String manipulations (reverse a string by characters, permutation)

I'm trying to solve simple Java problems in various ways. Could anyone comment about the code, like how it could be better, and what could be a problem?

// When using already-defined-method is allowed
static String reverseStringByChar1(String str) {
    return str == null ? null : new StringBuilder(str).reverse().toString();
}

// When using already-defined-method is not allowed
static String reverseStringByChar2(String str) {
    if (str == null)
        return null;

    int length = str.length();
    char[] charArray = new char[length];

    for (int i = length-1, j = 0; i >= 0; i--, j++)
        charArray[i] = str.charAt(j);

    return new String(charArray);
}

// When using extra data structure is not allowed 
// (Is there any way not to use extra memory? Here like the temp.)
static String reverseCharacterArray(char[] chars) {
    if(chars.length == 0)
        return null;

    for (int i = 0, j = chars.length-1; i < chars.length / 2; i++, j--) {
        /* char temp = chars[i];
        chars[i] = chars[j];
        chars[j] = temp;
        */
        char temp = chars[i];
        chars[i] = chars[chars.length-1-i];
        chars[chars.length-1-i] = temp;

    }
    return new String(chars);
}

// When comparing two strings to see if they are permutations of each other.
static boolean arePermutations(String str1, String str2) {
    // can be trimmed
    // need to know what to do with uppercases and lowercase

    if (str1 == null || str2 == null || str1.length() != str2.length())
        return false;

    // assuming askii code
    boolean[] str1Array = new boolean[128];

    for (char c : str1.toCharArray())
        str1Array[c] = true;

    for (char c : str2.toCharArray()) {
        if (!str1Array[c])
            return false;
    }

    return true;
}