public boolean validateInput () {
		int i = 0, j = 0;
        /* Uses nested for loops to check that all input is valid */
		for (i = 0; i < 9; i++) {
			for (j = 0; j < 9; j++) {
				    try {
				        if (Integer.parseInt(entries[i][j].getText()) < 1) {
				        	return false;
				        }
				    } catch (NumberFormatException e) {
				        JOptionPane.showMessageDialog(null, "Invalid input. Enter an integer at row " + (i+1) + " column " + (j+1), "Error", JOptionPane.ERROR_MESSAGE);
				        return false;
				    }
			}
		}
		return true;
	}