的JPanel呼籲重繪（X，Y，寬，高）之後逐漸清晰

Question

這裏是我的代碼的一部分：

public final class ImagePanel extends JPanel implements SwingConstants, Printable ,MouseListener { 

private static final long serialVersionUID = 1L; 
private static final int IMAGE_LEFT_INDENT = 30; 
private Image image; 
private int verticalAlignment = CENTER; 
private int horizontalAlignment = CENTER; 
private int image_dy; 
private Color color; 
private int vectorImageIndex = 0; 
public int tempIndex; 
    @Override 
    public Dimension getPreferredSize() { 
    if (image == null) { 
     return super.getPreferredSize(); 
    } else { 
     return new Dimension(image.getWidth(this) + 100, image.getHeight(this) + 100); 
    } 
} 

    @Override 
    public void update(Graphics g) 
    { 

    } 

    @Override 
    protected void paintComponent(Graphics g) { 
    super.paintComponent(g); 

    if (image == null) { 
     return; 
    } 

    Insets insets = getInsets(); 
    int x = insets.left; 
    int y = insets.top; 

    int w = getWidth() - insets.left - insets.right; 
    int h = getHeight() - insets.top - insets.bottom; 

    int src_w = image.getWidth(null); 
    int src_h = image.getHeight(null); 

    double scale_x = ((double) w)/src_w; 
    double scale_y = ((double) h)/src_h; 

    double scale = Math.min(scale_x, scale_y); 

    int dst_w = (int) (scale * src_w); 
    int dst_h = (int) (scale * src_h); 

    int dx = x + (w - dst_w)/2; 
    if (horizontalAlignment == LEFT) { 
     dx = x; 
    } else if (horizontalAlignment == RIGHT) { 
     dx = x + w - dst_w; 
    } 

    int dy = y + (h - dst_h)/2; 
    if (verticalAlignment == TOP) { 
     dy = y; 
    } else if (verticalAlignment == BOTTOM) { 
     dy = y + h - dst_h; 
    } 

    image_dy = dy;  

    if (previousRect != null) { 
     g.setColor(color); 
     g.drawRect((previousRect.x + getInsets().left), dy, previousRect.width, previousRect.height); 
    } else { 
     g.drawImage(image, dx, dy, src_w, src_h, null); 
     g.setColor(Color.YELLOW); 
     for (Rectangle rect : FontExtractor.vectorRectangle) { 
      g.drawRect((rect.x + getInsets().left), dy, rect.width, rect.height); 
     } 
    } 
} 
    public void mouseClicked(MouseEvent e) { 
    if (SwingUtilities.isLeftMouseButton(e)) { 
     int click_x = e.getX(); 
     int click_y = e.getY(); 
     for (Rectangle rect : FontExtractor.vectorRectangle) { 
      if (((rect.x + getInsets().left) <= click_x && (rect.x + getInsets().left + rect.width) >= click_x) && (image_dy <= click_y && (image_dy + rect.height) >= click_y)) { 
       if (previousRect != null) { 
        color = Color.YELLOW; 
        System.out.println("repaint 1"); 
        repaint((previousRect.x + getInsets().left), image_dy, previousRect.width + 1, previousRect.height + 1); 
       } 
       previousRect = rect; 
       color = Color.RED; 
       System.out.println("repaint 2"); 
       repaint((rect.x + getInsets().left), image_dy, rect.width + 1, rect.height + 1); 
       hasBreakCommandExecuted = true; 
       break; 
      } 
     } 
     if ((!hasBreakCommandExecuted) && (previousRect != null)) { 
      color = Color.YELLOW; 
      System.out.println("repaint 3"); 
      repaint((previousRect.x + getInsets().left), image_dy, previousRect.width + 1, previousRect.height + 1); 
      previousRect = null; 
     } else { 
      hasBreakCommandExecuted = false; 
     } 
    } 
} 

}

我做錯了什麼在這裏，everTime當我打電話重繪（Dimension），它調用PaintComponent（Graphics g）並首先清除jpanel背景。請說明問題在哪裏。

Answer 1

這就是AWT/Swing的工作原理。每次調用repaint時，AWT/Swing都會清除該區域，並開始從底部組件到頂部一直繪製該區域。現在，如果您的機器速度很慢，您實際上可以看到發生這種情況，這稱爲閃爍。爲了避免在Swing中閃爍，請嘗試JPanel.setDoubleBuffered（true）。