System.out.print unknown value

I am making a simple board for a game that I am making. It's printing out

[[Ljava.lang.String;@2a788b76

and i don't know where this is coming from. It might be something stupid that I am doing wrong.

public class AI {

public static String[][] board = new String[10][10];
    public static void addPiece(int x, int y, String r){

        board[x][y] = r;//no need for new String(), board is already made of Strings.
    }
    public static void showBoard(){
        //it's generally better practice to initialize loop counters in the loop themselves
        for (int row = 0; row < 9; row++)
        {
            System.out.println(" ");
            System.out.println("-------------------");    
            for(int col = 0; col < board[row].length; col++)
            {
                System.out.print("|"); //you're only printing spaces in the spots
                if(board[col][row] == null){
                  System.out.print(" ");
                }else{
                  System.out.print(board[col][row]);
                }
            }
        }
        System.out.println(" ");
        System.out.println("-------------------");
    }
    public static void main(String[] args) {
        System.out.println(board);
        addPiece(0,0," ");
        addPiece(0,1,"1");
        addPiece(0,2,"2");
        addPiece(0,3,"3");
        addPiece(0,4,"4");
        addPiece(0,5,"5");
        addPiece(0,6,"6");
        addPiece(0,7,"7");
        addPiece(0,8,"8");
        addPiece(1,0,"1");
        addPiece(2,0,"2");
        addPiece(3,0,"3");
        addPiece(4,0,"4");
        addPiece(5,0,"5");
        addPiece(6,0,"6");
        addPiece(7,0,"7");
        addPiece(8,0,"8");
        addPiece(1,1,"X");  
        addPiece(8,8,"O");
        showBoard();    
    }

}