JFrame not including certain variables

Question

I am new to using JFrame. I have a JFrame class that is separate from the main class. I have been having trouble getting it to work, like separating it from the main class. I have an issue though where a constructor in my JFrame class is not used, and cannot be used for whatever reason.

This is the class it is called in:

public class Main{ 

public static void main(String[] args) {

    DisplayWindow start = new DisplayWindow();
    start.setVisible(true);

This is the actual JFrame Class:

public class DisplayWindow extends JFrame {

class ButtonOKListener implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        JOptionPane.showMessageDialog(null, "OK Button Clicked");
    }
}

class ButtonCancelListener implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        lblMessage.setText("The Cancel button was clicked");
    }
}

JButton btnOK;
JButton btnCancel;
JMenuBar menubar;
JMenu menu;
JMenuItem menuitem;
JLabel lblMessage;
JPanel pnlHolder;
ButtonOKListener btnOKListener;
ButtonCancelListener btnCancelListener;

public DisplayWindow() {

    //
    menubar = new JMenuBar();
    menu = new JMenu("This is something");
    menuitem = new JMenuItem("File");
    pnlHolder = new JPanel();
    btnOKListener = new ButtonOKListener();
    btnCancelListener = new ButtonCancelListener();

    btnOK = new JButton("OK");
    btnOK.addActionListener(btnOKListener);
    btnOK.setBounds(50, 50, 20, 40);
    btnCancel = new JButton("Cancel");
    btnCancel.addActionListener(btnCancelListener);

    lblMessage = new JLabel();

    pnlHolder.add(menubar);
    menubar.add(menu);
    menu.add(menuitem);
    pnlHolder.add(btnOK);
    pnlHolder.add(btnCancel);
    pnlHolder.add(lblMessage);
    this.add(pnlHolder);
}    
public static void DisplaySet() {

    DisplayWindow GUI = new DisplayWindow();
    GUI.setTitle("Graphical User Interface");
    GUI.setSize(300, 200);
    GUI.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    GUI.setVisible(true);    
}}

DisplaySet is not being used for some reason and I don't know why. Also why do I have to name my public constructor DisplayWindow the same as the class name Display Window? I kind of want to change that. To sum things up. -DisplaySet is not used, and gives me errors when I try to change it. -Explanation: Why do I HAVE to name constructor DisplayWindow the same as the class DisplayWindow? -Am I calling this JFrame class correctly in the main?

Thanks

Answer 1

-DisplaySet is not used, and gives me errors when I try to change it

DisplaySet is static and from what you've described, you DON'T want to call it from within the class's constructor, instead, you could call it from the main method

public static void main(String[] args) {

    DisplayWindow.DisplaySet();

}

Why do I HAVE to name constructor DisplayWindow the same as the class DisplayWindow?

Because that's a requirement of the language, some languages use init as the constructor, Java uses the class's name.

You could have a look at Providing Constructors for Your Classes and Chapter 8.8. Constructor Declarations of the Java Language Specifications for more details

-Am I calling this JFrame class correctly in the main?

Yes, although general convention/wisdom would discourage you from extending directly from a top level container like JFrame, as it looks you into a single use case and you're not actually adding any functionality to the frame

You might also find Code Conventions for the Java TM Programming Language, useful, it will make it easier for people to read your code and for you to read others