Check whether a string has uniqe characters

Unique.java

package com.company;

public class Unique {
  public static boolean checkUnique(String word){
        if(word.length()>256) return false;// if a string is more than 256 characters long, it repeats any of the string 
        boolean[] charset = new boolean[256];
        for(int i = 0;i<word.length();i++){
            char c = word.charAt(i);
            if(charset[c]) return false; 
            charset[c]=true;
        }
            
        return true;
    }
    
    
    public static void main(String[] args) {
      System.out.println(checkUnique("Helow"));
    }
}