无法将ascii 121打印到字符y，而是打印特殊字符

Question

Given a String, I have to find the ASCII of each character in the string. If any of the ASCII's are not prime, it must be converted to the nearest prime ASCII. If two prime ASCII's are equidistant from the original ASCII, the lower is taken. After the above operation I need to convert the resulting ASCII to a String.

But my code's problem is it can't print y form ASCII 121

Please help me finding the error; Thanks in advance

    int T = s.nextInt();

    int len;
    byte lowerPrime, greaterPrime, temp;
    String word;
    byte[] b;
    for (int i = 0; i < T; i++) {
        len = s.nextInt();
        s.nextLine();
        word = s.nextLine();

        b = word.getBytes(StandardCharsets.US_ASCII);

        for (int k = 0; k < b.length; k++) {
            temp = b[k];
            if (!checkPrime(temp)) {
                lowerPrime = findPrimeSmaller(temp);
                greaterPrime = findPrimeGreater(temp);

                if ((temp - lowerPrime) == (greaterPrime - temp)) {
                    b[k] = lowerPrime;
                } else if ((temp - lowerPrime) < (greaterPrime - temp)) {
                    b[k] = lowerPrime;
                } else if ((temp - lowerPrime) > (greaterPrime - temp)) {
                    b[k] = greaterPrime;
                }
            }
        }

        System.out.println(new String(b, "UTF-8"));

    }

}

private static boolean checkPrime(byte n) {

    if (n == 1) {
        return false;
    }

    byte i = 2;

    while (i < (n / 2)) {
        if (n % i == 0) {
            return false;
        }
        i++;
    }
    return true;
}

private static byte findPrimeGreater(byte n) {
    while (!checkPrime(n)) {
        n++;
    }
    return n;
}

private static byte findPrimeSmaller(byte n) {
    while (!checkPrime(n)) {
        n--;
    }
    return n;
}

Answer 1

Your code is working as intended. Here an online compiler with a few added debug-lines.

As you can see, it will find 127 for the character y ( 121 ) in the input, being the first greater prime. The ASCII character with code-point 127 is an unprintable character however , which makes it appear as if there isn't a character in the output.

You may want to either restrict your code to the printable ASCII range [32,126] , by adding some checks to the findPrimeSmaller and findPrimeGreater methods so they don't go beyond this limit (of course, don't return that limit, since it isn't a prime, but return Integer.MIN_VALUE and Integer.MAX_VALUE or something like that instead when it has reached the boundary, so you know it will pick the other prime instead in your if-checks).
Or you may want to skip unprintable characters if you want to non-ASCII unicode characters in your program.

Also, a general note: using an IDE debugger or adding print lines like I did in the online version above is helpful in checking where the code goes wrong and what it does. In this case your code works completely fine, but the resulting character simply is an unprintable character (with unicode value 127 ).

PS: This part of your code can be simplified:

if ((temp - lowerPrime) == (greaterPrime - temp)) {
    b[k] = lowerPrime;
} else if ((temp - lowerPrime) < (greaterPrime - temp)) {
    b[k] = lowerPrime;
} else if ((temp - lowerPrime) > (greaterPrime - temp)) {
    b[k] = greaterPrime;
}

To:

if ((temp - lowerPrime) <= (greaterPrime - temp)) { // Single <=, instead of loose == and <
    b[k] = lowerPrime;
} else if ((temp - lowerPrime) > (greaterPrime - temp)) {
    b[k] = greaterPrime;
}