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())

Here's some Java 8 lambda to do so:

// original List<List<String>>
List<List<String>> nestedList = new ArrayList<>();
// adding test values
nestedList.add(new ArrayList<String>(){{add("a");add("b");}});
nestedList.add(new ArrayList<String>(){{add("c");add("c");}});
nestedList.add(new ArrayList<String>(){{add("e");add("f");}});
// printing the original List<List<String>>
System.out.println(nestedList);
String[][] nestedArray = nestedList
    // using the stream API
    .stream()
    // mapping each `List`...
    .map(
        // ... to a resulting array
        (l) -> l.toArray(new String[l.size()])
    )
    // collecting as a List<String[]>
    .collect(Collectors.toList())
    // converting the resulting List<String[]> to a String[][]
    .toArray(new String[nestedList.size()][]);
// printing the resulting 2-dimensional array
System.out.println(Arrays.deepToString(nestedArray));

Output

[[a, b], [c, c], [e, f]]
[[a, b], [c, c], [e, f]]

You want

String[] strarray = strlist.toArray(new String[0]);

See here for the documentation and note that you can also call this method in such a way that it populates the passed array, rather than just using it to work out what type to return. Also note that maybe when you print your array you'd prefer

System.out.println(Arrays.toString(strarray));

since that will print the actual elements.