Initialize Java list

In this article, we will take a look at different ways to initialize a list in one line containing elements with examples.

1. Arrays.asList()
java.util.Arrays has a static asList() method which accepts an array and returns a list with the elements of this array.
You can provide an array or a series of elements separated by a comma. Example,

List<String> letters = Arrays.asList("A", "B");
List<Integer> numbers = Arrays.asList(new Integer[]{1,2});

The list is backed by the array, which means that any changes made to the list will also be visible in the array and vice-versa. Example,

String[] letters = { "A", "B" };
List<String> list = Arrays.asList(letters);
list.set(0, "C");
System.out.println("Modified array:" + 
          Arrays.toString(letters));
letters[0] = "D";
System.out.println("Modified list:" + list);

Output is

As you can see, modifying the list also modifies the array and vice-versa.

Note that you cannot modify the size of list created with Arrays.asList(). That is, you cannot call add() or remove() method on it.
Doing so would result in UnsupportedOperationException. Example,

List<String> list = Arrays.asList("A", "B");
list.add("C");

Result will be

2. Java 8 stream
Java 8 stream provides a static of() method, which takes a var-args as parameter and returns a stream of those elements.
This stream can be converted to a list using collect() and Collectors.toList() method as shown below

List<String> letters = Stream.of("A", "B").
                       collect(Collectors.toList());
letters.add("C");
System.out.println(letters);

You can add and remove elements from this list.
3. Java 9 of() method
In java 9, of() method was added to List interface, which accepts multiple arguments and returns a list with those elements.
of() is a static interface method. Example,

List<String> letters = List.of("A", "B");

List returned by of() is an unmodifiable list, meaning that you cannot add or remove elements from it.

Hope the article was useful.