System Development with Java Introduction to Java Applets Budditha Hettige Department of Statistics and Computer Science What is an applet? applet: a Java program that can be inserted into