Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 8, Size: 0 at java.util.ArrayList.rangeCheckForAdd(ArrayList.java:665)

Question

Im trying to solve a problem which is stated as -> Given an array A, find the next greater element G[i] for every element A[i] in the array. The Next greater Element for an element A[i] is the first greater element on the right side of A[i] in array, A. Elements for which no greater element exists, consider the next greater element as -1.

public class Solution {
    public ArrayList<Integer> nextGreater(ArrayList<Integer> A) {
       Stack<Integer> stk = new Stack<>();
       if (A.size() == 1)
        {
            ArrayList<Integer> ans= new ArrayList<>();
            ans.add(0,-1);
            return ans;
        }
       ArrayList<Integer> ans= new ArrayList<>(A.size());
       ans.add(A.size()-1,-1); //error ->Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 8, Size: 0
       stk.push(0);
       for(int i=1;i<A.size()-1;i++){
               while(!stk.isEmpty()&&A.get(i)>A.get(stk.peek())){
                   ans.add(stk.pop(),A.get(i));
                //   stk.pop();
               }
               stk.push(i);
           }
       return ans;
    }
}

But while solving this Im not understanding why Im getting error while adding -1 in the arraylist at position A.size()-1 where A : [ 34, 35, 27, 42, 5, 28, 39, 20, 28 ]

Answer 1

A.size() is 0, since there is no element in the list yet. Arrays should be initialized with a starting value, Lists (ArrayLists) are not. You should initialize your ArrayList without capacity: ArrayList<Integer> ans= new ArrayList<>();

Then you can simply ans.add(-1);

or better, you can add -1 simply at the very end of your method.

Answer 2

The ArrayList constructor is for initial capacity, this not fill the ArrayList with elements, the size is 0 until insert or add an element.