要实现Android UDP返回随机字符的功能，可以使用以下代码示例：

1. 创建一个UDP服务器端的类，用于接收和处理UDP请求：

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;

public class UDPServer {
    private static final int PORT = 12345; // UDP服务器端口号

    public static void main(String[] args) {
        DatagramSocket socket = null;
        try {
            socket = new DatagramSocket(PORT);

            while (true) {
                byte[] buffer = new byte[1024];
                DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
                socket.receive(packet);

                String request = new String(packet.getData(), 0, packet.getLength());
                System.out.println("Received request: " + request);

                String response = generateRandomString(); // 生成随机字符
                byte[] responseData = response.getBytes();
                DatagramPacket responsePacket = new DatagramPacket(responseData, responseData.length, packet.getAddress(), packet.getPort());
                socket.send(responsePacket);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (socket != null) {
                socket.close();
            }
        }
    }

    private static String generateRandomString() {
        // 在这里生成随机字符的逻辑
        return "RandomString";
    }
}

2. 创建一个UDP客户端的类，用于发送UDP请求并接收服务器的响应：

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class UDPClient {
    private static final int PORT = 12345; // UDP服务器端口号

    public static void main(String[] args) {
        DatagramSocket socket = null;
        try {
            socket = new DatagramSocket();

            String request = "Hello Server"; // 请求消息
            byte[] requestData = request.getBytes();
            DatagramPacket requestPacket = new DatagramPacket(requestData, requestData.length, InetAddress.getLocalHost(), PORT);
            socket.send(requestPacket);

            byte[] buffer = new byte[1024];
            DatagramPacket responsePacket = new DatagramPacket(buffer, buffer.length);
            socket.receive(responsePacket);

            String response = new String(responsePacket.getData(), 0, responsePacket.getLength());
            System.out.println("Received response: " + response);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (socket != null) {
                socket.close();
            }
        }
    }
}

这里使用Java的UDP Socket编程实现了一个简单的UDP服务器和客户端。服务器端接收到客户端的请求后，生成随机字符作为响应，再发送给客户端。客户端发送请求后，等待接收服务器的响应，并打印出来。你可以根据自己的需求修改生成随机字符的逻辑。