I was going to point you to my earlier answer on this topic but it turns out that you've already linked to that question. The other answer also provides useful information.

If you want char values, you can use the IntStream returned by String.chars() and cast the int values to char without loss of information. The other answers explained why there's no CharStream primitive specialization for the Stream class.

If you really want boxed Character objects, then use mapToObj() to convert from IntStream to a stream of reference type. Within mapToObj(), cast the int value to char. Since an object is expected as a return value here, the char will be autoboxed into a Character. This results in Stream<Character>. For example,

Stream<Character> sch = "abc".chars().mapToObj(i -> (char)i);
sch.forEach(ch -> System.out.printf("%c %s%n", ch, ch.getClass().getName()));

a java.lang.Character
b java.lang.Character
c java.lang.Character

The exception asks you for an array instead comma-sepparated strings:

// incorrect
String.format("%02d:%02d:%02d",hour,minute,second);

// fast but correct
Object[] data = { hour, minute, second };
String.format("%02d:%02d:%02d", data);

But actually, method format(String,object[]) does not exists in String, it is: format(String pattern, Object... arguments) what should work with commas ,. There is something with your syntax, but not in the shown code.

Use Stream.concat inside the flatMap operation:

references.stream()
        .flatMap(user -> Stream.concat(
            Stream.of(user.getId()),
            user.getExternalIds().stream()
        ))
        .collect(Collectors.toList())