Collections.sort() explanation

Newbie & Debugging Questions
#1

I’m making a top down game and needed a way to render objects in the correct order based on their y-axis position. After a day of trying to hard code it, I ended up using Collections.sort() and it works perfectly, but I don’t understand why it does. The docs say it sorts by “natural ordering”, but I don’t understand what that actually is. Here is the code:


public static void sortAndRenderObjectsInYPositionOrder(
			ArrayList<GameObject> gameObjectList,
			SpriteBatch batch,
			ShapeRenderer shapeRenderer,
			ImageLoader imageLoader
			) {
		Collections.sort(gameObjectList);
		for (int i = 0; i < gameObjectList.size(); i++) {
			if (playerPositionIsWithinBoundsToRenderGameObjects(
					gameObjectList.get(0).getX(), 
					gameObjectList.get(0).getY(), 
					gameObjectList.get(i).getX(), 
					gameObjectList.get(i).getY(), 
					gameObjectList.get(i).getHeight(),
					gameObjectList.get(i).getWidth()
					)) {
				gameObjectList.get(i).renderObject(batch, shapeRenderer, imageLoader);
			}
		}
	}

My question is, how is the gameObjectList actually being sorted, and since each object has an x, y, width, and height, how is it seemingly sorting by the y-position of the objects without my specifying to sort that way?