错误：java.lang.NullPointerException

Question

I'm trying to code a text analysis program, and one of the methods is supposed to write a file containing every word started by a certain letter and the number of occurences of that word, using a text file as input ( BufferedReader and whatnot).

If input is: "O Pai Natal nao existe. Mas o coelho da Pascoa existe (ea Fada dos Dentes tambem)."

Output for e should be: EXISTE 2E 1 My code so far is as follows – I know I could have used a HashMap , but this is school work and we haven't learned that yet, so I'm trying to keep it within what was lectured –:

void WordList(char c, String file) throws IOException {

    BufferedWriter out = new BufferedWriter(new FileWriter(ficheiro+".txt"));
    String[] words = null;
    int indexVector = 0;
    String[] wordAlreadyWritten = null;
    int indexVector2 = 0;

    for (int i = 0; i < text.length; i++) {
        if (text[i].charAt(0) == c) {
            words[indexVector] = texto[i];  /*error here */
            indexVector++;
        }
    }
    for (int i = 0; i < words.length; i++) {   /*error here*/
        if (!wordWasAlreadyWritten(words[i], wordAlreadyWritten)) {
            out.write(words[i] + ": " + countNumberRepetitions(words[i], words));
            wordAlreadyWritten[indexVector2] = words[i];   /* error here*/
            indexVector2++;
        }
    }
    out.close();
}

I am also using some auxiliary methods as some may have noticed:

static boolean wordWasAlreadyWritten(String original, String[] array) {
    boolean was = false;

    for (int j = 0; j < array.length; j++) {
        if (original.equals(array[j])) {
            was = true;
        }
    }
    return was;
}

static int countNumberRepetitions(String word, String[] array) {
    int counter = 1;
    for (int i = 0; i < array.length; i++) {
        if (word.equals(array[i])) {
            contador++;
        }
    }
    return counter;
}

If someone can explain the java.lang.NullPointerException I'm getting and how to fix it, I would be very thankful. Also if someone has any idea on how to make my code better, it would also be greatly appreciated.

Answer 1

Initial both the String array

String[] words = new String[count];    // count any integer value 
String[] wordAlreadyWritten = new String[count];    // count any integer value

Due to which you are facing error.

Answer 2

Few Q's:

1. Where is text defined?
2. I do not see any memory allocation for 'words' String array. This will cause NPE.

Another suggestion:

You haven't studied HashMap but still you can write your own HashMap like implementation or closely resemble to that. I am sure you will learn something out of it.