Problem adding item to list using a function. Error - Null pointer

Question

Hi Guys,
I'm not sure what the problem is, but for some reason I am unable to add an item to a list using a function. I can add an item when I call it in the same function that the list is created, but I cannot add the item from anoter function. Here is the code:
Main:
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
public class main {
      * @param args
     public static void main(String[] args) {
          JFrame JFrame = new JFrame();
          JFrame.setSize(400,400);
          testpanel panel1 = new testpanel();
          panel1.setBackground(Color.red);
          JPanel panel2 = new testpanel();
          panel2.setBackground(Color.BLUE);
          JFrame.add(panel1);
          JFrame.add(panel2);
          JFrame.setLayout(new FlowLayout());
          //THIS GIVES ERROR:
          //NullPointerException
          panel1.addListItem("test");
          JFrame.setVisible(true);
}And here is the testpanel class:
* To change this template, choose Tools | Templates
* and open the template in the editor.
* testpanel.java
* Created on Apr 7, 2010, 6:10:24 PM
import java.awt.List;
import java.awt.ScrollPane;
import java.awt.event.ActionListener;
* @author Snowraver1
public class testpanel extends javax.swing.JPanel {
     public void addListItem(String string){
          list.add(string);
    /** Creates new form testpanel */
    public testpanel() {
        initComponents();
    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">//GEN-BEGIN:initComponents
    private void initComponents() {
        jButton1 = new javax.swing.JButton();
        ScrollPane scrollPane = new ScrollPane();
        List list = new List();
        jButton1.setText("jButton1");
        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(49, Short.MAX_VALUE)
                .addComponent(jButton1)
                .addGap(46, 46, 46))
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(123, 123, 123)
                .addComponent(jButton1)
                .addContainerGap(139, Short.MAX_VALUE))
        this.add(scrollPane);
        scrollPane.add(list);
    }// </editor-fold>//GEN-END:initComponents
    // Variables declaration - do not modify//GEN-BEGIN:variables
    private javax.swing.JButton jButton1;
    private ScrollPane scrollPane;
    private List list;
    // End of variables declaration//GEN-END:variables
}Thanks for the help!
-- Snow

Answer 1

You should check how you are initializing your variables in your testpanel class. You initialize your button correctly, but your other class variables are being overridden. As a result, your class variables still have a value of 'null'.