Referencing a Location in an Array rather than the Data within it. (Java)

Question

I'm writing a project that requires me to use a graph traversal algorithm over a grid-like structure made up of Units. I'm creating the grid so that each unit of the grid has references to the each of its neighbors:

class Unit {
    boolean type, visited;
    Unit up, left, down, right;
    Unit(Unit u, Unit l, Unit d, Unit r) {
        up = u;
        left = l;
        down = d;
        right = r;
    }
}

I create a grid of Units by adding a new Unit object to each position with the neighbors in the form of references to other locations in the grid:

Unit[][] grid = new Unit[height][width];

for (int i = 0; i < height; ++i) {
    for (int j = 0; j < width; ++j) {
        grid[i][j] = new Unit(
            i == 0 ? null : grid[i - 1][j],
            j == 0 ? null : grid[i][j - 1],
            i == height - 1 ? null : grid[i + 1][j],
            j == width  - 1 ? null : grid[i][j + 1]
        );
    }
}

Unfortunately, whenever some of the grid Units are created, their neighbors are null so they do not hold the correct reference. To solve this I just run this entire algorithm over again so that it has every grid element filled, but this seems like a very inefficient and ugly solution.

What I would like to do is hold the Unit object as a "dynamic" index of the grid so if a certain position gets updated then the neighbor reference in the appropriate Units would also get updated. This acts as a sort of promise by the grid saying that if a position (i, j) becomes filled then the neighboring Units will have the updated information.

Thank you to anyone who can help me solve this!

P.S. I realize that I can store the indexes to the grid in the Unit class for the neighbors but that would restrict me to the specific grid I have in this example and it would make it necessary to make the call to get the item everytime it is updated. I may not want to store it in a two-dimensional array like I do it here, so I want a generalizable system.

Mr Moose · Accepted Answer · 2016-11-21 04:51:28Z

1

When you are creating your very first Unit using your constructor, there are no adjacent units, so your references are going to refer to null. You could instead check if an adjacent cell contains a null reference (being mindful of the adjacent cells i and j values), and if so, create a new Unit().

answered Nov 21, 2016 at 4:51

Mr Moose

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2 Comments

Brandon H. Gomes Over a year ago

But when I create those new Unit()s they will also have no neighbors yet instantiated. Also there would be checks for almost every Unit because most of them will not have been created yet by the definition of the problem.

Mr Moose Over a year ago

You are correct that there will be checks for every unit. That is essentially what you are asking though isn't it? Basically, you want to say, if a neighbouring cell already has a unit assigned, then use that reference in my constructor, otherwise create a new instance of Unit. So I'd suggest adjusting the call to your constructor to perform those checks and instantiate objects where necessary

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