JComboBox Null pointer Exception. Need to get Int from box and set int as selected int from box

Question

My program right now needs to load up two text files and I'm sorting them by string of words. JcomboxBox is supposed to allow you to select int between 1 and 4 (the size of the string to compare) so 2 would return "I am" whereas 1 would return "I" I'm getting null pointed exception and I have never used combo boxes before. Please help/

import java.awt.GridLayout;

import javax.swing.JFrame;
import java.awt.event.*;
import javax.swing.*;
import java.awt.*;
import java.io.*;
import java.util.Scanner;
import java.util.Arrays;
import javax.swing.JComboBox;


public class Lab8 extends JPanel
{
  private JPanel text;
  private JComboBox input;
  private JLabel label;
  private JButton load, go,go2;
  private CountList<SuperString> words;
  private String filename;
  private int width = 400;
  private int height = 600;
  private TextArea textarea,textarea2;
  Scanner scan;

  public Lab8()
  {
    Integer [] select = {1,2,3,4};
    JComboBox input = new JComboBox(select);
    text = new JPanel(new GridLayout(1,2));
    go = new JButton("Select Text File");
    go2 = new JButton("Select 2nd Text File");
    label = new JLabel("How many sequenced words would you like to analyze? (Must be => 1)" );
    input.setSelectedIndex(0);

    ButtonListener listener = new ButtonListener();
    go.addActionListener(listener);
    go2.addActionListener(listener);
    input.addActionListener(listener);

    textarea = new TextArea("",0,0,TextArea.SCROLLBARS_VERTICAL_ONLY);
    textarea2 = new TextArea("",0,0,TextArea.SCROLLBARS_VERTICAL_ONLY);
    textarea.setFont(new Font("Helvetica",Font.PLAIN,24));
    textarea2.setFont(new Font("Helvetica",Font.PLAIN,24));
    textarea.setPreferredSize(new Dimension(width,height));
    textarea2.setPreferredSize(new Dimension(width,height));
    setPreferredSize(new Dimension(900,600));
    text.add(textarea);
    text.add(textarea2);
    add(input);
    add(go);
    add(go2);
    add(text);

    textarea.setText("No File Selected");
    textarea2.setText("No File Selected");


  }
    public class ButtonListener implements ActionListener //makes buttons do things
    {
      JFileChooser chooser = new JFileChooser("../Text");


      public void actionPerformed(ActionEvent event)
      {
       Integer N = input.getSelectedIndex();

        if(event.getSource() == go)
      {
        int returnvalue = chooser.showOpenDialog(null);

    if(returnvalue == JFileChooser.APPROVE_OPTION)
    {
      try
      {
        File file = chooser.getSelectedFile();
        filename = file.getName();
        System.err.println(filename);
        scan = new Scanner(file);
      }
      catch (IOException e)
      {
        System.err.println("IO EXCEPTION");
        return;
      }       
    }
    else
    {
      return;
    }

     String[] storage = new String[N];
    words = new CountLinkedList<SuperString>();
   for(int i=1;i<N;i++)
     storage[i] = scan.next().toLowerCase().replace(",","").replace(".","");

    while(scan.hasNext())
    {
      for(int i=0;i<=N-2;i++)
        storage[i] = storage[i+1];
      storage[N-1] = scan.next().toLowerCase();
      storage[N-1] = storage[N-1].replace(",","").replace(".","");
      SuperString ss = new SuperString(storage);
    //  System.out.println(ss);
      words.add(ss );
    }
    scan.close();
    textarea.append("    "+filename+" has wordcount: "+words.size()+
      "\n-------------------------\n\n");

    SuperString[] ss = new SuperString[words.size()];
    int i=0;
    for(SuperString word: words)
    {
      ss[i] = word;
      i++;
    }
    Arrays.sort(ss, new SuperStringCountOrder());
    for(SuperString word : ss)
    {
        textarea.append("  "+word+"\n");     
    }
  }


         if(event.getSource() == go2)
      {
        int returnvalue = chooser.showOpenDialog(null);

    if(returnvalue == JFileChooser.APPROVE_OPTION)
    {
      try
      {
        File file = chooser.getSelectedFile();
        filename = file.getName();
        System.err.println(filename);
        scan = new Scanner(file);
      }
      catch (IOException e)
      {
        System.err.println("IO EXCEPTION");
        return;
      }       
    }
    else
    {
      return;
    }
     String[] storage = new String[N];
    words = new CountLinkedList<SuperString>();
   for(int i=1;i<N;i++)
     storage[i] = scan.next().toLowerCase().replace(",","").replace(".","");

    while(scan.hasNext())
    {
      for(int i=0;i<=N-2;i++)
        storage[i] = storage[i+1];
      storage[N-1] = scan.next().toLowerCase();
      storage[N-1] = storage[N-1].replace(",","").replace(".","");
      SuperString ss = new SuperString(storage);
      words.add(ss );
    }
    scan.close();
    textarea2.append("    "+filename+" has wordcount: "+words.size()+
      "\n-------------------------\n\n");

    SuperString[] ss = new SuperString[words.size()];
    int i=0;
    for(SuperString word: words)
    {
      ss[i] = word;
      i++;
    }
    Arrays.sort(ss, new SuperStringCountOrder());
    for(SuperString word : ss)
    {
        textarea2.append("  "+word+"\n");     
    }
  }

    }
    }


   public static void main(String[] arg)
  {
    JFrame frame = new JFrame("Lab 8");
    frame.getContentPane().add(new Lab8());
    frame.pack();
    frame.setVisible(true);
  }
}

Answer 1

You're re-declaring your JComboBox variable in the constructor and thus shadowing the field found in the class. By doing this the field remains null:

public class Lab8 extends JPanel
{
  private JPanel text;
  private JComboBox input;  // this guy remains null

  // .... etc ....

  public Lab8()
  {
    Integer [] select = {1,2,3,4};

    // the line below initializes a local input variable.
    // this variable is visible only inside of the constructor
    JComboBox input = new JComboBox(select); // ***** here ****

Don't re-declare the variable:

public class Lab8 extends JPanel
{
  private JPanel text;
  private JComboBox input;

  // .... etc ....

  public Lab8()
  {
    Integer [] select = {1,2,3,4};
    // JComboBox input = new JComboBox(select); // ***** here ****
    input = new JComboBox(select); // ***** note difference? *****