Java Swing绘图：解决仅显示最后图形的引用问题

Point startPoint = new Point();
Point endPoint = new Point();

@Override
public void mousePressed(MouseEvent e) {
    startPoint.setLocation(e.getPoint()); // 修改现有startPoint实例的location
}

@Override
public void void mouseReleased(MouseEvent e) {
    endPoint.setLocation(e.getPoint()); // 修改现有endPoint实例的location

    if (object == 0) {
        canvas.addPrimitive(new Line(startPoint, endPoint, temp));
    }
    // ...
}

// Painter Class
public class Painter implements ActionListener, MouseListener, MouseMotionListener {
    // ... 其他成员变量 ...
    Point currentStartPoint; // 用于临时存储当前绘制的起始点
    Point currentEndPoint;   // 用于临时存储当前绘制的结束点

    @Override
    public void mousePressed(MouseEvent e) {
        // 创建一个新的Point实例来存储起始点
        currentStartPoint = new Point(e.getPoint());
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        // 创建一个新的Point实例来存储结束点
        currentEndPoint = new Point(e.getPoint());

        if (object == 0) {
            // 将新创建的Point实例传递给Line构造函数
            canvas.addPrimitive(new Line(currentStartPoint, currentEndPoint, temp));
        } else if (object == 1){
            // 将新创建的Point实例传递给Circle构造函数
            canvas.addPrimitive(new Circle(currentStartPoint, currentEndPoint, temp));
        }

        canvas.repaint();
    }
    // ... 其他方法 ...
}

// Line Class
public class Line extends PaintingPrimitive{
    Point startPoint; // 不再初始化，将在构造函数中赋值
    Point endPoint;   // 不再初始化，将在构造函数中赋值

    public Line(Point start, Point end, Color c) {
        super(c);
        // 对传入的Point对象进行防御性复制，确保Line对象拥有独立的坐标实例
        this.startPoint = new Point(start);
        this.endPoint = new Point(end);
    }

    public void drawGeometry(Graphics g) {
        System.out.println("draw geo called"); // 注意：在实际应用中应避免在绘图方法中进行System.out.println操作，因为它会影响性能
        g.drawLine(startPoint.x, startPoint.y, endPoint.x, endPoint.y);
    }
    // ... 其他方法 ...
}

// PaintingPanel Class
public class PaintingPanel extends JPanel {
    ArrayList<PaintingPrimitive> primitives = new ArrayList<PaintingPrimitive>();

    // ... 构造函数 ...

    public void addPrimitive(PaintingPrimitive obj) {
        primitives.add(obj);
        this.repaint(); // 添加新图形后请求重绘
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g); // 调用父类方法，清空背景

        // 遍历所有存储的图形对象并逐一绘制
        for (PaintingPrimitive shape : primitives) {
            // 示例中的 g.drawLine(0,0,100,100); 应该移除，因为它会每次都绘制一条固定的线
            // g.drawLine(0,0,100,100); 
            shape.draw(g); // 调用图形对象的draw方法进行绘制
        }
    }
}