What is the most efficient algorithm for reversing a String in Java?

You said you don't want to do it the easy way, but for those Googling you should use StringBuilder.reverse:

String reversed = new StringBuilder(s).reverse().toString();

If you need to implement it yourself, then iterate over the characters in reverse order and append them to a StringBuilder. You have to be careful if there are (or can be) surrogate pairs, as these should not be reversed. The method shown above does this for you automatically, which is why you should use it if possible.

The following does not deal with UTF-16 surrogate pairs.

public static String reverse(String orig)
{
    char[] s = orig.toCharArray();
    int n = s.length;
    int halfLength = n / 2;
    for (int i=0; i<halfLength; i++)
    {
        char temp = s[i];
        s[i] = s[n-1-i];
        s[n-1-i] = temp;
    }
    return new String(s);
}

You say you want to know the most efficient way and you don't want to know some standard built-in way of doing this. Then I say to you: RTSL (read the source, luke):

Check out the source code for AbstractStringBuilder#reverse, which gets called by StringBuilder#reverse. I bet it does some stuff that you would not have considered for a robust reverse operation.