Trouble with CardLayout and actionPerformed; null exception thrown

Question

Hello. I'm writing my first GUI, and I am using the CardLayout class to do so. I have two panels, and a button on each that "should" navigate to the other. However, when the button is pushed on the first card, I get this exception:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
     at emerensi_BMI.actionPerformed(emerensi_BMI.java:149)
There are many lines after that, but I figured I would start with the first??
Here is the code associated with that line:
public void actionPerformed(ActionEvent e){
     if (e.getSource() == btnReCalculate){
          ((CardLayout)cardPanel.getLayout()).show(cardPanel, "one");
     } else if (e.getSource() == btnCalculate){
          ((CardLayout)cardPanel.getLayout()).show(cardPanel, "two");The last line is the actual line referenced. I am wondering if the "two" is what is null? Here is the code that it's associated with:
String one = new String();
String two = new String();
JPanel cardPanel = new JPanel();
cardPanel.setLayout(new CardLayout());
cardPanel.add(one, Page1);
cardPanel.add(two, Page2);
contentPane.add(cardPanel, BorderLayout.CENTER);If I understand it correctly, the string in the (string, container) is supposed to reference its container, right?
I'm not sure if this will make a difference, but also when the "calculate" button is pressed, I have it so that there will be calculations done with text that is entered on the first page, and the second page will display these calculations. Here is the code for that:
} else if (e.getSource() == btnCalculate){
          ((CardLayout)cardPanel.getLayout()).show(cardPanel, "two");
     String feet = txtFeet.getText();
     String inches =     txtInches.getText();
     String pounds =     txtPounds.getText();
     float f = Float.valueOf(feet);
     float i = Float.valueOf(inches);
     float p = Float.valueOf(pounds);
     float height = (f * 12 + i);
     float math = (p * 703) / (height * height);
     String BMI = String.valueOf(math);
     lblBMI.setText(BMI);
     if (math <= 18.5){
          lblHealthEval.setText("Underweight");
     } else if (math <= 24.9){
          lblHealthEval.setText("Normal");
     } else if (math <= 29.9){
          lblHealthEval.setText("Overweight");
     } else if (30 <= math){
          lblHealthEval.setText("Obese");
     } // end else ifI have labels defined for the second panel that should get the set strings. But since the null exception is thrown right before it regarding the card layout stuff, this probably isn't an issue (yet).
Sorry if this is verbose; just wanted to be thorough. Thanks for any and all help...

Answer 1

Well... I took out as much code as I could. Here's my simplified program:
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
@SuppressWarnings("serial")
public class emerensi_BMI extends JFrame implements ActionListener{
     JLabel lblHeading1;
     JLabel lblHeading2;
     JButton btnCalculate;
     JButton btnReCalculate;
     JPanel Page1;
     JPanel Page2;
     String one;
     String two;
     JPanel cardPanel;
public static void main(String args[]){
          new emerensi_BMI();
     public emerensi_BMI(){
          super("Body Mass Index Calculator");
          setupGUI();
          registerListeners();
     } // end constructor
     public void setupGUI(){
          Container contentPane = getContentPane();
          JPanel pnlHeading1 = new JPanel();
          lblHeading1 = new JLabel("Page 1");
          pnlHeading1.setLayout(new FlowLayout());
          pnlHeading1.add(lblHeading1);
          // north panel for page1
          JPanel pnlCalculate = new JPanel();
          btnCalculate = new JButton("Calculate");
          pnlCalculate.setLayout(new FlowLayout());
          pnlCalculate.add(btnCalculate);
          // south panel for page1
          JPanel Page1 = new JPanel();
          Page1.setLayout(new BorderLayout());
          Page1.add(pnlHeading1, BorderLayout.NORTH);
          Page1.add(pnlCalculate, BorderLayout.SOUTH);
          // page1 panel
          JPanel pnlHeading2 = new JPanel();
          lblHeading2 = new JLabel("Page 2");
          pnlHeading2.setLayout(new FlowLayout());
          pnlHeading2.add(lblHeading2);
          // north panel for page2
          JPanel pnlReCalculate = new JPanel();
          btnReCalculate = new JButton("<-- Recalculate with different data");
          pnlReCalculate.setLayout(new FlowLayout());
          pnlReCalculate.add(btnReCalculate);
          // south panel for page2
          JPanel Page2 = new JPanel();
          Page2.setLayout(new BorderLayout());
          Page2.add(pnlHeading2, BorderLayout.NORTH);
          Page2.add(pnlReCalculate, BorderLayout.SOUTH);
          // page2 panel
          String one = new String();
          String two = new String();
          JPanel cardPanel = new JPanel();
          cardPanel.setLayout(new CardLayout());
          cardPanel.add(Page1, one);
          cardPanel.add(Page2, two);
          contentPane.add(cardPanel, BorderLayout.CENTER);
          // card panel with page1 and page2
          this.setSize(500, 350);
          this.setVisible(true);
          this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     public void itemStateChanged(ItemEvent evt) {
             CardLayout cl = (CardLayout)(cardPanel.getLayout());
             cl.show(cardPanel, (String)evt.getItem());
     public void registerListeners() {
          btnCalculate.addActionListener(this);
          btnReCalculate.addActionListener(this);
          // registered actions
     public void actionPerformed(ActionEvent e){
          if (e.getSource() == btnReCalculate){
               ((CardLayout)cardPanel.getLayout()).show(Page1, one);
          } else if (e.getSource() == btnCalculate){
               ((CardLayout)cardPanel.getLayout()).show(Page2, two);
          } // end else if else if
          // handling action events
     public void textValueChanged(TextEvent e) {
          // TODO Auto-generated method stub
}// end main