java building game : snap to grid

I am making a game in where it's topdown and you can place blocks how can I have a snap to grid game so its like you place a block and it snaps to a grid so you cant place squares on top of eachother. Im not asking you to write my code (I don't mind if u do) I just want to know how to do this. my code so far that would be helpful to luck at and get an idea of what I mean:

public static ArrayList<hellstone> hpf = new ArrayList<hellstone>();

for (int i = 0; i < hpf.toArray().length; i++) {
    hpf.get(i).render(g);
}

public void mouseReleased(MouseEvent e) {
        if (e.getButton() == MouseEvent.BUTTON1) {
            Comp.ml = false;
            if (Comp.play && Comp.money >= 100) {

                if(inventory.sel == 0){
                Comp.hpf.add(new hellstone(Comp.mx / 2 - 10, Comp.my / 2 - 10));
                Comp.money -= 100;
                }else if(inventory.sel == 1){
                    Comp.b.add(new board(Comp.mx / 2 - 10, Comp.my / 2 - 10));
                    Comp.money -= 100;
                }

            }
        } else if (e.getButton() == MouseEvent.BUTTON3) {
            Comp.mr = false;
            if (Comp.play) {

            }

        }

    }