Hints

Java

Calculate hamming distance by comparing bits: Integer.bitCount(x ^ y)
Utilize split() to count appearance of characters, add blank space in the beginning and end of the String first
string.replaceAll("regular expression","")
Count prime numbers: Sieve of Eratosthenes

public int countPrimes_1(int n)
{
    boolean[] isPrime = new boolean[n];
    for (int i = 2; i < n; i++)
        isPrime[i] = true;
    for (int i = 2; i * i < n; i++)
    {
        if (!isPrime[i])
            continue;
        for (int j = i * i; j < n; j += i)
            isPrime[j] = false;
    }
    int count = 0;
    for (int i = 0; i < n; i++)
        if (isPrime[i])
            count++;
    return count;
}

Always consider boundary or overflow numbers
When we want to log the appearance of some characters, initialize an integer array with size 256 — since each char maps to a number, use char as index and value as count:

int[] chars = new int[256];
for (char c : s.toCharArray())
{
    chars[c]++;
}