Java Constructor with object array NullPointerException

Question

I've tried to implement a hash table myself without data structures provided by JDK. (such as Linkedlist...) So I tried to implement a hash table by implementing an array of linked lists. However, it returns NullPointerExcetion.

This is main code.

public static void main(String[] args) {
    HTable test = new HTable(500000);
    for(int i = 0; i < 50000; i++) {
        BigInteger s = BigInteger.ONE;
        for(int j = 0; j <= 0; j++) {
            s = s.multiply(BigInteger.valueOf(37));
        }
        if(test.insert(s, String.valueOf(s))) {}
    }
}

and HTable is below.

class HTable {
static int length;
Linkedlist[] HT;
HTable(int size) {
    ...
    Linkedlist[] HT = new Linkedlist[sample];
    for(int j = 0; j < sample; j++) {
        HT[j] = new Linkedlist();
    }
    length = sample;
}
public boolean insert(BigInteger i, String s) {
    int sum = 0;
    String[] i_toString = (i.toString()).split("");
    System.out.println(i_toString[0]);
    int[] data = new int[i_toString.length];
    int[] multiple = new int[i_toString.length];
    data[0] = Integer.parseInt(i_toString[0]);
    multiple[0] = (int) Math.pow(data[0], (int)Math.log10(length));
    ...
    int index = sum % length;
    if(HT[index].isEmpty()) {
        HT[index].insertAtFront(i, s);
        return true;
    }
    for(Iterator v = HT[index].getIterator(); !v.atEnd(); v.next()) {
        if(v.getdata() == i) {
            return false;
        }
    }
    HT[index].insertAtFront(i, s);
    return true;
}

and finally, this is Linkedlist code.

class Linkedlist {
public Node first;

public Linkedlist() {
    first = null;
}

public boolean isEmpty() {
    return (first == null);
}

public void insertAtFront(BigInteger x, String y) {
    Node newnode = new Node();
    newnode.data = x;
    newnode.payload = y;
    newnode.next = first;
    first.prev = newnode;
    first = newnode;
}

I have been struggling to identify the problem, but I could not find a clear solution. However, I found that in inner constructor, HF[index] can be used, but in HTable.insert can't.

Why is that?