Sorting individual elements in a String Array by their alphabetical order

Question

I have to write a program that reads a list of dictionary words from a file. Subsequently, the characters of each word are put into alphabetical order and stored in the original array. (For example: Batman would become aabmnt). Now, here is what I've done so far:

public static String[] alphabeticalOrder(String[] s)
{
    //
    // Sort each individual string element by alphabetical order
    //
    for (int i = 0; i < s.length; i++)
    {
        String wordSt = s[i];
        char[] word = wordSt.toCharArray();
        Arrays.sort(word);
        s[i] = new String(word);
    }
    return s;
}

The call in main is simply: String[] alphaOrder = alphabeticalOrder(dictionary);

However, whenever I run the program, I get a NullPointerException and I can't seem to figure out why.

Changing s[i] to s[0] made me skip the error, but I need to convert all elements in the String, not just the first.

What is going wrong?

Answer 1

This can only happen if your string array has null elements, as Zavior commented.

Change your code to this:

public static String[] alphabeticalOrder(String[] s)
{
    //
    // Sort each individual string element by alphabetical order
    //
    for (int i = 0; i < s.length; i++)
    {
        String wordSt = s[i];
        if(wordSt == null) continue;

        char[] word = wordSt.toCharArray();
        Arrays.sort(word);
        s[i] = new String(word);
    }
    return s;
}

Answer 2

From what I see, NullPointerEception can happen only if one of the elements of the passed String array is null . To debug it, just do a null check before calling wordSt.toCharArray() :

if (wordSt == null) {
    System.out.println("Null encountered at index: " + i + ". Skipping this element...");
    continue;
}
char[] word = wordSt.toCharArray();

This will help you figure out what is wrong with the input, and take any necessary steps to prevent it if this is not expected.

Answer 3

As everyone has pointed out, you might encounter NullPointerException when wordSt is null. Yet another point where you might encounter null is when String[] s itself is null(rare but possible). So I would suggest-

public static String[] alphabeticalOrder(String[] s)
{
    if(s == null || s.length == 0) return s;
    for (int i = 0; i < s.length; i++) {
        if(s[i] != null) {
            char[] word = s[i].toCharArray();
            Arrays.sort(word);
            s[i] = new String(word);
        }
    }
    return s;
}

Hope this helps.