public String toString(){
		String res="     0   1   2   3   4   5   6   7\n   ---------------------------------\n";
		for(int row=0; row<8;row++){
			res += row+"  |";
			for(int column=0; column<8; column++){
				if ((column+row)%2==1)
					res+="   |";
				else{
					if(board[row][column]==null)
						res+=" _ |";
					else res+= " " + board[row][column] + " |" ; // O = bianco
				}
			} 
			res+="\n   ---------------------------------\n";
		}
		
		return res;
	}