Ajuda em swing

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

public class Calculadora 
{
  public JFrame f;
  private JButton botao[];
  private JButton soma, div, mult, sub;
  private JLabel result;
  private int number[];
 
 
  public Calculadora()
  {
     f= new JFrame("Calculadora");
  	
     number= new int[10];
  	
     for(int a=0; a<number.length; a++)
      {
       botao[a]= new JButton("a");  	 
      }
  } 	  
  	  
  public void mostra()
  {
     f.setLayout(new GridLayout(4,4));
  	
     for(int a= 0; a<10 ; a++)
     f.add(botao[a]);
   
     f.pack();
     f.setVisible(true);
  }
  
  public static void main(String [] args)
  {
	Calculadora calc= new Calculadora();
	calc.mostra();	  
  }
  	  
}