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Activity #2: Packages and Events

Java source code and packages

A package is a set of classes; all the class file are stored together; while you are compiling and developing all the class files must be in a directory the same name as the package. (for purposes of installing a developed package the class files may be stored in a '.zip' file or a '.jar' file; we'll get to that later). The directory is stored relative to your CLASSPATH. All classes defined are visible to each other; classes defined a public are also visible to classes in other packages. By default, the CLASSPATH consists of the builtin java classes and the working directory. Packages can be nested by nesting the directories containing them.

Any class in any package reachable from the CLASSPATH can be used in a Java program by using the fully qualified name. For example, in JEA#1, HelloWeb was defined as: public class HelloWeb extends java.applet.Applet. This declares the HelloWeb is a public class (i.e. available to all packages) and is derived from the class Applet in the package java.applet. The import statement, allows you to refer to classes with just the class name. For example, including the statement import java.applet.Applet in HelloWeb.java, would let you define the class as: public class HelloWeb extends Applet. Notice, the import statement does not change the visibility of the class; it only makes typing the fully qualified name unnecessary. The statement import java.applet.*; allows short references to all the public classes in the package java.applet.

If you do not declare otherwise, classes belong to the default package and are assumed to be located at the root relative to the class path. In order to place a class in a named package, the first non-comment statement must be a package statement. Adding package activity to HelloWeb.java places the class HelloWeb in the package activity. The -d option on the java compiler command (javac) will instruct the compiler where to place the classes and the directories required for the files being compiled. If you change HelloWeb.java to place the class in package activity and compile it with the command: javac -d . HelloWeb.java then the HelloWeb will be put in a subdirectory activity the working directory (it will create the directory if necessary). For more examples, look at The Java Package Tutorial.

Member visibility

Events

// Steve Hirst, NCS 28-Aug-97 
import java.awt.event.*; 
class MousePrintln implements MouseListener { 
   public void mouseClicked(MouseEvent e) { 
      System.out.println("mouseClicked"); 
   } 
   public void mouseEntered(MouseEvent e) { 
      System.out.println("mouseEntered"); 
   } 
   public void mouseExited(MouseEvent e) { 
      System.out.println("mouseExited"); 
   } 
   public void mousePressed(MouseEvent e) { 
      System.out.println("mousePressed"); 
   } 
   public void mouseReleased(MouseEvent e) { 
      System.out.println("mouseReleased"); 
   } 
}

Activity: at last

1. Copy and compile the MousePrintln code above.
2. Modify your HelloWeb.java code to include the following statement in its init() method: addMouseListener(new MousePrintln());
3. Run the applet in the appletviewer and notice the various kinds of events.  When does a mouseClicked event happen?

Combining the steps:

// The Hello World for Java
// Steve H. 19-Aug-97
package activity;

public class HelloWeb extends java.applet.Applet {
   private String message;
   int xPosition;
   int yPosition;
   int xBall;
   int yBall;
   void setPosition( int x, int y ) {
      xPosition=x;
      yPosition=y;
      repaint();
      }
   public void init() {
      message = getParameter("message");
      addMouseListener( new OnClick() );
      xPosition = 25;
      yPosition = 60;
      xBall = 50;
      yBall = 10;
      }
   public void paint( java.awt.Graphics gc ) {
     gc.fillOval(xBall,yBall,10,10);
     gc.drawString(message, xPosition, yPosition);
     gc.drawLine(1,1,xPosition,yPosition);
     }
}

1. It has been placed in a package activity
2. It defines a new method setPosition that changes the position of the message and causes a redraw
3. And, it creates an instance of the class OnClick and adds it to the list of MouseListeners.

// OnClick listens for mouse clicks in HelloWeb
// Steve Hirst; 21-Aug-97
package activity;
import java.awt.event.*;

class OnClick extends MouseAdapter {
   public void mouseReleased( MouseEvent e) {
     int newX = e.getX();
     int newY = e.getY();
     HelloWeb theApplet = (HelloWeb) e.getComponent();
     theApplet.setPosition(newX,newY);
   }
}

More Acitivity

1. Modify HelloWeb.java to match the above code; create OnClick.java; compile both with '-d .' option (you'll have to compile OnClick.java first). Then modify your html document to refer to activity.HelloWeb.class and run appletviewer.
2. When that works, add the following lines to HelloWeb's init() method:

java.awt.Button change = new java.awt.Button("Change Color");
add(change);
3. Recompile and run appletviewer.
4. Finaly, create a method to set Text color and use it to control the color of the message text (Go look at: Class java.awt.Graphics ); create a MouseListener for Button change and add it it's MouseListener list; test it our with appletviewer
5. Beyond finally, look at the other listener interfaces; mess with a couple of them, maybe create Listeners like MousePrintln and attach them to either the applet or the button.

Looking ahead