How to quickly and conveniently create a one element arraylist

Question

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)

Answer 1

Fixed size List

The easiest way, that I know of, is to create a fixed-size single element List with Arrays.asList(T...) like

// Returns a List backed by a varargs T.
return Arrays.asList(s);

Variable size List

If it needs vary in size you can construct an ArrayList and the fixed-sizeList like

return new ArrayList<String>(Arrays.asList(s));

and (in Java 7+) you can use the diamond operator <> to make it

return new ArrayList<>(Arrays.asList(s));

Single Element List

Collections can return a list with a single element with list being immutable:

Collections.singletonList(s)

The benefit here is IDEs code analysis doesn't warn about single element asList(..) calls.

Answer 2

Collections.singletonList(object)

the list created by this method is immutable.

Answer 3

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.

Answer 4

With Java 8 Streams:

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

or if you need a set:

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

Answer 5

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!");

Answer 6

Very simply:

Arrays.asList("Hi!")

Answer 7

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.

Answer 8

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.