I am trying to implement a level-order traversal (BFS) on a binary tree in Java. I understand that level-order traversal visits nodes level by level from left to right, but I’m not sure how to implement it efficiently.

Here’s the basic structure of my binary tree:

class TreeNode {
    int value;
    TreeNode left;
    TreeNode right;

    TreeNode(int value) {
        this.value = value;
        left = null;
        right = null;
    }
}

How can I implement a level-order traversal for this binary tree in Java? Should I use a queue to achieve this, and if so, how?
What are the time and space complexities of this approach?
Is there a way to modify the traversal to return the nodes level by level as separate lists, instead of in a single list?