How to find duplicate characters in a string and concat them to a new string

Question

• I take in a string aabullc

• returingStrin should be assigned aall after the loop since a and l are duplicates

• strings with more then 1 dupe like example: (3x a). aaabullc will never be passed in

  public static String rearrangeLetters(String S)
  {
  
  String returnString;
  
   for(int i = 0; i<S.length()-1; i++){
       for(int j = i+1; i<S.length()-1-i; j++){
           if(S.charAt(i) == (S.charAt(j))){
           String char1 = String.valueOf(S.charAt(i));
           returnString = returnString.concat(char1);
           }
        }
    }
  
    System.out.println(returnString);
  
      return returnString;
  }
  

Code briefing:

• nested for loop starts at first letter in string and compares to the second letter until it reachs the end.
• the inner loop length gets sorter depending on the outer loops.
• if character matches we add it to returningString

Issue: does not like String char

Answer 1

char is a reserved keyword in JAVA. So it cannot be used as a variable name.