Detect if number is a multiple of 7 or contains the number 7

Question

I read a book about challenges in Java, and it gives the next question:

create a function, which get a number as argument, and detect if number is a multiple of 7 or contains the number 7.

The signature is: public boolean find7(int num)

I create this function, when the number is between 0 to 99, by the next condition:

if (num mod 7 == 0 || num / 10 ==7 || num mod 10 == 7)
   return true;

But what with number which is greater than 99? like 177, or 709? How can I detect it?

Answer 1

It's probably best to leave strings out of this:

public static boolean check(final int n) {
    int m = Math.abs(n);
    while (m > 0) {
        if (m % 10 == 7)
            return true;

        m /= 10;
    }

    return n % 7 == 0;
}

The while-loop checks each digit and tests if it is 7; if it is, we return true and if it isn't, we continue. We reach the final return statement only if none of the digits were 7, at which point we return whether the number is a multiple of 7.

Answer 2

if (num % 7 ==0 || ("" + num).contains("7") ){
    return true;
}