public byte[] keys() {
	int length = 0;
	for(int i = 0; i < array.length; i++) {
	    if (array[i] != null)
		length++;
	}
	byte[] result = new byte[length];
	int j = 0;
	for(int i = 0; i < array.length; i++) {
	    if (array[i] != null) {
		result[j] = (byte) i;
		j++;
	    }
	}
	return result;
    }