Question

Hey guys I’m trying to make a GUI which can navigate through JTextAreas when a Next or Previous button is pushed. I’m using CardLayout to switch between the JTextArea. I can get the program to compile but every time I push the next or previous button I always get a NullPointerException. I don’t know why the CardLayout object is null in the next and previous methods and its making me really confused.

The error is coming from this piece of code

cl.show(cardPanel, "" + (currentCard));

Any help is appreciated thx guys!

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;


public class AsciiCanvas extends JTextArea
{

private JPanel jp1;
private JPanel jp2;
private JPanel jp3;
private JPanel jp4;
private JPanel jp5;
private JPanel jp6;
private JPanel jp7;
private JPanel jp8;
private JPanel jp9;
private JPanel jp10;
private JTextArea text1;
private JTextArea text2;
private JTextArea text3;
private JTextArea text4;
private JTextArea text5;
private JTextArea text6;
private JTextArea text7;
private JTextArea text8;
private JTextArea text9;
private JTextArea text10;

private JPanel cardPanel;
private CardLayout cl;
private int currentCard = 1;

private JFileChooser fc;


public AsciiCanvas()
{
   Font mono = new Font("Monospaced", Font.PLAIN, 12);

    cardPanel = new JPanel();
    CardLayout cl = new CardLayout();
    cardPanel.setLayout(cl);


    JTextArea text1 = new JTextArea();
    JTextArea text2 = new JTextArea();
    JTextArea text3 = new JTextArea();
    JTextArea text4 = new JTextArea();
    JTextArea text5 = new JTextArea();
    JTextArea text6 = new JTextArea();
    JTextArea text7 = new JTextArea();
    JTextArea text8 = new JTextArea();
    JTextArea text9 = new JTextArea();
    JTextArea text10 = new JTextArea();



   JPanel jp1 = new JPanel();
   JPanel jp2 = new JPanel();
   JPanel jp3 = new JPanel();
   JPanel jp4 = new JPanel();
   JPanel jp5 = new JPanel();
   JPanel jp6 = new JPanel();
   JPanel jp7 = new JPanel();
   JPanel jp8 = new JPanel();
   JPanel jp9 = new JPanel();
   JPanel jp10 = new JPanel();

   jp1.add(text1);
   jp2.add(text2);
   jp3.add(text3);
   jp4.add(text4);
   jp5.add(text5);
   jp6.add(text6);
   jp7.add(text7);
   jp8.add(text8);
   jp9.add(text9);
   jp10.add(text10);

   cardPanel.add(jp1, "1");
   cardPanel.add(jp2, "2");
   cardPanel.add(jp3, "3");
   cardPanel.add(jp4, "4");
   cardPanel.add(jp5, "5");
   cardPanel.add(jp6, "6");
   cardPanel.add(jp7, "7");
   cardPanel.add(jp8, "8");
   cardPanel.add(jp9, "9");
   cardPanel.add(jp10, "10");

   setBorder(BorderFactory.createTitledBorder("Animat ion here"));
   setFont(mono);

   fc = new JFileChooser();
}


public void Next()
{
  if(currentCard < 10)
  {

     currentCard +=1;
     cl.show(cardPanel, "" + (currentCard));
  }


}

public void Previous()
{
  if(currentCard > 1)
  {
     currentCard -= 1;
     cl.show(cardPanel, "" + (currentCard));

  }
}

Error Message***

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at AsciiCanvas.Next(AsciiCanvas.java:108)

Answer 1

The problem is that cl is null, and here’s why:

private CardLayout cl;

...

public AsciiCanvas()
{
    ...
    CardLayout cl = new CardLayout();

In that constructor you’ve declared a new local variable, rather than assigning a value to the instance variable. The local variable shadows or hides the instance variable. Your consructor code should be:

public AsciiCanvas()
{
    ...
    cl = new CardLayout();

(As an aside, I’d strongly recommend using String.valueOf(...) instead of "" + ... to convert a value to a string in a null-safe way. You’re not really interested in string concatenation, are you?)