How to replace all elements of an ArrayList with another ArrayList which is of different size?

Given two sorted integer arrays A and B, merge B into A as one sorted array.

Note: You have to modify the array A to contain the merge of A and B. Do not output anything in your code. TIP: C users, please malloc the result into a new array and return the result.

If the number of elements initialized in A and B are m and n respectively, the resulting size of array A after your code is executed should be m + n

Example :

Input : A : [1 5 8] B : [6 9]

Modified A : [1 5 6 8 9]

My Solution:

    public class Solution {
      public void merge(ArrayList<Integer> a, ArrayList<Integer> b) {

         int i=0,j=0;
             ArrayList<Integer> al= new ArrayList<Integer>();

       while(i<a.size() && j<b.size()){
          if(a.get(i)<b.get(j)){
            al.add(a.get(i));
            i++;
        }

        else{
            al.add(b.get(j));
            j++;
        }
    }

    while(i<a.size()){
        al.add(a.get(i));
        i++;
    }

    while(j<b.size()){
        al.add(b.get(j));
        j++;
    }
}

I created a 3rd ArrayList where I merged all elements of both 1st and 2nd ArrayLists. Now I have to copy all the elements of 3rd ArrayList into 1st one. I'm stuck now. Because, when I resized it by using

    public static void ensureSize(ArrayList<Integer> list, int size){
    list.ensureCapacity(size);
    while(list.size()<size){
        list.add(null);
    }
}

ensureSize(a,al.size());
for(int k=0;k<al.size();k++){
    a.set(k,al.get(k));
}

It's giving me ArrayIndexOutOfBound error. I tried Collections.copy(a,al), it too didn't work.

Please suggest.