public class RoachPopulation
{
   public RoachPopulation(int initialPopulation)
   {
      pop = initialPopulation;
   }
 
   public void doublePopulation()
   {
      pop = pop * 2;
   }
 
   public int getPopulation()
   {
      return pop;
   }
 
   private int pop;
}

import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
 
/**
   This program displays the growth of a roach population.
*/
public class RoachPopulationViewer
{
   public static void main(String[] args)
   {
      JFrame frame = new JFrame();
 
      JButton button = new JButton("Double roach population!");
 
      final RoachPopulation roachPopulation = new RoachPopulation(2);
 
      // The label for displaying the results
      JLabel label = new JLabel(
            "population: " + roachPopulation.getPopulation());
 
      // The panel that holds the user interface components
      JPanel panel = new JPanel();
      panel.add(button);
      panel.add(label);
      frame.add(panel);
 
      //your code goes here
      class ButtonListener implements ActionListener
      {
		  public void actionPerformed(ActionEvent e)
		  {
			  roachPopulation.doublePopulation();
		  }
	  }
 
	  ActionListener listener = new ButtonListener();
	  button.addActionListener(listener);
 
      frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setVisible(true);
   }
 
   private static final int FRAME_WIDTH = 400;
   private static final int FRAME_HEIGHT = 100;
}

roachPopulation.doublePopulation();
lable.setText( roachPopulation.getPopulation() );