One simple way is to append your list items in a StringBuilder

List<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);

StringBuilder b = new StringBuilder();
list.forEach(b::append);

System.out.println(b);

you can also try:

String s = list.stream().map(e -> e.toString()).reduce("", String::concat);

Explanation: map converts Integer stream to String stream, then its reduced as concatenation of all the elements.

Note: This is normal reduction which performs in O(n²)

for better performance use a StringBuilder or mutable reduction similar to F. Böller's answer.

String s = list.stream().map(Object::toString).collect(Collectors.joining(","));

Ref: Stream Reduction

In Java 8 or later:

String listString = String.join(", ", list);

In case the list is not of type String, a joining collector can be used:

String listString = list.stream().map(Object::toString)
                        .collect(Collectors.joining(", "));

You are almost there. You just need to use Stream.collect.

Here is the modified code.

String list2 = IntStream.range(0, list.size())
        .mapToObj(i-> i+1 + ". " + list.get(i))
        .collect(Collectors.joining(System.lineSeparator()));

output:

1. book1
2. book2
3. book3
4. book4

Use Stream.concat inside the flatMap operation:

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