Ограничение ввода букв

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
public class test extends JFrame {
private JButton bSum;
private JButton bRaz;
private JLabel lA,lB,lC;
private JTextField tfA,tfB,tfC;
private int a,b,c;
test(String s)
{
super(s);
setLayout(null);
setSize(300, 250);
setVisible(true);
this.setDefaultCloseOperation(EXIT_ON_CLOSE);
this.setResizable(true);
lA=new JLabel("a:");
lA.setBounds(20, 85, 40, 20);
add(lA);
tfA=new JTextField();
tfA.setBounds(50, 85, 50, 20);
add(tfA);
lB=new JLabel("b:");
lB.setBounds(20, 125, 40, 20);
add(lB);
tfB=new JTextField();
tfB.setBounds(50, 125, 50, 20);
add(tfB);
lC=new JLabel("c:");
lC.setBounds(20, 165, 40, 20);
add(lC);
tfC=new JTextField();
tfC.setBounds(50, 165, 50, 20);
add(tfC);
bSum=new JButton("Сумма");
bSum.setBounds(140,45,130,30);
add(bSum);
bSum.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
a=Integer.parseInt(tfA.getText());
b=Integer.parseInt(tfB.getText());
c=a+b;
tfC.setText(String.valueOf(c));
}});
bRaz=new JButton("Разница");
bRaz.setBounds(140,90,140,30);
add(bRaz);
bRaz.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
a=Integer.parseInt(tfA.getText());
b=Integer.parseInt(tfB.getText());
c=a-b;
tfC.setText(String.valueOf(c));
}
});
}
 
public static void main(String[] args) {
new test("Калькулятор");
}
}