How can I initialize a collection and add data on the same line?

Question

In C# I can create a collection of some kind and initialize it with data on the same line.

var foo = new List<string> {"one","two","three"};

Is there an equivalent way to do this in Java?

likely because there are many duplicates of this question which are easily searchable. — Ryan Ransford, Oct 04 '13 at 15:41
I did search. for some reason nothing popped up. I'll remove it — , Oct 04 '13 at 15:42
http://stackoverflow.com/questions/13395114/how-to-initialize-liststring-object-in-java — My Stack Overfloweth, Oct 04 '13 at 15:43

score 49 · Accepted Answer · answered Oct 04 '13 at 15:33

49

If you need a read-only List

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

// Can't add since the list is immutable
numbers.add("four"); // java.lang.UnsupportedOperationException

If you would like to modify the List later on.

List<String> numbers2 = new ArrayList<String>(
                            Arrays.asList("one","two","three"));
numbers2.add("four");

System.out.println(numbers2); // [one, two, three, four]

answered Oct 04 '13 at 15:33

Ravi K Thapliyal

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It's worth noting that `Arrays.asList` produces a **MUTABLE** list. The documentations indicates that the resulting list has a fixed size but `numbers.set(0, "one")` will absolutely modify both the list and underlying array. – Chris Kerekes Apr 05 '18 at 15:55
For interface driven people.........this will work too: Collection inlineCollection = Arrays.asList( myObjectInstantiatedOne, myObjectInstantiatedTwo, myObjectInstantiatedThree ); Thanks for this answer Ravi! – granadaCoder Aug 31 '18 at 12:16

Daniel Imms · Answer 2 · 2013-10-04T15:45:47.693

10

You can use Arrays.asList(T... a)

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

As Boris mentions in the comments the resulting List is immutable (ie. read-only). You will need to convert it to an ArrayList or similar in order to modify the collection:

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

You can also create the List using an anonymous subclass and initializer:

List<String> foo = new ArrayList<String>() {
    {
        add("one");
        add("two");
        add("three");
    }
};

edited Oct 04 '13 at 15:45

answered Oct 04 '13 at 15:29

Daniel Imms

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This makes the the resulting List immutable. – Boris the Spider Oct 04 '13 at 15:30

Sebastiaan van den Broek · Answer 3 · 2013-10-04T15:41:11.083

I prefer doing this using the Guava (formerly called Google Collections) library, which both removes the need to write the type down again AND has all kinds of ways of adding data straight away.

Example: List<YourClass> yourList = Lists.newArrayList();

Or with adding data: List<YourClass> yourList = Lists.newArrayList(yourClass1, yourclass2);

The same works for all other kinds of collections and their various implementations. Another example: Set<String> treeSet = Sets.newTreeSet();

You can find it at https://code.google.com/p/guava-libraries/

Ryan Ransford · Answer 4 · 2013-10-04T15:37:00.467

1

The best that I've been able to come up with is:

final List<String> foo = new ArrayList<String>() {{
  add("one");
  add("two");
  add("three");
}};

Basically, that says that you are creating an anonymous sub-class of the ArrayList class which is then statically initialized using "one", "two", "three".

edited Oct 04 '13 at 15:37

answered Oct 04 '13 at 15:31

Ryan Ransford

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This question is intended for Java. – Sotirios Delimanolis Oct 04 '13 at 15:31
2

There is no reason to create an anonymous class to avoid a few lines of code. – Boris the Spider Oct 04 '13 at 15:31
sorry, I've been working on a little too much javascript lately. – Ryan Ransford Oct 04 '13 at 15:33
`List` is an interface in Java – Daniel Imms Oct 04 '13 at 15:34
Yeah, I guess that the anonymous implementation would require you to also implement all of the other `List` methods. Better to anonymously extend a concrete implementation. – Ryan Ransford Oct 04 '13 at 15:37

score 0 · Answer 5 · answered Oct 04 '13 at 15:29

0

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

answered Oct 04 '13 at 15:29

Nailgun

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This makes the the resulting `List` immutable. – Boris the Spider Oct 04 '13 at 15:30
Ok then `new ArrayList(Arrays.asList("one", "two", "three"))` – Nailgun Oct 04 '13 at 15:32
1

Now you've created a list and an array as well as the list wrapper for the array when all that was required was an array... – Boris the Spider Oct 04 '13 at 15:34
Create a static factory method or, if you're using Guava, there are plenty of methods there. – Boris the Spider Oct 04 '13 at 15:39
That's not plain java 'inline' solutions. Generally I'm using Guava too. – Nailgun Oct 04 '13 at 15:42

Suresh Atta · Answer 6 · 2013-10-04T15:38:52.973

0

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

As Boris commented, it makes your numbers immutable.

Yes,You can, but with two lines.

List<String> numbers= new ArrayList<String>();
Collections.addAll(numbers,"one","two","three");

If you still want in Only in one line ,With Gauva

List<String> numbers= Lists.newArrayList("one","two","three");

edited Oct 04 '13 at 15:38

answered Oct 04 '13 at 15:30

Suresh Atta

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How can I initialize a collection and add data on the same line?

6 Answers6