1. Why would you?
2. What String will you reverse if you don't use String?
  And how will you represent the result without using a String?
3. Are you thinking of extracting the chars into an array and
  reversing them there? Take care with multi-char Unicode characters :)

/L

This obviously an artificial exercise since no one in their right mind
would avoid the built-in StringBuffer/StringBuilder/String methods.

The basic idea is a create char array the correct length, the fill it
one by one from your original string in a loop.  Then when you are
done you convert the char[] to a String.

See http://mindprod.com/jgloss/conversion.html

What is the sound of one hand clapping?
You can't reverse a string unless you have a string, now can you?

If you already have a string, you can *print* the reverse if it by
counting backward from the end of it, and printing each character along
the way.

new StringBuilder( givenString ).reverse().toString();

piotr

 The statement

java.lang.System.out.println( "Hello!" );

 can be split in a model and a view.

 The model (»"Hello!«) is the internal storage of the code
 points H-e-l-l-o-!, the View (»println«) displays this as

Hello!

 One can as well adopt the point of view that for the string
 model it makes no sense to »reverse« anything, because the
 same model can also be viewed as a storage of the reversed
 string, because whether to display the stored code points
 H-e-l-l-o-! as

Hello!

 or as

!olleH

 is solely a question of the View, just as the color of the
 characters used to display it or the size of the font.

 From such a point of view, every string model already /is/
 also a model of the reversed string, without the need for
 an explicit computation to »determine« the reverse string.

 (Admittedly, this point of view is not helpful in some cases.)

String reverse (String in)
    throws IOException
{
    Reader reader = new StringReader(in);

    Stack<Character> stack = new Stack();

    int ch;

    while ((ch = reader.read()) != -1)
    {
        stack.push((char)ch);
    }

    Writer writer = new StringWriter();

    while (!stack.isEmpty())
    {
        writer.write(stack.pop());
    }

    String reversed = writer.toString();

    return reversed;
}