JTabbedPane shows itself randomly

the problem I encountered is weird for me, because I was doing everything step by step, correctly (in my opinion) and finally when I could say I finished one part of my program it appeared to make a fun of me. The actual problem is that in GUI I created I used a JPanel, then I've put it into a JTabbedPane which I've finally put int开发者_开发技巧o a JFrame. Everything is fine and works apart from times when it doesn't. I know it sounds strange, but after running program once I get what I wanted (Frame with tabbed pane containing panel with some stuff in it) and then when I run it again it either show the correct thing again or just empty frame. The worst thing is that it's so random, I haven't got a clue what can be wrong, I don't even know what exactly should I google to find it out. The code is:

import java.awt.Dimension;
import java.awt.Toolkit;
import javax.swing.*;

public class GUI extends JFrame {

JFrame frame = new JFrame("WakeOnLan script generator");
JPanel panel1 = new JPanel(null);
JTextArea text; //= new JTextArea("test");
JScrollPane scroll = new JScrollPane();
JButton but = new JButton("test");
JTabbedPane tab = new JTabbedPane();

public GUI() {

    Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();
    int w = frame.getSize().width;
    int h = frame.getSize().height;
    int x = (dim.width-w)/3;
    int y = (dim.height-h)/4;

    frame.setSize(500,500);
    frame.setLocation(x,y);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
    frame.setLayout(null);

    createTab1();

    tab.addTab("Tab 1", panel1);
    tab.setVisible(true);
    tab.setBounds(0, 0, 500, 500);

    frame.add(tab);

}

public void createTab1(){

    text = new JTextArea("test");
    text.setVisible(true);

    scroll.setViewportView(text);
    scroll.setBounds(10,10,465,300);

    panel1.setLayout(null);
    panel1.add(scroll);
    panel1.setVisible(true);
    panel1.setSize(500,500);
    //panel.setBackground(Color.blue);
    }

   }

And then I just run it in the main method in other class:

public class GUIStarter {

public static void main(String[] args) {

    GUI start = new GUI();

}

}

So could anyone give me an answer or just a hint? Thank you.

You should call frame.setVisible(true) after adding all your components to your JFrame. So try moving it to the end of your constructor.

Alternatively, you can call frame.validate() after all the components have been added.