2D阵列无法打印

Question

I'm trying to make a 2D array that essentially acts as a room/grid - like a top-down view.

This code creates an array of dimensions row x col based on user input. I then have it initialize every cell with a "." to represent a space.

My question is, why, when I use the printRoom() method does it not print anything out at all? The double nested for-loops are running through the array, but nothing gets printed.

I know it has to do with the user input, because I tried just initializing with row = 5 and col = 5, and it printed just fine.

Thank you for your help.

Here is the Room Class:

public class Room
{
    int row, col;   
    String[][] arr = new String[row][col];

    //Room Constructor
    public Room(int row, int col)
    {
        this.row = row;
        this.col = col;              
    }

    //Fills the Room with "."
    public void fillRoom()
    {
        for (int i = 0; i < arr.length; i++)
        {
            for (int j = 0; j < arr[i].length; j++)
                arr[i][j] = ".";
        }
    }

    //Prints the entire room/grid
    public void printRoom()
    {
        for (int i = 0; i < arr.length; i++)
        {
            for (int j = 0; j < arr[i].length; j++)
                System.out.print(arr[i][j]);

            System.out.println();
        }
    }
}

Here is my RoomTest class which contains main:

import java.util.*;

public class RoomTest
{
    public static void main (String[] args)
    {
        Scanner scan = new Scanner(System.in);
        int row, col;

        System.out.println ("Enter room dimensions (row col)");
        row = scan.nextInt();
        col = scan.nextInt();        

        Room room1 = new Room(row,col);
        room1.fillRoom();
        room1.printRoom();
    }
}

Here is the output when I run and enter user input:

Enter room dimensions (col row)

4 6

Answer 1

When you initialize your array, row and col are zero, so it is a 0x0 array.

As such, the guard condition in for (int i = 0; i < arr.length; i++) is immediately false, and so never executes.

You need to move the array initialization into the constructor:

arr = new String[row][col];

Answer 2

Try to add length of the 2D array here because before are initialized with zero:

  int row= 4, col= 6; 
  String[][] arr = new String[row][col];

Or you can initialize it in Constructor like this:

  String[][] arr;
//Room Constructor
public Room(int row, int col)
   {
   arr = new String[row][col];
    this.row = row;
    this.col = col;              
    }

And in your main method initialize the row and col :

 int row= 4, col= 6;

Answer 3

To solve the problem create the array at the constructor of Room object:

int row, col;   
String[][] arr = null

//Room Constructor
public Room(int row, int col)
{
    this.row = row;
    this.col = col;
    arr = new String[row][col];
}