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;

}

}