Events & Listeners

Question

Hi, I'm trying toget a handle on events and listeners and am studying the following classes for this purpose. I'm pretty comfortable with understanding regular applets, but am confused by a couple of things in the following.
1.) How is the Dots class able to instantiate a Point object if the Dots program does not import a Point class?
My guess here is that the Point class is a sub class of the MouseEvent class, which is made available to the program via the java.awt.event.* import statement. If this is correct, is this an example of the inheritance concept?
2.) How is the Dots class able to instantiate a DotsMouseListener object (in the init() method) if the Dots program doesn't import the DotsMouseListener class?
My guess here is that since the constructor in the DotsMouseListener class takes as its parameter the Dots applet, the Dots class is somehow able to instantiate DotsMouseListener objects. If this is true, though, I don't get it.
Thanks for your help . . .
DOTS.JAVA
import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
public class Dots extends Applet
   private final int APPLET_WIDTH = 200;
   private final int APPLET_HEIGHT = 100;
   private final int RADIUS = 6;
   private Point clickPoint = null;
   //  Creates a listener for mouse events for this applet.
   public void init()
      DotsMouseListener listener = new DotsMouseListener(this);
      addMouseListener(listener);
      setBackground (Color.black);
      setSize (APPLET_WIDTH, APPLET_HEIGHT);
   //  Draws the dot at the appropriate location.
   public void paint (Graphics page)
      page.setColor (Color.green);
      if (clickPoint != null)
         page.fillOval (clickPoint.x - RADIUS, clickPoint.y - RADIUS,
                        RADIUS * 2, RADIUS * 2);
   //  Sets the point at which to draw the next dot.
   public void setPoint (Point point)
      clickPoint = point;
DOTSMOUSELISTENER.JAVA
import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
class DotsMouseListener implements MouseListener
   private Dots applet;
   //  Stores a reference to the applet.
   public DotsMouseListener (Dots applet)
      this.applet = applet;
   //  Determines the point at which the mouse is clicked, sets the
   //  point in the applet, then forces the applet to repaint.
   public void mouseClicked (MouseEvent event)
      Point clickPoint = event.getPoint();
      applet.setPoint (clickPoint);
      applet.repaint();
   //  Provide empty definitions for unused event methods.
   public void mousePressed (MouseEvent event) {}
   public void mouseReleased (MouseEvent event) {}
   public void mouseEntered (MouseEvent event) {}
   public void mouseExited (MouseEvent event) {}
}

Answer 1

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1.) How is the Dots class able to instantiate a Point
object if the Dots program does not import a Point
class?It imports java.awt.*, which contains Point
The * implies that you are importing all classes in the java.awt package.
My guess here is that the Point class is a sub class
of the MouseEvent class, which is made available to
the program via the java.awt.event.* import statement.
If this is correct, is this an example of the
inheritance concept?This is not an example of inheritance, it is Composition. A basic explanation used very often is is a and has a
Is a = Inheritance
Has a = Composition
eg. Man is a Mammal. So, you can say that Man inherits from the base class Mammal.
On the other hand,
Man has a Dog
Dog also inherits from Mammal. But as related to Man, it is Composition.
2.) How is the Dots class able to instantiate a
DotsMouseListener object (in the init() method) if the
Dots program doesn't import the DotsMouseListener
class?You need to import a class/package only if the class is in a different package(directory) than the class in which it is being used.
i.e., If Dots and DotListener are in the same directory, say C:\java\code then you do not need to import DotListener.
The reason you need to import the API classes is that they are not in the same directory as your code.
Hope This Helps