Java : unique characters

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */

class nonStatic
{
public boolean isUnique(String str)
{
if(str.length() > 256)
return false;
 
   boolean[] charset = new boolean[256];
   for (int i =0;i<str.length();i++)
   {
    int val = str.charAt(i);
    if(charset[val])
    return false;
   
    charset[val] = true;
   
   
   }
   return true;
}
}

class Ideone
{

public static void main (String[] args) throws java.lang.Exception
{
boolean t;
nonStatic n = new nonStatic();
t = n.isUnique("sd");
System.out.println("hi" + t);
}



}