How to Efficiently Join a List of Strings in Java?

Converting a List to a Joined String in Java

In JavaScript, the Array.join() method offers a convenient way to concatenate elements of an array into a single string. Does Java offer a similar functionality?

If you want to create such a functionality from scratch, you can use StringBuilder:

static public String join(List<String> list, String conjunction) {
   StringBuilder sb = new StringBuilder();
   boolean first = true;
   for (String item : list) {
      if (first) first = false;
      else sb.append(conjunction);
      sb.append(item);
   }
   return sb.toString();
}

However, since Java 8, more efficient and elegant options are available:

String.join

To join a List specifically, you can use the String.join() method:

List<String> list = Arrays.asList("foo", "bar", "baz");
String joined = String.join(" and ", list); // "foo and bar and baz"

Collectors.joining

For collections with elements of types other than String, you can use the Collectors.joining collector with the Stream API:

List<Person> list = Arrays.asList(
  new Person("John", "Smith"),
  new Person("Anna", "Martinez"),
  new Person("Paul", "Watson ")
);

String joinedFirstNames = list.stream()
  .map(Person::getFirstName)
  .collect(Collectors.joining(", ")); // "John, Anna, Paul"

StringJoiner

You may also consider using the StringJoiner class, which offers additional customization options:

StringJoiner joiner = new StringJoiner(", ");
list.stream()
  .forEach(joiner::add);

String joinedFirstNames = joiner.toString(); // "John, Anna, Paul"

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