Problems calling a method from another Java class

Question

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:

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();
    }
}

It is call the an instance of Status (st) Here is the Status class

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<String> lastState;
   public String ls;

   public String rls;  
    //constructors

    public Status()
    {
       lastState = new ArrayList<String>(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;

    }
}

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.

Thanks for any help.

displayBox is inherited from the Calculator class which this class extends — Sizemj
– Sizemj, Commented Nov 23, 2009 at 17:30

David Hedlund · Accepted Answer · 2009-11-23 17:24:13Z

2

My guess is that displayBox is not set to anything by the time copyState is called. Where is displayBox even declared?

answered Nov 23, 2009 at 17:24

David Hedlund

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4 Comments

Chip Uni Over a year ago

I'm guessing that displayBox is created in the class Calculator.

David Hedlund Over a year ago

i'm guessing it's declared there, but not instantiated, as he gets an NPE at copyState, it clearly isn't there. If there was something in getText() that caused the error, that's where the error would be, in the stack trace, so it's not getting there, and if ls is null, that wouldn't matter. So it's definitely a case of displayBox being null.

Sizemj Over a year ago

Yes the displayBox class is created in the BasicCalculator class

PSpeed Over a year ago

Assuming the exception is accurate, we can state with 99.999% certainty that displayBox is null in copyState(). Stick a log or a debug println in to prove it and then work back from there. A full stack trace would be necessary to guess further... and access to the superclass where displayBox is created.

TofuBeer · Accepted Answer · 2009-11-23 19:00:38Z

As others have said it is likely that the displayBox variable is null.

The best way to deal with this is at compile time... if you get into the following habit you will catch that sort of mistake early:

public class TestFrame
    extends JFrame
{
    private final JButton aButton;
    private JButton bButton;

    public TestFrame()
    {
        // fails to compile since aButton is not instantiated
        // aButton = new JButton("A"); 
    }

    public init()
    {
        add(aButton);
        add(bButton);
        aButton.addActionListener(....);

        // crash at runtime since bButton is null
        bButton.addActionListener(....); 
    }
}

by declaring variables as "final" the compiler forces you to give them a value. Since the buttons (and other GUI items) are not likely to change you should be able to make them all final. Once you have that you will have the com;iler help you out to avoid this sort of thing.

Chip Uni · Accepted Answer · 2009-11-23 17:30:50Z

0

Try adding the following line as the first line of Status.Status():

super();

Let me know if that fixes the problem!

answered Nov 23, 2009 at 17:30

Chip Uni

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3 Comments

Andreas Dolk Over a year ago

... and the same in BasicCalculator, if BasicCalculator subclasses Calculator. If it's the other way round it could work ;)

Chip Uni Over a year ago

I didn't see that Status extended BasicCalculator, but Size_j only provided BetterCalculator. I shouldn't have guessed the hierarchy of classes.

PSpeed Over a year ago

super() will be called implicitly in subclass constructors if no other super(...) call is specified. It is good practice to call it but not required. It will make no difference here.

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