Science and Computers -- PHY307/607

More simple Java

Basics

1. Login in to SUnix and move to your PHY307 directory (type cd public_html/PHY307 at the SUnix prompt). Create a new directory with the command mkdir Simple. Change its permissions with command chmod 755 Simple.
2. Go to the Labs section of the PHY307 page and download the two Java source codes Simple.java and Map.java to the new Simple directory.
3. Compile them with the command javac *.java. Change permissions on the .class files with the usual chmod 644 *.class.
4. Create a page to launch this new applet from using pico: pico Simple.html. Put the following line in this file

    < applet code=Simple.class width=250 height=250 > < /applet >
   

5. Change its permissions chmod 644 Simple.html.
6. Point Netscape at web.syr.edu/~your SUnixuserid/PHY307/Simple/Simple.html.
   What do you see ?

 import java.awt.*;

 public class Simple extends java.applet.Applet{

The init() method

The paint() method

g.drawString("x is "+mymap.getX(),100,125);

The Map object

 public class Map{

Making some improvements

1. Edit Simple.java to add the following lines inside the paint() method (i.e between its opening and closing curly braces and before the call to g.drawString())

   g.setColor(Color.yellow);
   g.fillRect(25,25,200,200);
   g.setColor(Color.red);
   

   This changes the color to yellow and calls another method fillRect() to draw a rectangle of size 200 by 200 with its bottom lefthand corner at coordinates (25,25). It then changes the color to red in preparation for the call to drawString(). Exit pico saving your changes.
2. Recompile the Java codes (javac *.java) and reload the applet (holding down the shift key while hitting Reload forces Netscape to reload Java class files). What do you see ?
3. OK, lets do something else now. Edit Simple.java and add the line

   mymap.iterate();

   just before the drawString()method call. Exit, recompile and reload the applet. What do you now see ? You should see that the value of x has now changed as it should and the new value has been written to the screen.
4. It would be nice to have a button on the screen which when you click it automatically iterates the function and writes the new x value to the screen. Lets do this now.
5. First we need to define a button type. At the top of Simple.java underneath the line private Map mymap add the line

   private Button mybutton;

   .
6. Inside the init() method add lines to create the button and add it to the screen:

   mybutton = new Button("Iterate");
   add(mybutton);

7. We're almost there. Finally we must add code that registers when the button is clicked and calls iterate. To do that we must add a new method action() to the Simple object/class. Directly after the paint() method (and before the final closing curly brace of the class definition) add the code

   
   public boolean action(Event evt, Object arg){
   if(evt.target instanceof Button){
   mymap.iterate();
   repaint();
   return true;
   }
   return false;
   }
   

8. This catches clicks of the button, issues the method call mymap.iterate() and then calls repaint() which basically just calls the paint() method again. Finally, delete the original mymap.iterate() call you had put into the paint() method.
9. Exit pico, recompile and reload. What happens on clicking the button ?

1. Email me the url of your applet and (as an attachment) all your source .java files. If you use the email client pine you attach files using the ctrl-J key sequence once you have positioned the cursor over the attachment header line.