Reverse String without in built method

package com.nataraja.b;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class ReverseString {

public static void main(String[] args) throws IOException {
System.out.println("Enter a Number:");
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
String s=br.readLine();
StringBuffer sb=new StringBuffer();
for(int i=s.length()-1;i>=0;i--){
sb.append(s.charAt(i));
}
System.out.println(sb);
}
}