Null pointer error reading text file and storing it in an arraylist

Question

I've written a main method for it also, but I can't tell the source of the error. This is the code I'm using:

import java.io.*;
import java.util.*;
 public class WordList{
  private ArrayList<String> words;

 public WordList(String filename){
ArrayList<String> words = new ArrayList<String>();
}

public ArrayList<String> openFile(String filename) throws IOException{
FileReader fr= new FileReader(filename);
//create a Filereader object
BufferedReader textReader= new BufferedReader(fr);
//create a BR object
String line = textReader.readLine();
while (textReader.readLine() != null){
  words.add(line);
  textReader.readLine();
}
textReader.close();
return words;
}
Random r= new Random();
public String getRandomWord(){
String x= new String();
int y=r.nextInt(words.size());
x= words.get(y);
return x;

 }
}

This is the main method I used for testing my code:

import java.io.*;
import java.util.*;
 public class Test{
  public void main(String args[])throws IOException{
 String path= "C:/Users/Cyril/Desktop/COMP 202/Assignment 4/Text files/Majors.txt" ;

 try {
WordList list = new WordList(path);
ArrayList<String> majors = new ArrayList<String>();
majors = list.openFile(path);
System.out.println(majors);
}

catch (IOException e){
  System.out.println( e.getMessage());
  }
 }
}

I am getting a null pointer error. I can't find the source of it. The question I had was:

Write a class WordList with a private arraylist that reads a textfile and stores every line as an entry in the arraylist. I've added the random method to generate random words from the arraylist.

The instance should use the interface, e.g. List<String> not ArrayList. — Rob
– Rob, Commented Dec 3, 2013 at 19:25

Community · Accepted Answer · 2017-05-23 11:56:09Z

3

You've declared a local variable which shadows the instance member

public WordList(String filename){
    ArrayList<String> words = new ArrayList<String>();
}

change to

public WordList(String filename){
    words = new ArrayList<String>();
}

But also see Kugathasan's answer...which they just deleted.

This snippet

String line = textReader.readLine();
while (textReader.readLine() != null){
    words.add(line);
    textReader.readLine();
}

You are reading 3 lines from the input stream. Is that what you want?

edited May 23, 2017 at 11:56

CommunityBot

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answered Dec 3, 2013 at 19:23

Sotirios Delimanolis

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Comments

Jean Raymond Daher · Accepted Answer · 2014-10-03 18:44:37Z

0

private ArrayList<String> words;

public WordList(String filename){
ArrayList<String> words = new ArrayList<String>();
}

In this part of your code , the constructor is creating another array list and the one in your class(instance variable ) is not linked to it.

please change your code to :

private ArrayList<String> words;

public WordList(String filename){
this.words = new ArrayList<String>();
}

It should then work :)

answered Oct 3, 2014 at 18:44

Jean Raymond Daher

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Comments

Prabhaker A · Accepted Answer · 2013-12-03 19:26:00Z

0

You are declaring a local variable with same name as instance variable in the constructor instead of assigning the instance variable.
So Change the declaration to assignment in your constructor ,like this.

public WordList(String filename){
   words = new ArrayList<String>();
}

answered Dec 3, 2013 at 19:26

Prabhaker A

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Comments

Nishant · Accepted Answer · 2013-12-03 19:29:30Z

0

Change

public WordList(String filename) {
    ArrayList<String> words = new ArrayList<String>();
}

to

  public WordList(String filename) {
     words = new ArrayList<String>();
}

Because you have shadowed your instance member words in the constructor, words is not initialized and defaults to null and null.Something is a NullPointerException

answered Dec 3, 2013 at 19:29

Nishant

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