Why I get the null pointer exception with the array?

Question

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Scanner;

public class Test {
private static ArrayList<Countries> arr =null;
static String country;
static String capital;
static String cities;
static String answer;

public static void main(String args[]) throws IOException{
    Scanner keybord = new Scanner(System.in);
    ArrayList<Countries> d = read("data.txt");
    String res = "";
    for(int i = 0; i < d.size(); i++){
             res += d.get(i).toString();
        answer = keybord.nextLine();
        if(answer.equalsIgnoreCase(d.get(i).getCapital()))
            res+= "The answer is true!";
        else
            res += "The answer is not true" + "The answer is " + d.get(i).getCapital();
        System.out.println(res);
    }
}

public static ArrayList<Countries> read(String filename) throws IOException{
    Scanner scan = new Scanner(new File(filename));
    while(scan.hasNext()){
        country = scan.nextLine(); //System.out.println(country);
        String cities1 = scan.nextLine(); //System.out.println(cities1);
        String cities2 = scan.nextLine(); //System.out.println(cities2);
        String cities3 = scan.nextLine(); //System.out.println(cities3);
        String cities4 = scan.nextLine(); //System.out.println(cities4);
        String capital = scan.nextLine(); //System.out.println(capital);
        Countries c = new Countries(cities1, cities2, cities3, cities4, capital); 
        arr.add(c); // where i get the exception
        scan.nextLine();
    }
    return arr;

}

I could not understand why I'm getting the null pointer exception while I'm trying to add the countries to the array list? should not I instantiate it as a null before creating it?

Why is arr a static field and not just part of the scope of the read method? — Mark Rotteveel
– Mark Rotteveel, Commented Jul 7, 2012 at 20:00

Jigar Joshi · Accepted Answer · 2012-07-07 20:07:37Z

1

You haven't initialized arr so by default it is null

Initialize it by

private static ArrayList<Countries> arr = new ArrayList<Countries>();

Also,

You check if next element exist by

 while(scan.hasNext()){

and then you are reading next 7 line

country = scan.nextLine(); //System.out.println(country);
String cities1 = scan.nextLine(); //System.out.println(cities1);
String cities2 = scan.nextLine(); //System.out.println(cities2);
String cities3 = scan.nextLine(); //System.out.println(cities3);
String cities4 = scan.nextLine(); //System.out.println(cities4);
String capital = scan.nextLine(); //System.out.println(capital);

scan.nextLine();

Which can fail

edited Jul 7, 2012 at 20:07

answered Jul 7, 2012 at 19:57

Jigar Joshi

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5 Comments

sergerde Over a year ago

Now in the top of the code I initialized it but there is still exception error?

Jigar Joshi Over a year ago

Please post the updated code and the new exception stacktrace

sergerde Over a year ago

How should I initialize the instance d for reading the data from the txt file?

sergerde Over a year ago

Exception in thread "main" java.util.NoSuchElementException: No line found at java.util.Scanner.nextLine(Scanner.java:1516) at Test.read(Test.java:41) at Test.main(Test.java:18) it gives this and the line's are String cities4 = scan.nextLine(); //System.out.println(cities4); and ArrayList<Countries> d = read("data.txt");

Pshemo Over a year ago

From what I see you are using scan.nextLine() 7 times in your read(String filename). Does your file contains multiplication of 7 lines (7,14,21,...)?

Oleksi · Accepted Answer · 2012-07-07 19:58:02Z

0

You need to initialize arr:

private static ArrayList<Countries> arr = new ArrayList<Countries>();

If you omit the new ArrayList<Countries>(); part, the default value will be null, and then when you call arr.add(), you get that NullPointerException.

answered Jul 7, 2012 at 19:58

Oleksi

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1 Comment

sergerde Over a year ago

I made it now but there is still something like that. Exception in thread "main" java.util.NoSuchElementException: No line found at java.util.Scanner.nextLine(Scanner.java:1516) at Test.read(Test.java:41) at Test.main(Test.java:18) first error is for this line String cities4 = scan.nextLine(); //System.out.println(cities4); the other one is ArrayList<Countries> d = read("data.txt");

dbf · Accepted Answer · 2012-07-07 19:58:32Z

0

arr variable is not initialized, replace

private static ArrayList<Countries> arr

with

private static ArrayList<Countries> arr = new ArrayList<Countries>();

answered Jul 7, 2012 at 19:58

dbf

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Comments

Yossi · Accepted Answer · 2012-07-07 20:00:09Z

0

You never initialized arr.

Add arr = new ArrayList<Countries>(); to your main.

answered Jul 7, 2012 at 20:00

Yossi

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