NullPointer Exception in a java client?

Question

I am coding a simple client, but I always get a NullPointerException at:

            while ((input = in.readLine()) != null) {

Here is my code:

public class ClientTest {

public static String server = "127.0.0.1";
public static int sport = 11111;
public static int cport = 11111;
private static String clientname="";
public static ExecutorService pool = Executors.newCachedThreadPool();
public static BufferedReader in = null;
public static PrintWriter out = null;
public static Socket socket = null;

public ClientTest(){
    this.clientname="";
}


public static void main(String[] args) {

    try {
        socket = new Socket(server, sport);
        out = new PrintWriter(socket.getOutputStream(), true);
        in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
    } catch(IOException e) {
        System.out.println("could not create connection:\n" + e);
        try {
            if(socket != null) socket.close();
        } catch(Exception ex) {
            ex.printStackTrace();
        }
    }
    pool.execute(new Communicator());

}

public static class Communicator implements Runnable {
    @Override
    public void run() {
        String input = "";
        try {
            while ((input = in.readLine()) != null) {//here  i get the NPException
                StringTokenizer tokenizer = new StringTokenizer(input);
                tokenizer.nextToken();

                if(input.startsWith("!open")) {
                    if(tokenizer.hasMoreTokens()) System.out.print(tokenizer.nextToken() + "> ");
                    else System.out.print("> ");
                } 
                else if(input.startsWith("!exit")) {
                    System.out.print("Hallo hallo");
                } 
            }
        } catch(IOException e) {
            System.out.println("An error as occurred while reading from server - \n" + e);
        }
    }

}


}

Why do I get the NullPointerException ? I am still initializing the String input.

Answer 1

My question is, why do I get the NP Exception, I am still initializing the String input!

You would get a NullPointerException there if in is null... which it certainly could be. After all, if anything in the first part of main throws an exception, you catch it, print it out... and then keep going! So if socket.getOutputStream() throws an exception, both in and out will be null.

Moral: catching an exception and just continuing as if nothing had happened is very rarely a good idea.

Answer 2

我认为try块中的某些操作失败并且in尚未初始化，然后在异常之后运行Comunicator并且in仍然为null。