【1】Detect if a string has all unique characters

Question: Implement an algorithm to determine if a string has all unique characters. What if you can not use additional
data structures?

package CareerCup;

public class UniqueChar
{
public UniqueChar(){}

public boolean Detect(String str)
{
boolean[] charList = new boolean[256];
for(int i=0;i<str.length();i++)
{
if(charList[str.charAt(i)]) return false;
else charList[str.charAt(i)] = true;
}
return true;
}

public static void main(String[] args)
{
String str = "abcdaf";
UniqueChar uqc = new UniqueChar();
if(uqc.Detect(str)) System.out.println("String: "+str+" has all unique characters!");
else System.out.println("String: "+str+" has not all unique characters!");
}
}