JTextField将用户输入另存为字符串

Question

So I am attempting to create a login screen that prompts the user with a text box and 2 buttons (Login and Cancel). When the user hits login, I want the value of the JTextField to be stored in a variable or at least be usable. When I attempt to do anything with the playerNameTxt.getText() method, I get an error as if playerNameTxt doesn't exist.

public class GUI extends JPanel implements ActionListener {

protected JTextField playerNameTxt;

public GUI() {
    JTextField playerNameTxt = new JTextField(20);

    JLabel playerNameLbl = new JLabel("Enter Player Name");

    JButton loginBtn = new JButton("Login");
    loginBtn.setVerticalTextPosition(AbstractButton.BOTTOM);
    loginBtn.setHorizontalTextPosition(AbstractButton.LEFT);
    loginBtn.setMnemonic(KeyEvent.VK_D);
    loginBtn.setActionCommand("login");
    loginBtn.addActionListener(this);
    loginBtn.setToolTipText("Click this to Login");

    JButton cancelBtn = new JButton("Cancel");
    cancelBtn.setVerticalTextPosition(AbstractButton.BOTTOM);
    cancelBtn.setHorizontalTextPosition(AbstractButton.RIGHT);
    cancelBtn.setMnemonic(KeyEvent.VK_M);
    cancelBtn.setActionCommand("cancel");
    cancelBtn.addActionListener(this);
    cancelBtn.setToolTipText("Click this to Cancel");

    add(playerNameLbl);
    add(playerNameTxt);
    add(loginBtn);
    add(cancelBtn);
}

public void actionPerformed(ActionEvent e) {
    if ("login".equals(e.getActionCommand())) {
        System.out.println(playerNameTxt);
    } else {
        System.exit(0);
    }
}

private static void createAndShowGUI() {
    JFrame frame = new JFrame("-- Munitions Login --");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLocation(400, 200);

    GUI newContentPane = new GUI();
    newContentPane.setOpaque(true);
    frame.setContentPane(newContentPane);

    frame.pack();
    frame.setVisible(true);
}

public static void main(String[] args) {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            createAndShowGUI();
        }
    });
}
}

Answer 1

You firstly declare the field outside of the constructor, then you declare it inside the constructor again, so that it will be deleted after the constructor returns and the GUI was destroyed, and it will not be usable for your class after the constructor has finished. You should change this line:

JTextField playerNameTxt = new JTextField(20);

to this:

playerNameTxt = new JTextField(20);

Answer 2

In your constructor, you aren't referencing the instance variable playerNameTxt - you're making a new local variable. You should change JTextField playerNameTxt = new JTextField(20); to playerNameTxt = new JTextField(20); (or this.playerNameTxt = new JTextField(20); ) to properly initialize the variable. You should then be able to call methods on it without warnings or errors, assuming everything else is correct.