Finding whether string has repeated chars

//finding if a string has repeated chars
// no additional data structures

public class HelloWorld{

     public static void main(String []args){
        System.out.println(charComp("7536481"));
     }

     /*public static Boolean unique (String s){
        if (s.length() > 100)
            return false;
        else
            return charComp(s);
     }*/

     public static Boolean charComp(String s){
        if (s.isEmpty())
            return true;

        char c = s.charAt(0);
        System.out.println(c);
        int index = s.indexOf(c,1);
        System.out.println(index);

        if (index!=-1)
            return false;
        else
            return(charComp(s.substring(1)));

     }
}