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.

Solution 1

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.

Solution 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(",");

Solution 3

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.

Solution 4

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.

Solution 5

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).

View more solutions

Comments

[BegemoT]

why not just Arrays.asList("s1", "s2", "s3")?

[Jon Skeet]

@BegemoT: I was adding that - personally I prefer the explicit nature of the Guava version.

[aioobe]

The resulting list is fix-sized though.

[Mathias Schwarz]

@The Elite Gentleman: The result of Arrays.asList is immutable. If a new ArrayList is constructed as in str2 then it is possible to add to the list afterwards.

[aioobe]

@Mathias Schwarz, not immutable, fixed sized.

[Mathias Schwarz]

@aioobe: You are right! It is possible to overwrite existing entries.

[aioobe]

which is unmodifiable -- It's actually modifiable (but fixed-sized).

[Paŭlo Ebermann]

You should note that the initialization block variant creates an anonymous subclass of the ArrayList class.

[Jon Skeet]

@Matt: No problem. I'll delete the one replying to you, and then delete this one soon, if you want to delete your second one too :)

[Ashish Lohia]

With all respect to Jon Skeet. Isn't it better to use Arrays.asList() instead of ImmutableList.of(), as the later, will maintain the copied array. Can you please share some light on this?

[Jon Skeet]

@AshishLohia: Not if you want the list to be immutable. That's why I give multiple options - the best approach depends on what you want the result to be.

[Ashish Lohia]

I completely agree with you @JonSkeet . Thanks for clarifying.