Question

I’m trying to implement a dictionary with a hash table (not using Java’s provided hash table classes, but rather made from scratch). Below is the insert() method from my Dictionary class, used to insert an element into a linked list contained in the particular array position.

I am running a supplied test program to determine if my Dictionary class works, but I am encountering a ArrayIndexOutOfBoundsException: -5980 when reaching a certain point. Included below is the particular test. Why would this exception be coming up? (I can provide more code if needed!)

Insert:

public int insert(DictEntry pair) throws DictionaryException {
    String entryConfig = pair.getConfig();
    int found = find(entryConfig);

    if (found != -1) {
        throw new DictionaryException("Pair already in dictionary.");
    }

    int entryPosition = hash(entryConfig);

    if (dict[entryPosition] == null) { //Dictionary.java:54
        LinkedList<DictEntry> list = new LinkedList<DictEntry>();
        dict[entryPosition] = list;
        list.add(pair);
        return 0;
    } else {
        LinkedList<DictEntry> list = dict[entryPosition];
        list.addLast(pair);
        return 1;
    }
}

The test:

    // Test 7: insert 10000 different values into the Dictionary
    // NOTE: Dictionary is of size 9901
    try {
        for (int i = 0; i < 10000; ++i) {
            s = (new Integer(i)).toString();
            for (int j = 0; j < 5; ++j) s += s;
            collisions += dict.insert(new DictEntry(s,i)); //TestDict.java:69
        }
        System.out.println("   Test 7 succeeded");
    } catch (DictionaryException e) {
        System.out.println("***Test 7 failed");
    }

Exception stack trace:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -5980
    at Dictionary.insert(Dictionary.java:54)
    at TestDict.main(TestDict.java:69)

Hash function:

private int hash(String config) {
int expBase = 41;
int exp;
int ASCIIChar;
int hashedConfig = 0;

    for (int i = 0; i < config.length(); i++) {
        ASCIIChar = (int)config.charAt(i);
        exp = (int)Math.pow(expBase, i);
        hashedConfig = hashedConfig + (ASCIIChar * exp);
    }

    hashedConfig = hashedConfig % dictSize;
    return hashedConfig;
}

Answer 1

Your

exp = (int)Math.pow(expBase, i);
hashedConfig = hashedConfig + (ASCIIChar * exp);

will overflow integer range, therefore generate negative numbers. Add a Math.abs before returning hashedConfig.

You probably should do an analysis how this affects the distribution of your hash function.