Math.random() problem: Index 10000 out of bounds for length 10000

There is a task for using BubbleSort algorythm: create array and add there 10000 numbers randomly. How to fix this method:

class Bubble{
public void randomInsert(long n) {
    for(int j=0; j<nElems; j++)
        n = (long)(java.lang.Math.random()*(nElems-1));
        a[nElems] = n;
        nElems++;
}}

To use here:

class NewBubbleApp {
public static void main(String[] args) {
    int maxSize = 10000;
    Bubble bubarr = new Bubble(maxSize);
    
    bubarr.randomInsert();
    bubarr.bubbleSort();
    bubarr.display();

}}

Here's the whole code:

 class Bubble{
private long[] a;
private int nElems;

public Bubble(int nElems) {
    this.nElems = nElems;
    a = new long[nElems];
    nElems = 0;
}

public void randomInsert(long n) {
    for(int j=0; j<nElems; j++)
        n = (long)(java.lang.Math.random()*(nElems-1));
        a[nElems] = n;
        nElems++;}

public void display(){
    for(int j=0; j<nElems; j++)
        System.out.println(a[j] + " ");
        System.out.println(" ");}

public void bubbleSort() {
    int out, in;
    for(out = nElems-1; out>1; out--)
        for(in=0; in<out; in++)
            if(a[in+1]<a[in])
                swap(in, in+1);}

public void swap(int one, int two) {
    long temp = a[one];
    a[one] = a[two];
    a[two] = temp;}}

class NewBubbleApp {
public static void main(String[] args) {
    int maxSize = 10000;
    Bubble bubarr = new Bubble(maxSize);
    bubarr.randomInsert();
    bubarr.bubbleSort();
    bubarr.display();}}

Eclipse offers to use as: bubarr.randomInsert(maxSize). Anyone who had such this before? Help me, please