单击JButton时退出JFrame

Question

I am writing a Login GUI and want the cancel button to close the entire program when clicked. I'm a freshman computer science major and still semi-new to java and programming in general. Here is my code:

Main Class:

public class loginGui
{
    public static void main(String[] args)
    {
        lGui gui = new lGui();
        lGui.gui();
    }
}

GUI class:

public class lGui
{
    public static void gui()
    {
        JFrame frame;
        JTextField field;
        JLabel l;
        JPasswordField p;
        JButton login, cancel;
        JCheckBox check;

        frame = new JFrame("Login");
        frame.setSize(300, 150);
        frame.getContentPane().setBackground(Color.LIGHT_GRAY);
        frame.setLocation(300, 200);
        frame.setLayout(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);

        l = new JLabel("Username: ");
        l.setLocation(15, 14);
        l.setSize(l.getPreferredSize());
        frame.add(l);

        field = new JTextField("Username");
        field.setColumns(15);
        field.setSize(field.getPreferredSize());
        field.setBackground(Color.DARK_GRAY);
        field.setForeground(Color.LIGHT_GRAY);
        field.setLocation(90, 10);
        field.setToolTipText("Enter User Name");
        frame.add(field);

        l = new JLabel("Password: ");
        l.setLocation(15, 54);
        l.setSize(l.getPreferredSize());
        frame.add(l);

        p = new JPasswordField("Password");
        p.setColumns(15);
        p.setSize(p.getPreferredSize());
        p.setBackground(Color.DARK_GRAY);
        p.setForeground(Color.LIGHT_GRAY);
        p.setLocation(90, 50);
        p.setToolTipText("Enter Password");
        frame.add(p);

        login = new JButton("Login");
        login.setSize(login.getPreferredSize());
        login.setLocation(195, 78);
        login.setToolTipText("Login");
        frame.add(login);
        login.addActionListener(new loginAction());

        cancel = new JButton("Cancel");
        cancel.setSize(cancel.getPreferredSize());
        cancel.setLocation(95, 78);
        cancel.setToolTipText("Cancel");
        frame.add(cancel);
        cancel.addActionListener(new cancelAction());

        check = new JCheckBox("Remember me?");
        check.setSize(check.getPreferredSize());
        check.setLocation(120, 100);
        check.setToolTipText("Remember your username for next time");
        frame.add(check);

        frame.setVisible(true);
    }

    static class cancelAction implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            frame.dispose();            
        }
    }

    static class loginAction implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {

        }
    }
}

I keep getting a "cannot find symbol" error here in the cancel button ActionListener:

frame.dispose();

Answer 1

frame only has context from within your static method gui . Start by getting rid of the static declaration and make frame an instance field of the class

public class lGui
{
    private JFrame frame;
    private JTextField field;
    private JLabel l;
    private JPasswordField p;
    private JButton login, cancel;
    private JCheckBox check;

    public void gui()
    {
        //...

You also won't need the static declarations on the inner classes...

protected class cancelAction implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {
        frame.dispose();            
    }
}

protected class loginAction implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {

    }
}

You might find it easier to make initialise the UI from within the classes constructor instead of the gui method, leaving it to show the window

You should also avoid using null layouts, pixel perfect layouts are an illusion within modern ui design. There are too many factors which affect the individual size of components, none of which you can control. Swing was designed to work with layout managers at the core, discarding these will lead to no end of issues and problems that you will spend more and more time trying to rectify

Instead, have a look at Laying Out Components Within a Container for some more ideas

Answer 2

gui方法中的代码必须在构造函数中，并且您的JFrame对象必须在任何方法之外定义为类的字段：)