Compiler error "Bad return type in lambda expression" when reducing a list of values to a delimited string

In a static method I've implemented inner class:

 class Inner implements  Comparable<Inner>
    {
        Integer v;
        String s;

        public Inner(Integer a, String st) {v = a; s= st; }

        @Override
        public int compareTo(Inner o) {
            int result = Integer.compare(this.v,o.v);

            if (result == 0)
                return (String.CASE_INSENSITIVE_ORDER.compare(this.s,o.s));
        else
            return result;
        }
    }

Its purpose is to hold String and Integer values. Additionally, the class implements the Comparable interface so that it's sortable.

The main container for Inner class instances is ArrayList:

ArrayList<Inner> cont = new ArrayList<>();

After filling this list I want to sort it and next concat String values, all of this using streams.

String  res =  cont.stream().sorted(Inner::compareTo).reduce(" ",(String r, Inner e)->{return r+=e.s+" ";});

Unfortunately, the compiler produces an error:

Bad return type in lambda expression: String cannot be converted to Inner

Is there any way to do this, still using streams?

You should take the s of Inner using .map() before reduce. And it's better to use Collectors.joining to join the string with delimiter

cont.stream().sorted(Inner::compareTo).map(e -> e.s).collect(Collectors.joining(" "));