多人聊天室的那个视频上的代码，为什么创建内部类对象时，显示错误  cannot be resolved to a type

public void startServer(){

while(true) {

Socket socket = server.accept();

System.out.println("用户进入聊天室");

allSocket.add(socket);

ServerThread t = new ServerThread(socket);//ServerThread cannot be resolved to a type

t.start();

}

class ServerThread extends Thread{

Socket socket;

public void run() {

BufferedReader br = null;

br = new BufferedReader(new InputStreamReader(socket.getInputStream()));

while(true) {

String str = br.readLine();

if(str.contains("EXIT")) {

String tmp = str.split(":")[1]+"用户退出聊天室";

sendMessageToAllClient(tmp);

allSocket.remove(socket);

socket.close();

return;//停止线程

}

}

}

}