How to determine if String has all unique characters

It is a most basic question asked in interviews to check if string contains all unique characters or not.
Below sample program is a basic solution to this problem.

 /**  
  *   
  */  
 /**  
  * Program to determine if a string has all unique characters.  
  *   
  * @author Dixit  
  *   
  */  
 public class StringUniqueCharacters {  
      /**  
       * @param args  
       */  
      public static void main(String[] args) {  
           String firstString = "America";  
           String secondString = "India";  
           String thridString = "Italy";  
           System.out  
                     .println("Result:" + isUniqueChars(firstString.toLowerCase()));  
           System.out.println("Result:"  
                     + isUniqueChars(secondString.toLowerCase()));  
           System.out  
                     .println("Result:" + isUniqueChars(thridString.toLowerCase()));  
      }  
      public static boolean isUniqueChars(String str) {  
           //Initialise a boolean array  
           boolean[] char_set = new boolean[256];  
           for (int i = 0; i < str.length(); i++) {  
                //get ASCII value of characters  
                int val = str.charAt(i);  
                //Check in the boolean array if it is unique then set the value to true at corresponding index position  
                //if value is already set at the required index position then return false  
                if (char_set[val])  
                     return false;  
                char_set[val] = true;  
           }  
           return true;  
      }  
 }  

Enjoy Programming.