Problam Error

package calc ;
/**
 *
 * @author me
 */

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;


public class Calcu1 extends JFrame implements ActionListener  {

  // private double mun1,mun2,mun3,mun4,s1,s2;

  private JPanel p1 ;

  private JTextField t1 , t2 ,t3,t4;
  private JLabel l1, l2 ,l3;
  private JLabel   l4,l5,l6 ;

  private JButton b1 , b2 ;
   String ss1= "" ;
   String ss2= "" ;
   String ss3= "" ;
   String ss4= "" ;
   double num1=0;
   double num2=0 ;
   double num3=0;
   double num4=0;
   double s1  =0  ;
   double s2  =0  ;


   Calcu1 ( ){

    this.setTitle("Calcul 4 N");
    this.setSize(500,350);



    p1.setLayout(new BorderLayout());
    b1 = new JButton ("Calcu");
    b2 = new JButton ("Clear");

    b2.addActionListener(this);
    b1.addActionListener(this);

    t1 = new JTextField () ;
    t1.setBackground(Color.ORANGE);
    t1.setFont(new Font("Arial",2,20));
    t1.setHorizontalAlignment(JTextField.CENTER);
    l1 = new JLabel() ;
    l1.setText("+");
    t2 = new JTextField () ;
    t2.setBackground(Color.ORANGE);
    t2.setFont(new Font("Arial",2,20));
    l2 = new JLabel() ;
    l2.setText("=");
    l3 = new JLabel() ;
    l3.setText("");

    t3 = new JTextField () ;
    t3.setBackground(Color.ORANGE);
    t3.setFont(new Font("Arial",2,20));
    l4 = new JLabel() ;
    l4.setText("*");
    t4 = new JTextField () ;
    t4.setBackground(Color.ORANGE);
    t4.setFont(new Font("Arial",2,20));
    l5 = new JLabel() ;
    l5.setText("=");
    l6 = new JLabel() ;
    l6.setText("");

    p1.add(t1,BorderLayout.WEST);
    p1.add(l1,BorderLayout.WEST);
    p1.add(t2,BorderLayout.WEST);
    p1.add(l2,BorderLayout.WEST);
    p1.add(l3,BorderLayout.WEST);

    p1.add(t3,BorderLayout.EAST);
    p1.add(l4,BorderLayout.EAST);
    p1.add(t4,BorderLayout.EAST);
    p1.add(l5,BorderLayout.EAST);
    p1.add(l6,BorderLayout.EAST);

    p1.add(b1, BorderLayout.CENTER);
    p1.add(b2, BorderLayout.CENTER);
    this.setContentPane(p1);
    this.setVisible(true);
    this.setDefaultCloseOperation(EXIT_ON_CLOSE);

   }

 @Override
 public void actionPerformed(ActionEvent e) {
    // TODO Auto-generated method stub
     if (e.getSource() == b1) {
    ss1 = t1.getText() ;
    ss2 = t2.getText() ;
    ss3 = t3.getText() ;
    ss4 = t4.getText() ;
      num1 = Double.parseDouble(ss1);
      num2 = Double.parseDouble(ss2);
      num3 = Double.parseDouble(ss3);
      num4 = Double.parseDouble(ss4);
      s1 = num1+num2 ;
      s2 = num3*num4 ;

     l3.setText(Double.toString(s1));
     l6.setText(Double.toString(s1));}
     if (e.getSource() == b2 )
     {
         String c = "" ;
         t1 = new JTextField () ;
            t1.setText(c);


            t2.setText(c);
            l3.setText(c);
            t3.setText(c);
            t4.setText(c);
            l6.setText(c);
     }
}

}