title: Problems calling a method from another Java class
body: <p>I am having a runtime error that states Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
      at Status.copyState(Status.java:29) This is a class that is call the an instance of the Status calls (st): Main class is:</p>
  
  <pre><code>import javax.swing.*;
  import java.awt.event.*;
  import java.awt.*;
  /**
   *
   * 
   * @author (Jason Sizemore) 
   * @version (11-20-09 HW09)
   */
  public class BetterCalculator extends Calculator 
  {   
      //attributes
      private JButton undo; 
      private String undoText;
      private Status st;
  
      public BetterCalculator()
      {
          super();
          st = new Status();
  
      }
      public void createUserInterface3()
      {
          createUserInterface2();
          undo = new JButton("undo");
          jPanel.add(undo);
          undo.setBackground(Color.red);
         undo.setToolTipText("This is the undo feature");
         undo.addActionListener(this);
  
      }
       public void actionPerformed(ActionEvent e)
       {
       super.actionPerformed(e);
       while(displayBox.getText() != null)
       {
           st.copyState();
  
          }
       if(e.getSource() == undo)
        {
  
          Status st;
          st = new Status();
         undoText = st.returnState();
          displayBox.setText(undoText);
  
  
        }
  
      }
      public static void main(String[] args)
      {
  
          BetterCalculator myCalc;
          myCalc = new BetterCalculator();
          myCalc.createUserInterface3();
      }
  }
  </code></pre>
  
  <p>It is call the an instance of Status (st) Here is the Status class</p>
  
  <pre><code>import java.util.*;
  import java.awt.event.*;
  import java.awt.*;
  /**
   * Write a description of class Status here.
   * 
   * @author (Jason Sizemore ) 
   * @version (HW09 11-21-09)
   *  This is a class to get the status for the undo feature
   */
  public class Status extends BasicCalculator
  {   
      //attributes
     private ArrayList&lt;String&gt; lastState;
     public String ls;
  
     public String rls;  
      //constructors
  
      public Status()
      {
         lastState = new ArrayList&lt;String&gt;(10);
  
      }
  
      //Methods
      public void copyState()
      {
           //operand1 = Double.parseDouble(displayBox.getText());
            ls = displayBox.getText();
          lastState.add(ls);
  
      }
      public String returnState()
      {
         int sizeOfArrayList; 
         sizeOfArrayList = lastState.size();
          rls = lastState.get(sizeOfArrayList);
          return rls;
  
      }
  }
  </code></pre>
  
  <p>I know the issue is with the line ls = displayBox.getText(); I have a object take the return of a method on a object. what am I missing here.</p>
  
  <p>Thanks for any help.</p>