ISBN 13 java代码

Question

public class ISBN_13 {
    public static int checkDigit (String isbn12){
        int finaldigit = 0;

        int stringLength = isbn12.length();
        if (stringLength!=12){
            return -1;  
        }
        try {
            for (int x = 0; x < 12; x = x +1){
                if (x % 2 ==1)
                    finaldigit = finaldigit + 3*Character.getNumericValue(isbn12.charAt(x));
                else {
                    finaldigit = finaldigit + Character.getNumericValue(isbn12.charAt(x));

                }
            }
        }
        catch (RuntimeException e) { 
            return -1;
        }
        finaldigit = finaldigit % 10;
        return 10 - finaldigit; 
    }
}

当我通过一个检查代码的程序运行它时（CodeJudge）它继续告诉我运行时错误，缺少类isbn 13.我的代码有问题吗？