// the only variables used are the char[] representing the string and
// the iterator variable i
public static void reverseInPlace(char[] str) {
    // for each character in the first half of the string, rounded down
    for(int i = 0; i < str.length / 2; i++) {
        // if opposite characters are equal, no transformation is needed
        // if the characters differ, swap them
        if(str[i] != str[str.length - i - 1]) {
            // this bit-xoring trick works as long as the values being
            // swapped are not equivalent
            str[i] ^= str[str.length - i - 1];
            str[str.length - i - 1] ^= str[i];
            str[i] ^= str[str.length - i - 1];
        }
    }
}

public static void main(String[] args) {
    String s = "abcdefg";
    char[] arr = s.toCharArray();
    System.out.println("arr: " + new String(arr));
    reverseInPlace(arr);
    System.out.println("arr rev: " + new String(arr));
}