Valid Sudoku

Determine if a Sudoku is valid, according to: Sudoku Puzzles - The Rules.

The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

A partially filled sudoku which is valid.

Note:
A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.

public boolean isValidSudoku(char[][] board){
        if (board.length!=9||board == null)
            return false;
        Set<Character> rowset = new HashSet<>();
        Set<Character> columset = new HashSet<>();
        Set<Character> gridset = new HashSet<>();

        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                //row
                if (board[i][j]!='.'){
                    if (rowset.contains(board[i][j])){
                        return false;
                    }
                    else {
                        rowset.add(board[i][j]);
                    }
                }

                //colum
                if (board[j][i]!='.'){
                    if (columset.contains(board[j][i])){
                        return false;
                    }
                    else {
                        columset.add(board[j][i]);
                    }
                }
            }
            rowset.clear();
            columset.clear();
        }

        //3*3
        for (int i=0;i<3;i++){
            for (int j = 0; j < 3; j++) {
                for (int row = i*3;row<i*3+3;row++){
                    for (int colum = j*3;colum<j*3+3;colum++){
                        if (board[row][colum]!='.'){
                            if (gridset.contains(board[row][colum])){
                                return false;
                            }
                            else {
                                gridset.add(board[row][colum]);
                            }
                        }
                    }
                }
                gridset.clear();
            }
        }

        return true;
    }