[java] How to quickly and conveniently create a one element arraylist

Is there a Utility method somewhere that can do this in 1 line? I can't find it anywhere in Collections, or List.

public List<String> stringToOneElementList(String s) {
    List<String> list = new ArrayList<String>();
    list.add(s);
    return list;
}

I don't want to re-invent the wheel unless I plan on putting fancy rims on it.

Well... the type can be T, and not String. but you get the point. (with all the null checking, safety checks...etc)

Yet another alternative is double brace initialization, e.g.

new ArrayList<String>() {{ add(s); }};

but it is inefficient and obscure. Therefore only suitable:

• in code that doesn't mind memory leaks, such as most unit tests and other short-lived programs;
• and if none of the other solutions apply, which I think implies you've scrolled all the way down here looking to populate a different type of container than the ArrayList in the question.

With Java 8 Streams:

Stream.of(object).collect(Collectors.toList())

or if you need a set:

Stream.of(object).collect(Collectors.toSet())

Collections.singletonList(object)

the list created by this method is immutable.

You can use the utility method Arrays.asList and feed that result into a new ArrayList.

List<String> list = new ArrayList<String>(Arrays.asList(s));

Other options:

List<String> list = new ArrayList<String>(Collections.nCopies(1, s));

and

List<String> list = new ArrayList<String>(Collections.singletonList(s));

• ArrayList(Collection) constructor.
• Arrays.asList method.
• Collections.nCopies method.
• Collections.singletonList method.

With Java 7+, you may use the "diamond operator", replacing new ArrayList<String>(...) with new ArrayList<>(...).

Java 9

If you're using Java 9+, you can use the List.of method:

List<String> list = new ArrayList<>(List.of(s));

Regardless of the use of each option above, you may choose not to use the new ArrayList<>() wrapper if you don't need your list to be mutable.

Seeing as Guava gets a mention, I thought I would also suggest Eclipse Collections (formerly known as GS Collections).

The following examples all return a List with a single item.

Lists.mutable.of("Just one item");
Lists.mutable.with("Or use with");
Lists.immutable.of("Maybe it must be immutable?");
Lists.immutable.with("And use with if you want");

There are similar methods for other collections.

The other answers all use Arrays.asList(), which returns an unmodifiable list (an UnsupportedOperationException is thrown if you try to add or remove an element). To get a mutable list you can wrap the returned list in a new ArrayList as a couple of answers point out, but a cleaner solution is to use Guava's Lists.newArrayList() (available since at least Guava 10, released in 2011).

For example:

Lists.newArrayList("Blargle!");

Very simply:

Arrays.asList("Hi!")