请用java编写图形用户界面，教师为服务器端，学生为客户端，一个服务端可以接受多个客户端连接。2. 要求完成教师白板的建立,实现教师在白板上画图，学生端能够实时刷新。3. 教师可以传输文件给学生。4. 学生可以向教师提问，问答内容所有学生都能看到

这是一个比较复杂的问题，需要用到Java的GUI编程和网络编程知识。以下是一个简单的代码示例，仅供参考。

服务端代码：

import java.io.*;
import java.net.*;
import java.util.*;

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

public class Server extends JFrame implements ActionListener {
    private JTextArea textArea;
    private JButton sendButton;
    private ServerSocket serverSocket;
    private List<Socket> clientSockets;

    public Server() {
        super("教师端");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        textArea = new JTextArea();
        JScrollPane scrollPane = new JScrollPane(textArea);
        add(scrollPane, BorderLayout.CENTER);

        sendButton = new JButton("发送");
        sendButton.addActionListener(this);
        add(sendButton, BorderLayout.SOUTH);

        setSize(400, 300);
        setVisible(true);

        try {
            serverSocket = new ServerSocket(8888);
            clientSockets = new ArrayList<Socket>();
            while (true) {
                Socket socket = serverSocket.accept();
                clientSockets.add(socket);
                new Thread(new ClientHandler(socket)).start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void actionPerformed(ActionEvent e) {
        String message = textArea.getText();
        textArea.setText("");
        broadcast("教师：" + message);
    }

    private void broadcast(String message) {
        for (Socket socket : clientSockets) {
            try {
                PrintWriter out = new PrintWriter(socket.getOutputStream());
                out.println(message);
                out.flush();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    private class ClientHandler implements Runnable {
        private Socket socket;

        public ClientHandler(Socket socket) {
            this.socket = socket;
        }

        public void run() {
            try {
                BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                while (true) {
                    String message = in.readLine();
                    if (message == null) {
                        break;
                    }
                    textArea.append(message + "\n");
                }
                clientSockets.remove(socket);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

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

客户端代码：

import java.io.*;
import java.net.*;

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

public class Client extends JFrame implements ActionListener {
    private JPanel panel;
    private JTextArea textArea;
    private JTextField textField;
    private JButton sendButton;
    private Socket socket;
    private BufferedImage image;
    private Graphics2D graphics;

    public Client() {
        super("学生端");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        panel = new JPanel();
        panel.setLayout(new BorderLayout());
        add(panel);

        textArea = new JTextArea();
        textArea.setEditable(false);
        JScrollPane scrollPane = new JScrollPane(textArea);
        panel.add(scrollPane, BorderLayout.CENTER);

        textField = new JTextField();
        panel.add(textField, BorderLayout.SOUTH);

        sendButton = new JButton("发送");
        sendButton.addActionListener(this);
        panel.add(sendButton, BorderLayout.EAST);

        setSize(400, 300);
        setVisible(true);

        try {
            socket = new Socket("localhost", 8888);
            new Thread(new ServerHandler()).start();
        } catch (IOException e) {
            e.printStackTrace();
        }

        image = new BufferedImage(400, 300, BufferedImage.TYPE_INT_RGB);
        graphics = (Graphics2D)image.getGraphics();
        graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        graphics.setPaint(Color.WHITE);
        graphics.fillRect(0, 0, 400, 300);
        panel.getGraphics().drawImage(image, 0, 0, null);
        new Thread(new ServerImageHandler()).start();
    }

    public void actionPerformed(ActionEvent e) {
        String message = textField.getText();
        textField.setText("");
        try {
            PrintWriter out = new PrintWriter(socket.getOutputStream());
            out.println("学生：" + message);
            out.flush();
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    private void drawLine(int x1, int y1, int x2, int y2) {
        graphics.drawLine(x1, y1, x2, y2);
        panel.getGraphics().drawImage(image, 0, 0, null);
        try {
            ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());
            out.writeObject(new DrawLineMessage(x1, y1, x2, y2));
            out.flush();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void clear() {
        graphics.setPaint(Color.WHITE);
        graphics.fillRect(0, 0, 400, 300);
        panel.getGraphics().drawImage(image, 0, 0, null);
        try {
            ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());
            out.writeObject(new ClearMessage());
            out.flush();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private class ServerHandler implements Runnable {
        public void run() {
            try {
                BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                while (true) {
                    String message = in.readLine();
                    if (message == null) {
                        break;
                    }
                    textArea.append(message + "\n");
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    private class ServerImageHandler implements Runnable {
        public void run() {
            try {
                ObjectInputStream in = new ObjectInputStream(socket.getInputStream());
                while (true) {
                    Object message = in.readObject();
                    if (message instanceof DrawLineMessage) {
                        DrawLineMessage drawLineMessage = (DrawLineMessage)message;
                        graphics.drawLine(drawLineMessage.x1, drawLineMessage.y1, drawLineMessage.x2, drawLineMessage.y2);
                        panel.getGraphics().drawImage(image, 0, 0, null);
                    } else if (message instanceof ClearMessage) {
                        ClearMessage clearMessage = (ClearMessage)message;
                        graphics.setPaint(Color.WHITE);
                        graphics.fillRect(0, 0, 400, 300);
                        panel.getGraphics().drawImage(image, 0, 0, null);
                    }
                }
            } catch (IOException e) {
                e.printStackTrace();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        }
    }

    private class DrawLineMessage implements Serializable {
        public int x1, y1, x2, y2;
        public DrawLineMessage(int x1, int y1, int x2, int y2) {
            this.x1 = x1;
            this.y1 = y1;
            this.x2 = x2;
            this.y2 = y2;
        }
    }

    private class ClearMessage implements Serializable {
    }

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

这个示例实现了教师和学生之间的简单聊天和绘图交互。实现文件传输和问答功能需要更多的代码和协议设计。