public class ReverseString {

public static void main(String[] args) {

String str = "789";

System.out.println(reverseString(str));

}

public static String reverseString(String in) {

if (in == null)

throw new IllegalArgumentException("input null");

StringBuilder out = new StringBuilder();

char[] chars = in.toCharArray();

for (int i = chars.length - 1; i >= 0; i--)

out.append(chars[i]);

return out.toString();

}

}