Java: convert List<String> to a join()d String

JavaScript has Array.join()

js>["Bill","Bob","Steve"].join(" and ")
Bill and Bob and Steve

Does Java have anything like this? I know I can cobble something up myself with 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();
}

.. but there's no point in doing this if something like it is already part of the JDK.

Solution 1:

With Java 8 you can do this without any third party library.

If you want to join a Collection of Strings you can use the new String.join() method:

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

If you have a Collection with another type than String you can use the Stream API with the joining Collector:

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"

The StringJoiner class may also be useful.

Solution 2:

All the references to Apache Commons are fine (and that is what most people use) but I think the Guava equivalent, Joiner, has a much nicer API.

You can do the simple join case with

Joiner.on(" and ").join(names)

but also easily deal with nulls:

Joiner.on(" and ").skipNulls().join(names);

or

Joiner.on(" and ").useForNull("[unknown]").join(names);

and (useful enough as far as I'm concerned to use it in preference to commons-lang), the ability to deal with Maps:

Map<String, Integer> ages = .....;
String foo = Joiner.on(", ").withKeyValueSeparator(" is ").join(ages);
// Outputs:
// Bill is 25, Joe is 30, Betty is 35

which is extremely useful for debugging etc.