I am trying to implement grid-based collision in a 2d game with moving circles. The canvas is 400x400 pixels. Below you can see the code for my Grid class. What I want it to do is check inside which box the entities are located and then run a collision check if there are 2 or more entities in the same box.

Right now I do not know how to find the position of an entity in a specific box. I know there are many tutorials online, but I haven't been able to find an answer to my question, because they are either written in C/C++ or use the 2d array approach.

Code snippets and other help is greatly appreciated. Thanks.

public class Grid {

ArrayList<ArrayList<Entity>> boxes = new ArrayList<>();
double boxSize = 40;
double boxesAmount = 10;
...
...
public void checkBoxLocation(ArrayList<Entity> entities) {

      for (int i = 0; i < entities.size(); i++) {
// Get top left coordinates of each entity         
            double entityLeft = entities.get(i).getLayoutX() - entities.get(i).getRadius();
            double entityTop = entities.get(i).getLayoutY() + entities.get(i).getRadius();

// Divide coordinate by box size to find the approximate location of the entity
            for (int j = 0; j < boxesAmount; j++) {   //Select each box
                if ((entityLeft / boxSize <= j + 0.7) && (entityLeft / boxSize >= j)) {
                    if ((entityTop / boxSize <= j + 0.7) && (entityTop / boxSize >= j)) {
                        holdingBoxes.get(j).add(entities.get(i));
                        System.out.println("Entity " + entities.get(i) + " added to box " + j);
                    }
                }
            }
        }
    }
}