I got everything until to the point that check if the space is emprty or not. I dont know how to do it. can someone give me an example?

I was think something like this 
public static boolean checkforvaild (int number)
    {
        if (board[rows][cols] != " ")
        {
            return false;
        }
        else
        {
            return true;
        }
    }

but mine is in the switch withc conver from 1 to 9 into 2d array

Without knowing how your program and its data are structured, it's almost impossible to answer that question. What's the "switch" you refer to?
Anyway, converting an int 1-9 into row/col values is just simple arithmetic. You need the divide operator / and the remainder operator %

ps

        if (board[rows][cols] != " ")
        {
            return false;
        }
        else
        {
            return true;
        }

is a long way to say

return board[rows][cols] == " "

but also, note that != and == for Strings tests for being the same object; to test for twoo Strings having the same sequence of charatcters use String's equals method.

case 7: //check if the space is empty;
                    user = board [0][0] = 'X';
                    displayboard (board);
                    break;
            case 8: //check if the space is empty;
                    user = board [0][1] = 'X';
                    displayboard (board);
                    break;
            case 9: //check if the space is empty;
                    user = board [0][2] = 'X';
                    displayboard (board);
                    break;
            case 4: //check if the space is empty;
                    user = board [1][0] = 'X';
                    displayboard (board);
                    break;
            case 5: //check if the space is empty;
                    user = board [1][1] = 'X';
                    displayboard (board);
                    break;
            case 6: //check if the space is empty;
                    user = board [1][2] = 'X';
                    displayboard (board);
            break;
            case 1: //check if the space is empty;
                    user = board [2][0] = 'X';
                    displayboard (board);
                    break;
            case 2: //check if the space is empty;
                    user = board [2][1] = 'X';
                    displayboard (board);
                    break;
            case 3: //check if the space is empty;
                    user = board [2][2] = 'X';
                    displayboard (board);
                    break;
        }

I could not figure out how to pass it to other methor to check whether or not that space is empty for now

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