11           // Add mouse press listener         
   12   
   13         class MousePressListener implements MouseListener
   14         {  
   15            public void mousePressed(MouseEvent event)
   16            {  
   17               int x = event.getX();
   18               int y = event.getY();
   19               component.moveTo(x, y);
   20            }
   21   
   22            // Do-nothing methods
   23            public void mouseReleased(MouseEvent event) {}
   24            public void mouseClicked(MouseEvent event) {}
   25            public void mouseEntered(MouseEvent event) {}
   26            public void mouseExited(MouseEvent event) {}
   27         }
      
   

• Only the mousePressed event is of interest, but all 5 methods of the MouseListener interface must be implemented. So empty implementations are given for the other 4.

• The MousePressListener class is used to provide a controller for the RectangleComponent. So it needs a way of referencing the component. See line 19.

• The mousePressed method can reference component in this case because:

  • MousePressedListener is an inner class defined in method main
  • component is a final local variable also defined in main.

Note: Inner classes defined in a method can only access local variables of the method if they are declared with the final qualifier!

So component is defined in main:

    ...
    7      public static void main(String[] args)
    8      {        
    9         final RectangleComponent component = new RectangleComponent();
    ...