DataOutputStream错误

Question

I keep getting an NullPointerException, and I dont know why. what the class is supposed to do, is to send whatever I write in my jtextfield as writeUTF, but I keep getting a NullPointerException at

dos = new DataOutputStream(socket.getOutputStream());

The GUI code:

public class GUI {

private JFrame frame = new JFrame("Dryck & Ingrediens"); // GUI
private JTextField jtf = new JTextField();// GUI
private JTextArea jl1 = new JTextArea();// GUI
private JList jl = new JList();// GUI
private JScrollPane js = new JScrollPane(jl);// GUI
private DataOutputStream dos;// ServerHandler
private JLabel lab = new JLabel("Ange dryck");//GUI
private JLabel lab1 = new JLabel("Walid Shams");
private JLabel lab2 = new JLabel("Kushtrim Brahimi");
private HashMap<String, ArrayList<String>> drinkar = null;//Controller
private DataInputStream dis;
private Socket socket;
private ServerHandler serverH;

public GUI() {


    frame.getContentPane().setLayout(null);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setBounds(50, 300, 420, 400);
    frame.setResizable(false);
    frame.setVisible(true);
    js.add(jl);
    js.add(jl1);
    jl1.setEditable(false);
    lab.setBounds(170, 20, 130, 20);
    lab1.setBounds(300, 310, 130, 20);
    lab2.setBounds(300, 330, 130,20);
    jtf.setBounds(130, 40, 150, 40);
    jl.setBounds(50, 90, 150, 200);
    jl1.setBounds(210, 90, 150, 200);
    Container con = frame.getContentPane();
    con.setBackground(Color.cyan);

This is where the error accures and where i get the nullpointerexception everytime i type something in my jtextfield.

    jtf.addKeyListener(new KeyListener() {

        public void keyTyped(KeyEvent e) {
        }

        public void keyPressed(KeyEvent e) {
        }

        public void keyReleased(KeyEvent e) {

            if (dos == null) {
                if (jtf.getText().length() > 0) {
                    try {
                        dos = new DataOutputStream(socket.getOutputStream());
                        dos.writeUTF(jtf.getText());
                    } catch (IOException ex) {
                        Logger.getLogger(GUI.class.getName()).log(Level.SEVERE, null, ex);
                    }
                } else {
                    String[] empty = new String[]{""};
                    jl.setListData(empty);
                }
            }

        }
    }
);


    jl.addListSelectionListener(new ListSelectionListener() {

        public void valueChanged(ListSelectionEvent e) {
             if (jl.getSelectedIndex() != -1) {
                 String item = (String) jl.getSelectedValue();
                 jl1.setText("");
                 for (String ingrediens : drinkar.get(item)) {
                     jl1.append(ingrediens + "\n");
                 }
             }else{
                 jl1.setText("");
             }
        }
    });
    frame.add(jtf);
    frame.add(jl);
    frame.add(jl1);
    frame.add(lab);
    frame.add(lab1);
    frame.add(lab2);
    frame.add(js);
}


// tar emot arrayen, lagrar i ny array och visar i JList
public void getArrayList() {

    String[] arr = null;
    for(int i = 0; i < arr.length; i++){
        arr = serverH.setList();
    }
    jl.setListData(arr);


}

public static void main(String[] args) {
    GUI g = new GUI();

}
}

Answer 1

Regarding:

dos = new DataOutputStream(socket.getOutputStream());

The only variable that can throw an NPE on this line is socket which begs the question: where do you initialize socket?

Answer: you don't.

Solution: fix this.

Please always check the variables on the line that throws the NPE as that will show you the cause for the error 99% of the time. You haven't appeared to have tried to do this yet.

Answer 2

You define an instance variable »serverH«

private ServerHandler serverH;

but don't assign any object to it. Therefore you should be facing the NullPointerException. You should assign an object to it, eg

serverH = new ServerHandler();

This should solve your problem regarding the NullPointerException.