How do I fix Null Pointer Exception?

Question

I am trying to count the number of vowels in the string. The problem is that it compiles correctly, but when I run it the command window says "Exception in thread 'main' java.lang.NullPointerException at CountVowels.main(CountVowels.java:25).

import javax.swing.JOptionPane;

public class CountVowels
{
    public static void main(String[] args)
    {
        String thingy = JOptionPane.showInputDialog("Please enter a sentence, and I  will count the vowels.");
        int count = 0;

        char[] chars = new char[thingy.length()];
        String[] letters = new String[thingy.length()];

        for(int i = 0; i < thingy.length(); ++i)
        {
            chars[i] = thingy.charAt(i);
        }

        for(int i = 0; i < letters.length; ++i)
        {
            letters[i].valueOf(chars[i]);
        }

        for(int i = 0; i < chars.length; ++i)
        {
            if(letters[i].equals("a") || letters[i].equals("e") || letters[i].equals("i") || letters[i].equals("o") || letters[i].equals("u"))
            {
                ++count;
            }
        }
        System.out.println("There are " + count + " vowels in the string + " + thingy);
    }
}

What causes this, how can I fix it, and how can I prevent it from happening again in the future?

line 25 is my if statement:

if(letters[i].equals("a") || letters[i].equals("e") || letters[i].equals("i") || letters[i].equals("o") || letters[i].equals("u"))
                {
                    ++count;
                }

This line isn't doing what you think it is: letters[i].valueOf(chars[i]); — GriffeyDog
– GriffeyDog, Commented Apr 1, 2014 at 15:31

The Guy with The Hat · Accepted Answer · 2014-04-01 15:36:03Z

3

The line

letters[i].valueOf(chars[i]);

does nothing. If you change it to

letters[i] = String.valueOf(chars[i]);

you will set all the elements in letters to the corresponding characters. That should fix your NullPointerException.

You can also save a lot of code by using a bit of regex trickery:

import javax.swing.JOptionPane;

public class CountVowels
{
    public static void main(String[] args)
    {
        String input = JOptionPane.showInputDialog("Please enter a sentence, and I will count the vowels.");
        String[] letters = input.split("(?<=.)(?=.)");

        int count = 0;

        for(int i = 0; i < letters.length; i++)
        {
            if(letters[i].matches("[aeiou]"))
            {
                count++;
            }
        }
        System.out.println("There are " + count + " vowels in the string + " + input);
    }
}

edited Apr 1, 2014 at 15:36

answered Apr 1, 2014 at 15:30

The Guy with The Hat

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Comments

Community · Accepted Answer · 2017-05-23 10:27:02Z

1

Try using this instead:

letters[i] = "" + chars[i];

Also, you don't need to go through all the hassle of creating new arrays going from String to chars to String. Read this question for more information about accessing each character from a String (so you can just check the letters in the thingy object without creating more arrays)

Get string character by index - Java

edited May 23, 2017 at 10:27

CommunityBot

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answered Apr 1, 2014 at 15:30

jlewkovich

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1 Comment

Beninemm Over a year ago

Thanks for the info, this will definitely save me some time

Benjamin · Accepted Answer · 2014-04-01 15:51:31Z

1

Try This :

      import javax.swing.JOptionPane;
      class CountVowels
     {
       public static void main(String[] args)
     {
      String thingy = JOptionPane.showInputDialog("Please enter a sentence, and I  will count the vowels.");
     int count = 0;
     char[] c=thingy.toCharArray();
     for(int i = 0; i < c.length; ++i)
     {
        if(c[i]=='a' || c[i]=='e' || c[i]=='i' || c[i]=='o' || c[i]=='u')
        {
            ++count;
         }
      }
    System.out.println("There are " + count + " vowels in the string + " + thingy);
   }
 }

output : enter image description here

edited Apr 1, 2014 at 15:51

answered Apr 1, 2014 at 15:40

Benjamin

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Comments

PetrS · Accepted Answer · 2014-04-01 15:30:04Z

0

Array letters is empty. It contains null values.

answered Apr 1, 2014 at 15:30

PetrS

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3 Comments

Beninemm Over a year ago

for(int i = 0; i < letters.length; ++i) { letters[i].valueOf(chars[i]); } Didn't this assign the values?

The Guy with The Hat Over a year ago

@Beninemm No, it didn't.

PetrS Over a year ago

No. You can check it in debug mode.

AJJ · Accepted Answer · 2014-04-01 15:35:56Z

0

There is a mis in your code. When you manipulate with String, the return string should be taken for furthur processing.. See docs

letters[i] = String.valueOf(chars[i]);

answered Apr 1, 2014 at 15:35

AJJ

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