Question

This is the code that i am using (its not mine)

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


public class SimpleChatClient {
JTextArea incoming;
JTextField outgoing;
BufferedReader reader;
PrintWriter writer;
Socket sock;

public void go() {
    JFrame frame = new JFrame("Ludicrously Simple Chat Client");
    JPanel mainPanel = new JPanel();
    incoming = new JTextArea(15, 50);
    incoming.setLineWrap(true);
    incoming.setWrapStyleWord(true);
    incoming.setEditable(false);
    JScrollPane qScroller = new JScrollPane(incoming);
    qScroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
    qScroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
    outgoing = new JTextField(20);
    JButton sendButton = new JButton("Send");
    sendButton.addActionListener(new SendButtonListener());
    mainPanel.add(qScroller);
    mainPanel.add(outgoing);
    mainPanel.add(sendButton);
    frame.getContentPane().add(BorderLayout.CENTER, mainPanel);
    setUpNetworking();

    Thread readerThread = new Thread(new IncomingReader());
    readerThread.start();

    frame.setSize(650, 500);
    frame.setVisible(true);

}

private void setUpNetworking() {
    try {
        sock = new Socket("127.0.0.1", 5000);
        InputStreamReader streamReader = new InputStreamReader(sock.getInputStream());
        reader = new BufferedReader(streamReader);
        writer = new PrintWriter(sock.getOutputStream());
        System.out.println("networking established");
    }
    catch(IOException ex)
    {
        ex.printStackTrace();
    }
}

public class SendButtonListener implements ActionListener {
    public void actionPerformed(ActionEvent ev) {
        try {
            writer.println(outgoing.getText());
            writer.flush();

        }
        catch (Exception ex) {
            ex.printStackTrace();
        }
        outgoing.setText("");
        outgoing.requestFocus();
    }
}

public static void main(String[] args) {
    new SimpleChatClient().go();
}

class IncomingReader implements Runnable {
    public void run() {
        String message;
        try {
            while ((message = reader.readLine()) != null) {
                System.out.println("client read " + message);
                incoming.append(message + "\n");
            }
        } catch (IOException ex)
        {
            ex.printStackTrace();
        }
    }
}

}

My question is in the incoming reader class
Why does this line—- while ((message = reader.readLine()) != null)
never return null?
Supposedly, the thread went to check this line and on the other side there are no messages for the client, then shouldn’t the above line return null?

Can anyone explain a bit of what’s going on? I do know about socket connections and all
I just want to know what’s going on with getting incoming messages.

Answer 1

You are using blocking functions such as BufferedReader.readLine(). This will block until there is something to read. It won’t “check the device and return null if nothing is returned”.

You can check if there is any incoming data using BufferedReader.available() and call readLine() if the result is > 0.