Create eventlistener in java pgm

Create eventlistener in Applet

import java.applet.*;

import java.awt.event.*;

import java.awt.*;

public class EventListeners extends Applet

implements ActionListener

{

   TextArea txtArea;

   String Add, Subtract;

   int i = 10, j = 20, sum =0,Sub=0;

   public void init()

  {

      txtArea = new TextArea(10,20);

      txtArea.setEditable(false);

      add(txtArea,"center");

      Button b = new Button("Add");

      Button c = new Button("Subtract");

      b.addActionListener(this);

      c.addActionListener(this);

      add(b);

      add(c);

   }

   public void actionPerformed(ActionEvent e)

  {

      sum = i + j;

      txtArea.setText("");

      txtArea.append("i = "+ i + "\t" + "j = " + j + "\n");

      Button source = (Button)e.getSource();

      if(source.getLabel() == "Add")

     {

         txtArea.append("Sum : " + sum + "\n");

      }

      if(i >j)

     {

         Sub = i - j;

      }

      else

      {

         Sub = j - i;

      }

      if(source.getLabel() == "Subtract")

      {

         txtArea.append("Sub : " + Sub + "\n");

      }

   }

}