如何在不设置维度的情况下初始化二维数组？

Question

I am using a method which reads in a file and sets the size of a 2D array based on the first 3 lines. My method returns a Room[][] object and the method also sets the dimensions of the Room[][] object so that I cannot set it outside before doing the method. For example:

public Room[][] readRooms(String filepath) throws IOException
    {
        int numberOfRooms;
        int numRows;
        int numCols;
        Room[][] grid;

    try
    {
        FileReader fr = new FileReader(filepath);
        BufferedReader br = new BufferedReader(fr);
        String next = null;

        numberOfRooms = Integer.parseInt(br.readLine());
        numRows = Integer.parseInt(br.readLine());
        numCols = Integer.parseInt(br.readLine());

        grid = new Room[numRows][numCols];

        while((next = br.readLine()) != null)
        {
            //do stuff
        }
    }
    catch{ //catch stuff }

return grid;
}

What I am attempting is to create a new Room[][] object in a main method in another class that does this:

Room[][] maze = readRooms(myfilepath);

However, Java is requiring that maze be defined before I can use it like this. Is it not possible to initialize a 2D array without explicitly declaring the dimensions?

Edit: The main method for those asking:

public static void main(String args[])
{
    Room[][] maze;
    maze = readRooms("C:/Users/Blaise/Programming/csc300/Maze");
}

Answer 1

The error is your function readRooms is not static . Make it static and you will not need an instance of Room to call it. But you do need to handle the IOException in your caller (or add throws IOException to main() )

public static Room[][] readRooms(String filepath) throws IOException

Also, you cannot double declare maze 's type.

public static void main(String args[])
{
  Room[][] maze;
  try {
    maze = Room.readRooms("C:/Users/Blaise/Programming/csc300/Maze");
  } catch (IOException e) {
    e.printStackTrace();
  }
}

or

public static void main(String args[]) throws IOException
{
  // Room[][] maze; // <-- not
  Room[][] maze = Room.readRooms("C:/Users/Blaise/Programming/csc300/Maze");
}

Answer 2

An array's size is always fixed in Java. So you can't....Typically instead of using an array, you'd use an implementation of List here - usually ArrayList, but with plenty of other alternatives available.

You can create an array from the list as a final step, of course - or just change the signature of the method to return a List to start with

Answer 3

Your main needs to either throw an IOException or surround maze = readRooms(myfilepath) with:

try {...} catch (IOException) {...}

Also you're declaring maze twice. Instead of writing

Room[][] maze;
Room[][] maze = readRooms(myfilepath);

Shorten it to

Room[][] maze = readRooms(myfilepath);

Like Eran said. Or if you really don't like that, do:

Room[][] maze;
maze = readRooms(myfilepath);

and Elliot is right too, readRooms() needs to be static.

I don't know if I answered your question or not, but at least it's some constructive criticism :).

Edit : I noticed you have try {...} catch {...} in your readRooms method... is this used to catch an IOException? If so, readRooms() doesn't need to throw an IOException at all.