Java: Finding neighbours of a start coordinate (0,0)

Question

I've been having an issue figuring out how I can search arround a coordinate to find its neighbours. It will make more sense when you see the picture.

I have 7 hexagons and they all have respective coordinates where the center one is (0,0). I wish to create a method in which I can add all neighbour hexagons to an arraylist, but I'm having a difficult time figuring out how I can determine that the added hexagon is in fact an neighbour.

Picture here:

Example: (Reference to picture)

I wish to know which neighbours the hexagon on position (0,-1) has. By looking at the picture I can see it has (1,0) , (0,0) and (-1,-1). But how would I loop through this and find its neighbours in java code?

Answer 1

I'm going to make some assumptions and try to present an answer. Please comment if something changes or doesn't sit right.

From the looks of it your grid is square. What I mean by that, both the X and the Y coordinate are specified in the same range. Consider the following class:

public class HexagonalGrid {
  // Helper class Cell
  public static class Cell {
    public int x;
    public int y;

    public Cell(int x, int y) {
      this.x = x;
      this.y = y;
    }
  }


  // ranges are
  // x -> [-width, width]
  // y -> [-height, height]
  private int width;
  private int height;

  public HexagonalGrid(int width, int height) {
    this.width = width;
    this.height = height;
  }

  public ArrayList<Cell> getNeighbours(Cell target)  {
    ArrayList<Cell> neighbours = new ArrayList<>();

    // These coordinates are predictable, so let's generate them
    // Each immediate 
    for (int x_offset = -1; x_offset <= 1; x_offset++) {
      for (int y_offset = -1; y_offset <= 1; y_offset++) {
        // No offset puts us back at target cell so skip
        if (x_offset == 0 && y_offset == 0) { 
          continue;
        }

        // Generate the cell with the offset
        int x = target.x + x_offset;
        int y = target.y + y_offset;

        // Check validity against bounds
        if (isValidCoordinate(x, y)) {
          // Add valid neighbour
          Cell neighbour = new Cell(x, y);
          neighbours.add(neighbour);
        }
      }
    }

    return neighbours;
  }


  private boolean isValidCoordinate(int x, int y) {
    // Enforcing the ranges specified above
    return -width <= x && x <= width
        && -height <= y && y <= height;
  }
}