Question

I’m writing a Java programm, according to MVC model.

So the problem is that the Frame doesn’t react to button click.
(The text, that I write isn’t added to the TextArea after click)

At first I call constructors for View and Controller

                MessageFrame mf = new MessageFrame(con);
                MessageFrameListener mfl = new MessageFrameListener(mf);

Here is the part of MessageFrameListener class (controller)

public class MessageFrameListener{
private MessageFrame mf;

public MessageFrameListener(MessageFrame m_f){
    mf = m_f;
    m_f.addButtonListener(new SButtonListener());
}

//@Override
public class SButtonListener implements ActionListener{
    public void actionPerformed(ActionEvent e) {
            String insert = mf.getInput();
            mf.addLine(insert);
            mf.refreshInput();
    }
}
}

Here is the part from MessageFrame class (View)

public class MessageFrame{

public JTextField messField;
public JTextArea dialogArea;
public JButton sendButton;
public JFrame frame;
public Contact con;
public MessageFrame (Contact con_get) {
    con = con_get;
    frame = new JFrame();

  frame.setSize(538, 299);

  JPanel panel_1 = new JPanel();
  frame.getContentPane().add(panel_1, BorderLayout.NORTH);

  JPanel panel_2 = new JPanel();
  frame.getContentPane().add(panel_2, BorderLayout.SOUTH);
  panel_2.setLayout(new BoxLayout(panel_2, BoxLayout.X_AXIS));

  messField = new JTextField();
  panel_2.add(messField);
  messField.setColumns(10);

  JButton sendButton = new JButton("Send");
  panel_2.add(sendButton);

  JPanel panel_3 = new JPanel();
  frame.getContentPane().add(panel_3, BorderLayout.EAST);

  JPanel panel_4 = new JPanel();
  frame.getContentPane().add(panel_4, BorderLayout.CENTER);
  panel_4.setLayout(new BorderLayout(0, 0));

  JTextArea dialogArea = new JTextArea();
  panel_4.add(dialogArea);
  frame.setVisible(true);


}

public String getInput(){
    return messField.getText();
}

public void refreshInput(){
    messField.setText("");
}

public void addLine(String line){
    dialogArea.append(line);
}

public void addButtonListener(ActionListener bal){
    sendButton.addActionListener(bal);
}


}

Answer 1

You will definately find the answer if you check the output of your program or debug it.

Exception in thread "main" java.lang.NullPointerException
    at test3.MessageFrame.addButtonListener(Main.java:93)
    at test3.MessageFrameListener.<init>(Main.java:28)
    at test3.Main.main(Main.java:18)

Your are hiding the reference to the JButton sendButton by declaring it again in the constructor so the field is never initialised.

JButton sendButton = new JButton("Send");
panel_2.add(sendButton);