Java: How to return object when I have its name as a string?

Question

Suppose I am building a chess game and creating board space objects. I am creating objects like so, precisely 64 for all spaces on the chess board:

BoardSpace a1 = new BoardSpace("black", 1, 1, true);

Here is the class I've created for the BoardSpace object:

public class BoardSpace {
    String color;
    int x_pos;
    int y_pos;
    boolean occupied;

    //constructor
    public BoardSpace (String color, int x_pos, int y_pos, boolean occupied) { 
        this.color = color;
        this.x_pos = x_pos; 
        this.y_pos = y_pos; 
        this.occupied = occupied;
    } 
}

I create all my BoardSpace objects prior to moving chess pieces on the board. My chess piece objects each have an x position and y position. What I want to do is convert their coordinates into a BoardPiece name, and then retrieve a previously-created BoardPiece object from that name.

This is what I want to do:

static String get_BoardSpace_color(int x_pos, int y_pos){
    int modified_x = x_pos + 96; //adjusting for ASCII
    char c = (char)(modified_x);
    String space_name = ""+c+y_pos;

    BoardSpace piece = (BoardSpace)(space_name); //PROBLEM AREA

    return piece.color;
}

How can I, using the correct string representation of the already existing object's name, actually RETRIEVE THAT OBJECT?

Answer 1

Again, objects don't have names. Yes variables do, but the name of a variable is not a String, and variable names almost don't exist in compiled code. What you need is a way to get a reference to the object of interest, and there are various ways to do this including:

• a Map<String, BoardSpace> such as a HashMap<String, BoardSpace>. This way you can associate a String with a unique object
• A collection such as an ArrayList<BoardSpace> which allows you to get your object by an int index
• a simple array of BoardSpace, such as BoardSpace[64]
• A 2 dimensional nested collection, such as a List<List<BoardSpace>>
• or a 2D array.

Since you appear to be making an 8 x 8 grid of BoardSpace, and since these dimensions likely will not change, simplest here is to create an 8x8 array of objects:

private BoardSpace[][] grid = new BoardSpace[8][8];

Then you can use your x and y (or row and column) indices to get the object of interest.

For example:

public class TestBoardSpace {
    public static void main(String[] args) {
        Board board = new Board();
        for (int y = 0; y < Board.ROWS; y++) {
            for (int x = 0; x < Board.COLS; x++) {
                System.out.printf("%8s ", board.getBoardSpace(x, y).getColor());
            }
            System.out.println();
        }
    }
}

class Board {
    public static final int ROWS = 8;
    public static final int COLS = ROWS;
    private BoardSpace[][] grid = new BoardSpace[ROWS][COLS];

    public Board() {
        for (int row = 0; row < grid.length; row++) {
            for (int col = 0; col < grid[row].length; col++) {
                MyColor color = row % 2 == col % 2 ? MyColor.BLACK : MyColor.WHITE;
                grid[row][col] = new BoardSpace(color, col, row, false);
            }
        }
    }

    public BoardSpace getBoardSpace(int x, int y) {
        // to get color, simply call getColor() on this
        return grid[y][x];
    }

}

// yes an enum here would be great and would protect against 
// bad Strings
enum MyColor {
    WHITE, BLACK
}

class BoardSpace {
    private MyColor color;
    private int x_pos;
    private int y_pos;
    private boolean occupied;

    // constructor
    public BoardSpace(MyColor color, int x_pos, int y_pos, boolean occupied) {
        this.color = color;
        this.x_pos = x_pos;
        this.y_pos = y_pos;
        this.occupied = occupied;
    }

    public boolean isOccupied() {
        return occupied;
    }

    public void setOccupied(boolean occupied) {
        this.occupied = occupied;
    }

    public MyColor getColor() {
        return color;
    }

    public int getX_pos() {
        return x_pos;
    }

    public int getY_pos() {
        return y_pos;
    }

}