What is ArrayList<Integer>[] or StringBuilder[]? What is this square bracket's role?

Explanation

It was hard to google it because google search doesnt work well with symbols. However, I found out that

ArrayList<Integer>[] G = new ArrayList[n];
StringBuilder[] st= new StringBuilder[numRows];

is actually just making an array of ArrayLists or StringBuilders.

The main difference between ArrayList[] and ArrayList is that the former is an array of ArrayList(s), whereas the latter is a single ArrayList. So when you instantiate the array, you have to assign the initial length, just like when you instantiate a regular array of int (=new int[3])

It’s important to note that this notation is considered as non-type-safe, hence it is not recommended to use them. Instead, you can use List<ArrayList> which is type-safe and provide more readability.

To convert the type with casting,

List<Integer>[] graph = (ArrayList<Integer>[]) new ArrayList[n];

When you are building arraylist in that array,

List<Integer>[] graph = (ArrayList<Integer>[]) new ArrayList[n];
for (int i = 0; i < n; i++) {
    graph[i] = new ArrayList<>();
    //not graph[i].add(new ArrayList<>())
}