don't answer this test question.what is this method used for?

Copy public class StringPrograms { public static void main (String args) { String str = "123"; System. out. print In( reverse(str) ) ; public static String reverse(String in) { if (in == null) throw new IllegalArgumentException("Null is not valid input"); StringBuilder out = new StringBuilder(); char chars = in. toCharArray ( ); for (int i = chars. length - 1; i >= 0; 1--) out . append ( chars [i] ) ; return out. toString( )