用Java打印2D对象数组？

Question

I am starting a simple project where the user plays a version of Battleship against the computer. I would like to simply print out the game board for now. However, after initializing the board as a 2D array of object composed of the cells of the grid, I have encountered an error. Rather than printing the cell type, which I have defined in the class, the code simply prints a grid of "null"s. Any help here is much appreciated.

public class main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        grid[][] gameBoard = new grid[9][9];

        for (int x = 0; x < 9; x++) {
            for (int y = 0; y < 9; y++) {
                System.out.print(gameBoard[x][y] + " ");
            }
            System.out.println("");
        }

    }

}

public class grid {

    public String type;
    public String owner;
    public boolean positionCalled;

    public grid() {
        type = "_";
        owner = "";
        positionCalled = false;
    }

    public String toString() {
        return type;
    }

}

Answer 1

You only created the array, you did not create the single cells inside it.

To do so, inside your loops:

gameBoard[x][y] = new grid();

Answer 2

You have to initialize your grid ,

for (int x = 0; x < 9; x++) {
    for (int y = 0; y < 9; y++) {
        gameBoard[x][y]=new grid();//initialize the grid
        System.out.print(gameBoard[x][y] + " ");
    }
    System.out.println("");
}

Answer 3

That's because you forgot to initialize 2d grid array objects after initialization. Add this line of code:

for (int i = 0; i < 9; i++)
    for (int j = 0; j < 9; j++)
        gameboard[i][j] = new grid();