import java.awt.Container;
import javax.swing.JButton;
import javax.swing.JFrame;
public class Prova02 extends JFrame{
    Prova02(){
        setSize(500, 500);      
        Container ctn = getContentPane();
        ctn.setLayout(null);
        for (int x = 0; x <= 400; x += 31) {
            for (int y = 0; y <= 400; y += 31) {
            JButton btnok = new JButton();
            btnok.setBounds(x, y, 30, 30);
            ctn.add(btnok);
            }
        }
        this.setVisible(true);
    }
    public static void main(String[] args) {
        Prova02 p = new Prova02();
    }
}