Write a program to String reverse without using any string Api?

package com.instaceofjava;

public class ReverseString {

public static void main(String[] args) {
String str="Hello world";

String revstring="";

for(int i=str.length()-1;i>=0;--i){
revstring +=str.charAt(i);
}
System.out.println(revstring);

}

}

Output: dlrow olleH