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.

Solution:
Trivial implementation question.

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

public class Solution {
    public boolean isValidSudoku(char[][] board) {
        //check rows
        for(int i = 0; i < 9; i++){
            int[] a = new int[10];
            for(int j = 0; j < 9; j++){
                if(board[i][j] == '.') continue;
                a[board[i][j] - '0']++;
                if(a[board[i][j] - '0'] > 1) return false;
            }
        }
        //check cols
        for(int j = 0; j < 9; j++){
            int[] a = new int[10];
            for(int i = 0; i < 9; i++){
                if(board[i][j] == '.') continue;
                a[board[i][j] - '0']++;
                if(a[board[i][j] - '0'] > 1) return false;
            }
        }
        //check sub-boxes
        for(int startRow = 0; startRow <= 6; startRow+=3){
            for(int startCol = 0; startCol <= 6; startCol+=3){
                int[] a = new int[10];
                for(int i = startRow; i < startRow+3; i++){
                    for(int j = startCol; j < startCol+3; j++){
                        if(board[i][j] == '.') continue;
                        a[board[i][j] - '0']++;
                        if(a[board[i][j] - '0'] > 1) return false;
                    }
                }
            }
        }
        return true;
    }
}