why use final here

class MouseEvents {
        public static void main(String args[]) {
                final Frame mainWindow = new Frame("Main Window");
                Button btnClr = new Button("Clear");
                btnClr.addMouseListener(new MouseAdapter() {
                                public void mouseClicked(MouseEvent
me) {
                                        mainWindow.repaint();
                                }
                        } );
                mainWindow.setSize(200,200);
                mainWindow.setLayout(new FlowLayout());
                mainWindow.add(btnClr);
                mainWindow.setVisible (true);
                mainWindow.addMouseListener(new
MyMouseAdapter(mainWindow));
        }
}

class MyMouseAdapter extends MouseAdapter {
        Frame FListener;
        MyMouseAdapter(Frame FListener) {
                this.FListener = FListener;
        }
        public void mouseClicked(MouseEvent me) {
 
FListener.getGraphics().drawString("*",me.getX(),me.getY());
        }
}

The problem is that use of mainWindow (as used for the mouseClick
event of the btnClr) requires mainWindow to be final because *too*
much nesting. Why? Couldn't get it.