public class Test extends JFrame{
private JPanel panel;
private JLabel display;
private int btnArray=16;

public Test(){
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setVisible(true);
this.setSize(400,400);

Container contena=this.getContentPane();

panel=new JPanel();
panel.setLayout(new GridLayout(4,4));

display=new JLabel("0");

JButton[] btn=new JButton[btnArray];
for(int i=0; i<btn.length; i++){
btn[i]=new JButton(Integer.toString(i));
panel.add(btn[i]);
}

contena.add(display,BorderLayout.NORTH);
contena.add(panel,BorderLayout.CENTER);
display.setHorizontalAlignment(JLabel.RIGHT);

this.pack();
this.setVisible(true);
}
}