Detect Infinite Loop?

Question

I'm working on a Sudoku solver program. The idea is that the user inputs the Sudoku puzzle and the program solves it for them.

The program works fine when entering any normal puzzle. However, there are some puzzles that are impossible to solve. Here is one example that I entered: http://i.imgur.com/5L8pF8Q.png

The input is perfectly legal, but top right square cannot be filled in since all numbers 1-9 have been used in that row and column. If I hit "Solve", my program freezes as it enters an infinite loop.

So my question here is, how can I prevent this from happening?

I tried implementing the "boolean flag" approach, but on second thought I realized that it's probably not feasible.

Here is my solver method:

public boolean solve(int i, int j) {
    for (int a = 0; a < 9; a++) {
        for (int b = 0; b < 9; b++) {
            if (sudokuArray[a][b] == 0) {
                for (int k = 1; k <= 9; k++) {
                    sudokuArray[a][b] = k;
                    if (checkNumber(a, b, k) && solve(a, b)) {
                        return true;
                    }else {
                        sudokuArray[a][b] = 0;
                    }
                }
                return false;
            }
        }
    }
    return true;
}

The checkNumber() method checks if number k can be legally inserted in row a, column b and returns true/false accordingly.

Ideas? Tips?

Thanks.

PS: Added checkNumber() as requested:

public boolean checkNumber(int i, int j, int num) {
    for (int a = 0; a < 9; a++) {
        if (a != i) {
            if (sudokuArray[a][j] == num) {
                return false;
            }
        }
        if (a != j) {
            if (sudokuArray[i][a] == num) {
                return false;
            }
        }
    }
    for (int a = (i / 3) * 3; a < (i / 3) * 3 + 3; a++) {
        for (int b = (j / 3) * 3; b < (j / 3) * 3 + 3; b++) {
            if ((a != i) && (b != j)) {
                if (sudokuArray[a][b] == num) {
                    return false;
                }
            }
        }
    }
    return true;
}

Answer 1

This is just an idea (i would comment but this account is too new). Try making it so that if all numbers from 0-9 return false, then it returns an error of some sort. For example if it hasn't found an answer by 9, then move up to 10. If the program detects a 10, do some kind of System.out.println("Unsolvable") or something along those lines. Sorry if I'm confusing you by the way. I've been told I'm bad at explaining things.