exception in thread main java.lang.nullpointerexception

Question

(Sorry guys,  I dont have chinese input on this machine)
The following simple code can compile, but give the following error message after execution:exception in thread main java.lang.nullpointerexception
 at TCPserver.main(TCPServer.java:20)
import java.io.*;
import java.net.*;
class TCPServer {
    public static void main(String argv[]) throws Exception
    {
        String clientSentence = "";
        String capitalizedSentence = "";
        ServerSocket welcomeSocket = new ServerSocket(6789);
        
Socket connectionSocket = welcomeSocket.accept();

BufferedReader inFromClient = 
new BufferedReader(new InputStreamReader(
connectionSocket.getInputStream()));
DataOutputStream outToClient = 
new DataOutputStream(
connectionSocket.getOutputStream());
while(true) {
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
            outToClient.writeBytes(capitalizedSentence);
        }
    }
}
can someone help me identify what the problem is?Many thanks.

Answer 1

clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';  // Should be here.When "inFromClient.readLine()" reach the end of stream, it will return null.So "clientSentence" could be null!