with - java quick initialize arraylist




How to quickly and conveniently create a one element arraylist (6)

This question already has an answer here:

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)


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));

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.


Very simply:

Arrays.asList("Hi!")

With Java 8 Streams:

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

or if you need a set:

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

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));

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.


Collections.singletonList(object)

the list created by this method is immutable.





collections