public class Okno extends JFrame{
    JFrame frame = new JFrame("Kalkulator");
    public Okno(){
        super("Kalkulator");
        frame.setPreferredSize(new Dimension(160, 250));
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(null);
        frame.pack();
        frame.setVisible(true);
    }
}


public class MojPanel extends JPanel{
    JPanel panel = new JPanel();
    JTextField textField = new JTextField("Właśnie taaak");
    //Buttons
    JButton buttonStart = new JButton("S");
    JButton buttonZero = new JButton("0");
    JButton buttonOne = new JButton("1");
    JButton buttonTwo = new JButton("2");
    JButton buttonThree = new JButton("3");
    JButton buttonFour = new JButton("4");
    JButton buttonFive = new JButton("5");
    JButton buttonSix = new JButton("6");
    JButton buttonSeven = new JButton("7");
    JButton buttonEight = new JButton("8");
    JButton buttonNine = new JButton("9");
    JButton buttonPlus = new JButton("+");
    JButton buttonMinus = new JButton("-");
    JButton buttonMultiplication = new JButton("x");
    JButton buttonDivine = new JButton("/");
    JButton buttonEquals = new JButton("=");

    public MojPanel(){
        panel.add(buttonStart);
        }
    }
}


public class Kalkulator extends MojPanel {
    public static void main(String[] args) {
        Okno okno=new Okno(); // tworzymy nowy obiekt typu Okno (dalej będziemy o nim mówić "okienko"), który zaraz stworzymy
        okno.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // zamknięcie okienka ma być jednoznaczne z zamknięciem programu
    }
}