Java 滑鼠拖曳繪圖：MouseListener、MouseMotionListener、雙緩衝防閃爍

核心介面

• MouseListener：`mousePressed` (起點)、`mouseReleased` (終點)、`mouseClicked`、`mouseEntered`、`mouseExited`。
• MouseMotionListener：`mouseDragged` (拖曳中)、`mouseMoved` (單純移動)。

完整範例：拖曳畫線

public class MouseDrawDemo extends JFrame
        implements MouseListener, MouseMotionListener {

    private int startX, startY, endX, endY;
    private boolean dragging = false;

    public MouseDrawDemo() {
        addMouseListener(this);
        addMouseMotionListener(this);
        setSize(600, 400);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }

    public void mousePressed(MouseEvent e) {
        startX = e.getX(); startY = e.getY();
        dragging = true;
    }

    public void mouseReleased(MouseEvent e) {
        endX = e.getX(); endY = e.getY();
        dragging = false;
        repaint(); // 確保最終線條繪製
    }

    public void mouseDragged(MouseEvent e) {
        endX = e.getX(); endY = e.getY();
        repaint(); // 即時回饋
    }

    // 未使用事件留空
    public void mouseMoved(MouseEvent e) {}
    public void mouseClicked(MouseEvent e) {}
    public void mouseEntered(MouseEvent e) {}
    public void mouseExited(MouseEvent e) {}

    // 雙緩衝防閃爍
    @Override
    public void update(Graphics g) {
        paint(g); // 直接呼叫 paint，JFrame 預設雙緩衝 (RepaintManager)
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g); // 清除背景
        g.setColor(Color.BLACK);
        if (dragging) {
            g.drawString("拖曳中: (" + endX + "," + endY + ")", 10, 20);
            g.drawLine(startX, startY, endX, endY); // 即時預覽線
        } else {
            g.drawString("最後線段: (" + startX + "," + startY + ") -> (" + endX + "," + endY + ")", 10, 20);
            g.drawLine(startX, startY, endX, endY); // 固定線
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(MouseDrawDemo::new);
    }
}

進階：多線段/多圖形/橡皮筋效果

• 多線段：`List` 存儲所有完成線段，`paint` 迴圈繪製；拖曳時只畫預覽線。
• 橡皮筋矩形/橢圓：`mouseDragged` 計算 `Rectangle` (min/max X/Y)，`paint` 畫 `drawRect`/`drawOval`。
• 雙緩衝手動實現 (舊版 JDK/JFrame 非預設)：
  

  private Image offscreen;
      public void update(Graphics g) {
          if (offscreen == null) offscreen = createImage(getWidth(), getHeight());
          Graphics og = offscreen.getGraphics();
          og.setColor(getBackground()); og.fillRect(0, 0, getWidth(), getHeight());
          paint(og);
          g.drawImage(offscreen, 0, 0, this);
      }

事件處理最佳實踐