Reverse String - keeping performance in mind (without StringBuffer) in java

package org.cmania;

public class ReverseString
{
    public static void main(String[] args)
{

String str="abcde"; //String to be reversed
       
                System.out.println("Original String: "+str);
                System.out.println("Reversed String: "+reverseString(str));        
        }

 /*
  * return reversed String with best performance.
  * Best case: O(n/2)- (1/2), when we have odd number of characters in string.
  * Average case: O(n/2) , generally when we have even number of characters in string.
  * Worst case: O(n/2).
  */

public static String reverseString(String str)
{
        char ar[]=str.toCharArray();
        char temp;
        for(int i=0,j=ar.length-1; i<(ar.length/2); i++,j--){
               temp=ar[i];
               ar[i]=ar[j];
               ar[j]=temp;
        }
        return new String(ar);
 }
}