@Override
    public Set<K> keySet() {
        Set<K> keys = new HashSet<K>();

        for (int i = 0; i < this.map.length; i++) {
            if (this.map[i] != null && this.map[i].getKey() != null) {
                keys.add(this.map[i].getKey());
            }
        }
        
        return keys;
    }