why I can't add int type value to an array

Question

import java.util.ArrayList;
import java.util.Collection;


public class TreeNode {
     int val;
     TreeNode left;
     TreeNode right;
     TreeNode(int x){val=x;}
}

public class SortedArrayToBalancedBST {

  public TreeNode sortedArrayToBST(int[] nums) {
        if(nums.length==0) return null;
        else return sortedArrayToBST(nums, 0, nums.length-1);
    }
    public TreeNode sortedArrayToBST(int[] nums, int left, int right){
        ArrayList<TreeNode> list = New ArrayList<TreeNode>()
        TreeNode root =null;
        int mid=(right+left)/2;
        if(right>=left){
            root= new TreeNode(nums[mid]);
            root.left=sortedArrayToBST(nums, left, mid-1);
            root.right=sortedArrayToBST(nums, mid+1, right);
            list.add(root);
            //System.out.print(list);
        }
        if(left>right) return null;
        return root;

    }

    public static void main(String[] args) {
        int nums[]= {1,3,5,7,9};
        SortedArrayToBalancedBST s = new SortedArrayToBalancedBST();
        s.sortedArrayToBST(nums);
        System.out.print(s.toString())
    }

I would like create a balanced tree by using the code above. But java return me an error that told me:Cannot invoke add(TreeNode) on the array type int[], so what should I do, why I can't add int type value to an array? And also I wish the output looks like [5,3,7,1,null,null,9] if I'm right.

Answer 1

An array type doesn't have an add(...) method. You must specify the index to which you are replacing the element (ie arr[0] = 1; ). Also, if your array will contain null values, consider using Integer[] type instead of the primitive.