Use String.join to turn values you already have into a string with a delimiter. Use StringJoiner when you want to add values incrementally or wrap the result with a prefix and suffix. For values in a stream pipeline, use Collectors.joining.
Choose the right Java 8 join API
| Situation | Use | Why |
|---|---|---|
| Values are already available as individual arguments or an iterable | String.join |
It directly returns a String with the delimiter between values. |
| Values arrive one at a time, or the output needs a prefix and suffix | StringJoiner |
It supports incremental additions and configurable wrappers. |
| Values are flowing through a stream pipeline | Collectors.joining |
It collects stream elements into a string in encounter order. |
These APIs accept CharSequence values, such as String; they do not apply custom formatting to arbitrary objects. Convert or map other values to text before joining. See Oracle’s Java 8 CharSequence API.
Join values with String.join
Java 8 adds two String.join overloads: one accepts a delimiter and varargs, and the other accepts a delimiter and an Iterable<? extends CharSequence>. Both return a new string with the delimiter placed between elements. The methods are marked “Since: 1.8” in Oracle’s Java SE 8 String API.
Join individual values
String message = String.join("-", "Java", "is", "cool");
// Java-is-coolThe first argument is the delimiter; the remaining arguments are the values to join.
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List<String> words = Arrays.asList("Java", "is", "cool");
String message = String.join(" ", words);
// Java is coolThe collection overload accepts an iterable of character sequences. The result follows the iterable’s iteration order, so use an ordered collection when output order matters.
Build a result incrementally with StringJoiner
Create a StringJoiner with a delimiter, then call add for each value. To surround the joined content, use the constructor that also takes a prefix and suffix. Calling toString() produces the result.
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StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.add("red").add("green").add("blue");
String result = joiner.toString();
// [red, green, blue]add returns the same joiner, which allows calls to be chained. Oracle documents the constructors, additions, and string representation in its Java SE 8 StringJoiner API.
Control the empty result
An empty joiner with no wrappers produces an empty string. An empty joiner constructed with a prefix and suffix produces those two strings together. Use setEmptyValue to choose a different representation while the joiner has no elements:
StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.setEmptyValue("[]");
String result = joiner.toString();
// []Once add has been called, the joiner is no longer considered empty—even if the added value is an empty string—so the empty-value replacement no longer applies.
Merge joiners when combining accumulated results
merge appends the other joiner’s contents without its prefix or suffix. If the joiners use different delimiters, the other joiner’s accumulated text is appended as a single element, so its internal delimiter remains part of that text.
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Join values from a stream
For a stream pipeline, collect with Collectors.joining. Its overloads concatenate stream values in encounter order, with optional delimiter, prefix, and suffix, as documented in Oracle’s Java SE 8 Collectors API.
String result = words.stream()
.collect(Collectors.joining(", "));To wrap the joined output, provide all three strings:
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String result = words.stream()
.collect(Collectors.joining(", ", "[", "]"));Nulls and other edge cases
- Null elements:
String.joinandStringJoiner.addrender a null element as the textnull; they do not skip it. - Null delimiter:
String.joinand theStringJoinerconstructors reject a null delimiter withNullPointerException. The three-argumentStringJoinerconstructor also rejects a null prefix or suffix. - Null input container:
String.joinrejects a null varargs array or a null iterable. - Empty input: Joining no values adds no element text. For
StringJoiner, the rendered result depends on its wrappers and any configured empty value.
Oracle’s Java 8 String documentation states: “Note that if an element is null, then “null” is added.”
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