Java-像二维数组一样填充多维（2d）ArrayList

Question

A while ago before I got used to object object oriented programming I created a basic TicTacToe game and to create the board I used an array.

The code is a complete mess because I didn't properly understand how to use objects, but I did initialize the board correctly:

char[][] board = new char[3][3];
for (int i = 0; i < board.length; i++){
    for (int j = 0; j < board[i].length; j++){
        board[i][j] = '[]' //or something like that...don't remember exactly
    }
}

My question is how would you this with an ArrayList?

     ArrayList <ArrayList<Character>> board = new ArrayList(); // this initialization is not
    // wrong using Java 8 but for earlier versions you would need to state the type on both 
//sides of the equal sign not just the left
        for (int i = 0; i < board.size(); i++){
                for (int j = 0; j < board.get(i).size(); j++){
                    board.get(i).get(j).add('[]');
                }
            }

but that does not work.

It does not have to be exactly like this, I just generally want to understand how to handle multidimensional ArrayLists.

-thanks

Answer 1

Unlike arrays, you can't initialize an entire ArrayList directly. You can specify the expected size beforehand (this helps performance when you are using very large lists, so it is a good practice to do it always).

int boardSize = 3;
ArrayList<ArrayList<Character>> board = new ArrayList<ArrayList<Character>>(boardSize);
for (int i = 0; i < boardSize; i++) {
    board.add(new ArrayList<Character>(boardSize));
    for (int j = 0; j < boardSize; j++){
        board.get(i).add('0');
    }
}

Answer 2

The main difference is that in your original code you had a multi-dimensional array of primitives (in this case, char ) and all you had to do was assign a new primitive value to each slot in the array.

However what you want now is an ArrayList of ( ArrayList of Character ). When you create the ArrayList it is empty. In order to procede you are going to need to fill it with several ( ArrayList of Character ) before you can begin to start adding Character s themselves.

So for example,

ArrayList <ArrayList<Character>> board = new ArrayList<>();

for (int i=0; i<3; i++) {
  board.add(new ArrayList<Character>());
}

Now you can start adding Character s to your lists:

for (int i=0; i<3; i++) {
  for (int j=0; j<3; j++) {
    board.get(i).add('A');
  }
}

Hope this helps.

Answer 3

First you have to initialize the ArrayList in your first line correctly and than you have to initialize an new ArrayList in each run of your first loop:

ArrayList <ArrayList<Character>> board = new ArrayList<ArrayList<Character>>();
for (int i = 0; i < 3; i++){
    ArrayList<Character> innerList = new ArrayList<Character>();
    board.add(innerList);
    for (int j = 0; j < 3; j++){
        innerList.add('[');
    }
}