



Basic Events


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Sometimes it's easiest way to enable events is to just have a class implement a Listener interface or
extend an Adapter class. The key restriction to keep in mind here is that
a class can only extend one class, but classes may implement any number of
interfaces.


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Implementing An Interface: 
import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class type extends Applet implements KeyListener{
  Label title, letter, win;
  char let;

  public void init(){
    let='a';
    title=new Label("Type the letter: ");
    title.setFont(new Font("Serif", Font.BOLD, 24));
    letter=new Label(""+let);
    letter.setFont(new Font("Serif", Font.BOLD, 24));
    win=new Label("Type the letters presented to the right!");
    setBackground(Color.yellow);
    addKeyListener(this);
    requestFocus();
    add(title);
    add(letter);
    add(win);
  }

  public void keyPressed(KeyEvent k){ }
  public void keyReleased(KeyEvent k){ }
  public void keyTyped(KeyEvent k) {
    char x = k.getKeyChar();
    char y = letter.getText().charAt(0);
    if(x==y && let<122) letter.setText(""+(++let));
    else if(let>=122){
      win.setText("YOU HAVE COMPLETED THIS ACTIVITY!");
    }          
  }
}
//<applet code=type.class height=200 width=400></applet>





There are two items you should pay special attention to in this little
applet:
1. When you add a key listener you write "addKeyListener(this);" The "this"
refers to the class you are in which in this case implements KeyListener. So,
the class serves as its own key listener. You can see that it provides
method definitions for keyTyped(), keyPressed(), and keyReleased().

2. Also take notice of the "requestFocus();" line. This simply lets the
applet know that it is the focus of keyboard events. Without this line it
doesn't realize that it needs to pay attention to keyboard stuff.

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Another Example:
import java.awt.event.*;
import java.awt.*;
import java.applet.*;

public class m1 extends Applet implements MouseListener{
  int r, g, b;
  Label l;

  public void init(){
    setBackground(new Color(r,g,b));
    l=new Label("Click Many Times!");
    l.setForeground(Color.cyan);
    add(l);
    addMouseListener(this);
  }

  public void mouseClicked(MouseEvent m){
    if(r<233) r+=20;
    else if(r>220 && g<233) g+=20;
    else if(r>220 && g>220) {
      r=0;
      g=0;
    }
    setBackground(new Color(r,g,b));
    l.setBackground(new Color(r,g,b));
    repaint();
  }    
  public void mouseEntered(MouseEvent m){}
  public void mouseExited(MouseEvent m){}
  public void mousePressed(MouseEvent m){}
  public void mouseReleased(MouseEvent m){}
}
//<applet code=m1.class height=200 width=200></applet>
<br>
You should take note that in this example it was necessary
to call the repaint() method. Without this method call, the
background of the applet does not get changed. The interesting thing
is that this method call is not necessary when you run the applet in
the appletviewer, BUT when the applet is viewed with Internet Explorer
4.0 it is necessary. The question is, what about Navigator 4.0??? The
repaint() method is defined in the Component class.





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Assignment: You will create an applet which listens for key
strokes and responds by changing the background color when certain keys
are entered. For instance, the background color should change to blue
when the "b" key is pressed. Here are the color to letter mappings you
should implement:

         Key     |    Color
       -----------------------
         y       |    yellow
         b       |    blue
         g       |    green
         r       |    red
         p       |    purple
         m       |    magenta
         o       |    orange
         x       |    black
         w       |    white
         c       |    cyan

To create a couple of these colors you will need to use the Color constructor
which takes three int arguments corresponding to red, green, and blue color
values. See the above code sample to see how it is used.