What is the shortest way to initialize List of strings in java?

Question

I am searching for the shortest way (in code) to initialize list of strings and array of strings, i.e. list/array containing "s1", "s2", "s3" string elements.

Answer 1

With Eclipse Collections, you can write the following:

List<String> list = Lists.mutable.with("s1", "s2", "s3");

You can also be more specific about the types and whether they are Mutable or Immutable.

MutableList<String> mList = Lists.mutable.with("s1", "s2", "s3");
ImmutableList<String> iList = Lists.immutable.with("s1", "s2", "s3");

You can also do the same with Sets, Bags and Maps:

Set<String> set = Sets.mutable.with("s1", "s2", "s3");
MutableSet<String> mSet = Sets.mutable.with("s1", "s2", "s3");
ImmutableSet<String> iSet = Sets.immutable.with("s1", "s2", "s3");

Bag<String> bag = Bags.mutable.with("s1", "s2", "s3");
MutableBag<String> mBag = Bags.mutable.with("s1", "s2", "s3");
ImmutableBag<String> iBag = Bags.immutable.with("s1", "s2", "s3");

Map<String, String> map = 
    Maps.mutable.with("s1", "s1", "s2", "s2", "s3", "s3");
MutableMap<String, String> mMap = 
    Maps.mutable.with("s1", "s1", "s2", "s2", "s3", "s3");
ImmutableMap<String, String> iMap = 
    Maps.immutable.with("s1", "s1", "s2", "s2", "s3", "s3");

There are factories for SortedSets, SortedBags and SortedMaps as well.

SortedSet<String> sortedSet = SortedSets.mutable.with("s1", "s2", "s3");
MutableSortedSet<String> mSortedSet = SortedSets.mutable.with("s1", "s2", "s3");
ImmutableSortedSet<String> iSortedSet = SortedSets.immutable.with("s1", "s2", "s3");

SortedBag<String> sortedBag = SortedBags.mutable.with("s1", "s2", "s3");
MutableSortedBag<String> mSortedBag = SortedBags.mutable.with("s1", "s2", "s3");
ImmutableSortedBag<String> iSortedBag = SortedBags.immutable.with("s1", "s2", "s3");

SortedMap<String, String> sortedMap =
        SortedMaps.mutable.with("s1", "s1", "s2", "s2", "s3","s3");
MutableSortedMap<String, String> mSortedMap =
        SortedMaps.mutable.with("s1", "s1", "s2", "s2", "s3","s3");
ImmutableSortedMap<String, String> iSortedMap =
        SortedMaps.immutable.with("s1", "s1", "s2", "s2", "s3","s3");

Note: I am a committer for Eclipse Collections.

Answer 2

Java 9+

Java 9 introduces a convenience method List.of used as follows:

List<String> l = List.of("s1", "s2", "s3");

Java 8 and older

Here are a few alternatives:

// Short, but the resulting list is fixed size.
List<String> list1 = Arrays.asList("s1", "s2", "s3");

// Similar to above, but the resulting list can grow.
List<String> list2 = new ArrayList<>(Arrays.asList("s1", "s2", "s3"));

// Using initialization block. Useful if you need to "compute" the strings.
List<String> list3 = new ArrayList<String>() {{
    add("s1");
    add("s2");
    add("s3");
}};

When it comes to arrays, you could initialize it at the point of declaration like this:

String[] arr = { "s1", "s2", "s3" };

If you need to reinitialize it or create it without storing it in a variable, you do

new String[] { "s1", "s2", "s3" }

If the string constants are may though, it would look like

String[] arr = { "s1", "s2", "s3", "s4", "s5", "s6", "s7", "s8", "s9", "s10",
                 "s11", "s12", "s13" };

In these cases I usually prefer writing

String[] arr = "s1,s2,s3,s4,s5,s6,s7,s8,s9,s10,s11,s12,s13".split(",");

Answer 3

JDK2

List<String> list = Arrays.asList("one", "two", "three");

JDK7

//diamond operator
List<String> list = new ArrayList<>();
list.add("one");
list.add("two");
list.add("three");

JDK8

List<String> list = Stream.of("one", "two", "three").collect(Collectors.toList());

JDK9

List<String> list = List.of("one", "two", "three");

Plus there are lots of other ways supplied by other libraries like Guava.

Answer 4

I am not familiar with Guava, but almost all solutions mentioned in other answers and using the JDK suffer have some flaws:

1. The Arrays.asList method returns a fixed-size list.

2. {{Double-brace initialization}} is known to create classpath clutter and slow down execution. It also requires one line of code per element.

3. Java 9: the static factory method List.ofreturns a structurally immutable list. Moreover, List.of is value-based, and therefore its contract does not guarantee that it will return a new object each time.

---

Here is a Java 8 one-liner for gathering multiple individual objects into an ArrayList, or any collection for that matter.

List<String> list = Stream.of("s1", "s2", "s3").collect(Collectors.toCollection(ArrayList::new));

Note: aioobe's solution of copying a transitory collection (new ArrayList<>(Arrays.asList("s1", "s2", "s3"))) is also excellent.

Answer 5

There are various options. Personally I like using Guava:

List<String> strings = Lists.newArrayList("s1", "s2", "s3");

(Guava's a library worth having anyway, of course :)

Using just the JDK, you could use:

List<String> strings = Arrays.asList("s1", "s2", "s3");

Note that this will return an ArrayList, but that's not the normal java.util.ArrayList - it's an internal one which is mutable but fixed-size.

Personally I prefer the Guava version as it makes it clear what's going on (the list implementation which will be returned). It's also still clear what's going on if you statically import the method:

// import static com.google.common.collect.Lists.newArrayList;
List<String> strings = newArrayList("s1", "s2", "s3");

... whereas if you statically import asList it looks a little odder.

Another Guava option, if you don't want a modifiable-in-any-way list:

ImmutableList<String> strings = ImmutableList.of("s1", "s2", "s3");

I typically want to either have a completely mutable list (in which case Lists.newArrayList is best) or a completely immutable list (in which case ImmutableList.of is best). It's rare that I really want a mutable-but-fixed-size list.

Answer 6

You can use the Arrays class in the standard Java API: http://download.oracle.com/javase/6/docs/api/java/util/Arrays.html#asList(T...)

List<String> strings = Arrays.asList("s1", "s2", "s3");

Be aware that the resultning list is fixed-size (you cannot add to it).

Answer 7

List<String> stringList = Arrays.asList("s1", "s2", "s3");

All these objects exists in the JDK.

PS: As aioobe stated, this makes the list fixed-sized.