用mousemotionlistener画出曲线

好的，下面是一个使用`MouseMotionListener`绘制曲线的例子：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class DrawPanel extends JPanel implements MouseMotionListener {
    private int x, y;
    private Graphics2D g2d;
    private boolean isDrawing = false;

    public DrawPanel() {
        addMouseMotionListener(this);
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (g2d == null) {
            g2d = (Graphics2D) g;
            g2d.setStroke(new BasicStroke(2));
        }
    }

    public void mouseDragged(MouseEvent e) {
        if (!isDrawing) {
            x = e.getX();
            y = e.getY();
            isDrawing = true;
        } else {
            int x2 = e.getX();
            int y2 = e.getY();
            g2d.drawLine(x, y, x2, y2);
            x = x2;
            y = y2;
        }
    }

    public void mouseMoved(MouseEvent e) {
        isDrawing = false;
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("Draw Panel");
        DrawPanel panel = new DrawPanel();
        frame.getContentPane().add(panel);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 400);
        frame.setVisible(true);
    }
}

这个例子创建了一个`DrawPanel`类，它继承了`JPanel`并实现了`MouseMotionListener`接口。`DrawPanel`类在构造函数中注册了自身作为`MouseMotionListener`，并重写了`paintComponent`方法来设置`Graphics2D`对象的线条宽度。当鼠标被拖动时，`mouseDragged`方法会被调用，它绘制了一条线段来连接之前的鼠标位置和当前鼠标位置。当鼠标移动时，`mouseMoved`方法会被调用，它将`isDrawing`标记为`false`，以便在下一次鼠标拖动时开始绘制新的曲线。

当你运行这个例子时，你可以按住鼠标并拖动来绘制曲线。