Question

I cannot seem to find the correct way to write an array of a class. In this form no errors are thrown on compiling, but I receive an error when I try to make use of the array / class. The array of classes is in a class named HashTable (I’m required to write my own for an assignment) and I am testing it with the code below:

theHashTable.insert("aa", "ab");

Here is the HashTable class:

Edit: As pointed out by Aniket, fileNames was not being initialized. I corrected this below but receive the same error.

private class HashTable {       
    private class Value {
        ArrayList<String> fileNames;
        String word;
        Value() {
            fileNames = new ArrayList<String>();
        }
    }

    private int currentSize = 101;
    private Value[] items;
    private HashTable() {
        items = new Value[currentSize];
        for (int i = 0; i < currentSize; i++) items[i] = new Value();
    }

    private int hash(String in) {
        int out = 0;
        for (int i = 0; i < in.length(); i++) out += 37*out+in.charAt(i);
        out %= currentSize;
        if (out < 0) out += currentSize;
        return out;
    }

    public void insert(String inW, String inF) {
        int index = hash(inW);
        index = 0;
        if (items[index].word.length() == 0) {
            items[index].word = inW;
            items[index].fileNames.add(inF);
        }
        else if (items[index].word.compareTo(inW) == 0) items[index].fileNames.add(inF);
        else System.out.println("Collision");
    }
}

Answer 1

fileNames ArrayList in your Values class is never initialized. Write a constructor for Values and Initialize fileNames.