Erro JPanel

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



public class Soma implements ActionListener
{
private JFrame f;
private JTextField tf1,tf2,tf3;
private JButton bt1,bt2;



public Soma()
{
f = new JFrame("Soma");
tf1 = new JTextField(20);
tf2 = new JTextField(20);
tf3 = new JTextField(20);
bt1 = new JButton("Soma");
bt1.addActionListener(this);
bt2 = new JButton("Limpar");
bt2.addActionListener(this);
tf3.setEditable(false);
}
public void actionPerfomed(ActionEvent e)
{
double a,b,x;
if(e.getSource()==bt1)
{
a = Double.valueOf(tf1.getText()).doubleValue();
b = Double.valueOf(tf2.getText()).doubleValue();
{
x = a + b;
tf3.setText(String.valueOf);
 }
}
if(e.getSource()==bt2)
{
tf1.setText(""); 
tf2.setText("");
tf3.setText("");
 }
}
public void launchFrame()
{
JLabel l1 = new JLabel("Valor 1");
JLabel l2 = new JLabel("Valor 2");
JLabel l3 = new JLabel("Resultado"); 



f.setLayout(new GridLayout(3,2)
JPanel p1 = new JPanel();
JPanel p2 = new JPanel();
f.add(l1);
f.add(tf1);
f.add(l2);
f.add(tf2);
f.add(l3);
f.add(tf3);
p1.add(bt1);
p2.add(bt2);
f.add(p1);
f.add(p2);



f.addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent ev)
{System.exit(0);
 }
});
f.pack();
f.setVisible(true);
}
public void main(String args []){
Soma s=new Soma();
s.launchFrame();
 }
}