数据结构使所有元素处于特定位置

Question

I have a square Grid with a certain height and width. It contains members of the class Item . Every Item has a certain Position . I'd like to be able to get all the items at a certain position in constant time, and I'd like to be able to place an item on the grid in amortized constant time.

What (Java) structure can do this while using an amount of memory proportional to the amount of used positions (positions with at least 1 item on it)?

Answer 1

If your Grid has a fixed size I would use an array.

Item[][] itemArray = new Item[3][3];
itemArray[0][0] = new Item();
System.out.println(itemArray[0][0]);

I would wrap that into the class Grid

public final class Grid {

    private Item[][] grid;

    public Grid(int width, int height) {
        grid = new Item[width][height];
    }

    public void setItemAt(Position position, Item item) {
        int x = position.getX();
        int y = position.getY();

        grid[x][y] = item;
    }

    public Item getItemAt(Position position) {
        int x = position.getX();
        int y = position.getY();

        return grid[x][y];
    }
}

Answer 2

The user above is right that internally you can use an array. Of course it should be of type List<Item>[][] . No matter if you use ArrayList or LinkedList , adding will be amortized constant time. Though I would suggest going for a LinkedList , as your Grid is likely to be sparsely populated ( ArrayList always starts with a couple of spaces already allocated).

If the Grid needs to be able to grow, just use the same trick ArrayList uses: keep track of current width/height, and if you need more just multiply available space by a suitable value (for a 2D array, doubling might be a bit much, but it depends on the type of growth that is possible).