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

public class Swing2 extends JFrame {
    private JButton[] buttons;

    public Swing2() {
        setTitle("Swing2");
        setSize(400,400);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        getContentPane().setLayout(new GridLayout(4,3));

        buttons = new JButton[12];
        for (int i = 0; i < buttons.length; ++i) {
            buttons[i] = new JButton(Integer.toString(i));
            getContentPane().add(buttons[i]);
        }
    }

    public static void main(String[] args) {
        Swing2 s = new Swing2();
        s.setVisible(true);
    }
}