2. 涂鸦板是一个具有趣味性的小程序，它的实现原理是通过鼠标事件，控制鼠标移动结合画图功能实现。鼠标拖拽才开始绘制，松开鼠标则停止绘制。 请用Java图形界面的知识，编写以上软件。

以下是使用Java Swing库创建一个简单的涂鸦板的示例代码：

import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;

public class DoodleBoard extends JFrame {
    private int lastX, lastY;
    private Color currentColor = Color.BLACK;
    private JPanel canvas;

    public DoodleBoard() {
        setTitle("Doodle Board");
        setSize(800, 600);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        canvas = new JPanel() {
            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);
            }
        };

        canvas.setBackground(Color.WHITE);
        canvas.addMouseListener(new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                lastX = e.getX();
                lastY = e.getY();
            }

            @Override
            public void mouseReleased(MouseEvent e) {
                lastX = -1;
                lastY = -1;
            }
        });

        canvas.addMouseMotionListener(new MouseMotionAdapter() {
            @Override
            public void mouseDragged(MouseEvent e) {
                if (lastX != -1 && lastY != -1) {
                    Graphics g = canvas.getGraphics();
                    g.setColor(currentColor);
                    g.drawLine(lastX, lastY, e.getX(), e.getY());
                    lastX = e.getX();
                    lastY = e.getY();
                }
            }
        });

        JButton blackButton = new JButton("Black");
        blackButton.addActionListener(e -> currentColor = Color.BLACK);

        JButton redButton = new JButton("Red");
        redButton.addActionListener(e -> currentColor = Color.RED);

        JButton greenButton = new JButton("Green");
        greenButton.addActionListener(e -> currentColor = Color.GREEN);

        JButton blueButton = new JButton("Blue");
        blueButton.addActionListener(e -> currentColor = Color.BLUE);

        JPanel buttonPanel = new JPanel(new GridLayout(1, 4));
        buttonPanel.add(blackButton);
        buttonPanel.add(redButton);
        buttonPanel.add(greenButton);
        buttonPanel.add(blueButton);

        add(canvas, BorderLayout.CENTER);
        add(buttonPanel, BorderLayout.SOUTH);
        setVisible(true);
    }

    public static void main(String[] args) {
        new DoodleBoard();
    }
}

这个程序创建了一个简单的涂鸦板窗口，可以通过鼠标拖拽来绘制线条，并且可以通过按钮来改变绘制的颜色。运行程序后，你将看到一个具有绘画功能的窗口，可以在上面进行涂鸦。