NullPointerException adding substring from string to array list

Question

What I'm trying to do is add a substring from a String to an ArrayList. basically adding every letter in the string to an index in the ArrayList. After that i have a print statement just to see if the letters were added to the ArrayList (thats the second for loop under makearraylisOfChosenWord). However, when i run this with or without the print statement, it gives me a NullPointerException. Is it because I'm adding the letters to the arraylist in a wrong way in the first for loop? thanks for the help heres the code:

String[] wordList = {"apple", "orange", "strawberry", "banana"};
String chosenWord;

//Make an array list to hold one letter of the chosen word at each index
void makeArrayListOfChosenWord(){
    ArrayList<String> lettersOfChosenWord = new ArrayList<String> ();
    for (int i = 0; i < chosenWord.length(); i++) {
        lettersOfChosenWord.add(chosenWord.substring(i, i+1));
    }
    for (int i = 0; i < lettersOfChosenWord.size(); i++) {
        System.out.println((lettersOfChosenWord.get(i)).toString());
    }

}

//Let the game pick a random word from the word list
void setRandomWord(){
    int wordListLength = wordList.length;
    int pickRandomWord = (int) (Math.random() * wordListLength);
    String createRandomWord = wordList[pickRandomWord];
    chosenWord = createRandomWord;
    System.out.printf("the word is %s letters long", chosenWord.length());
}

Answer 1

I was just thinking about your problem and would try to use an ArrayList of Character instead of String. This is imo ok, because you mentioned that you are splitting up a String into single Characters, so ArrayList<Character>() seems a reasonable approach.

For splitting up the String I would make use of the method toCharArray():

String str = "abcd...";
ArrayList<Character> chars = new ArrayList<Character>();
for (char c : str.toCharArray()) {
  chars.add(c);
}

Answer 2

If you call your setRandomWord method before you call makeArrayListOfChosenWord method no NullPointerException will be thrown. No if-check is needed in your code for this condition.