Java reference

Collectors.joining()

A collector that joins the strings of a stream into one.

Available since: Java 8

Syntax

static Collector<CharSequence, ?, String> joining()
static Collector<CharSequence, ?, String> joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)

Parameters

Parameter	Type	Description
delimiter	CharSequence	Optional delimiter.
prefix	CharSequence	Optional string at the start.
suffix	CharSequence	Optional string at the end.

Return value

Collector<CharSequence, ?, String>A collector into a single string.

Examples

A delimiter, a prefix and a suffix:

void main() {
    List<String> tags = List.of("java", "streams", "api");
    IO.println(tags.stream().collect(Collectors.joining()));
    IO.println(tags.stream().collect(Collectors.joining(", ")));
    IO.println(tags.stream().collect(Collectors.joining(" | ", "[", "]")));
}

Output:

javastreamsapi
java, streams, api
[java | streams | api]

Numbers must be turned into strings first:

void main() {
    String csv = Stream.of(3, 1, 4).map(String::valueOf).collect(Collectors.joining(","));
    IO.println(csv);
}

Output:

3,1,4

Notes

It works only with a stream of strings (CharSequence), so numbers and objects are first converted with map(String::valueOf). For an existing collection of strings without a stream, String.join(", ", list) works.

Related recipes

In the tutorial

Collectors — The collect method and the Collectors class — toList, toSet, toMap, joining, counting, summingInt, averagingDouble and the duplicate key error.

Thanks for reading!