String has all unique characters

//Implement an algorithm to determine if a string has all unique characters. What if you cannot use additional data structures?

import java.util.Arrays;
class Main {
    public static void main(String[] args) {
        System.out.println(isUnique("ABCDEFGIJKLMNOPQRSTWXYZ"));
        /*
        
        */
    }
    
    public static boolean isUnique(String str){
        if(str == null && str.length() == 0){
            return false;
        }
        
        char tempArray[] = str.toCharArray();
        Arrays.sort(tempArray);
        for(int i=1;i<tempArray.length;i++){
            if(tempArray[i] == tempArray[i-1]){
                return false;
            }
        }
        return true;
        
    }
}