分层Java JPanels

Question

我已经覆盖了JPanels，背景有一个图像可以阻止前景在applet的加载上绘制。 setOpaque（false）工作正常（重新绘制两个面板二十次你[？]），但我需要它不透明有BG颜色，不透明的背景导致重新涂抹自己3次，这抹去了顶部面板：

public class LayeredPanelsTest extends JApplet
{
private Image _image;

@Override
public void init()
{
    _image = getImage( getDocumentBase(), "./images/flag.GIF" );
    setSize( 450, 450 );
    setLayout( new OverlayLayout( getContentPane() ) );

    final BottomPanel background = new BottomPanel();
    background.setBounds( 10, 10, 400, 400 );

    final TopPanel foreGround = new TopPanel();
    foreGround.setBounds( 10, 10, 400, 400 );

    add( foreGround );
    add( background );
}

public class BottomPanel extends JPanel
{
    public BottomPanel()
    {
        setBackground(Color.CYAN);
        setLayout( new FlowLayout() );
        setOpaque( true );//with false foreground is painted just fine, but they get repainted like 20 times.
    }

    @Override
    public void paintComponent( final Graphics g )
    {
        super.paintComponent( g );
        g.drawImage( _image, 10, 10, this );
        System.out.println( "Bottom" );
    }
}

public class TopPanel extends JPanel
{
    int _mouseX = 10;

    public TopPanel()
    {
        setLayout( new FlowLayout() );
        setOpaque( false );

        addMouseListener( new MouseAdapter()
        {
            @Override
            public void mousePressed( final MouseEvent evt ) {
                _mouseX = evt.getX();
                repaint();
            }
        });
    }

    @Override
    public void paintComponent( final Graphics g )
    {
        super.paintComponent( g );
        g.drawLine( _mouseX, 0, _mouseX, 80);
        System.out.println( "Top" );
    }
}
}