Initialization of an Arraylist in One Line

Initialization of an ArrayList in one line

Actually, probably the "best" way to initialize the ArrayList is the method you wrote, as it does not need to create a new List in any way:

ArrayList<String> list = new ArrayList<String>();
list.add("A");
list.add("B");
list.add("C");

The catch is that there is quite a bit of typing required to refer to that list instance.

There are alternatives, such as making an anonymous inner class with an instance initializer (also known as an "double brace initialization"):

ArrayList<String> list = new ArrayList<String>() {{
    add("A");
    add("B");
    add("C");
}};

However, I'm not too fond of that method because what you end up with is a subclass of ArrayList which has an instance initializer, and that class is created just to create one object -- that just seems like a little bit overkill to me.

What would have been nice was if the Collection Literals proposal for Project Coin was accepted (it was slated to be introduced in Java 7, but it's not likely to be part of Java 8 either.):

List<String> list = ["A", "B", "C"];

Unfortunately it won't help you here, as it will initialize an immutable List rather than an ArrayList, and furthermore, it's not available yet, if it ever will be.

Initializing an ArrayList with an array in a single line

You can use Arrays.asList method, which takes a var-args, and returns a fixed-size List backed by an array. So, you can't add any element to this list. Further, any modification done to the elements of the List will be reflected back in the array.

String[] arrayOfStrings = {"this", "is", "an", "array", "of", "strings"};
List<String> list = Arrays.asList(arrayOfStrings);

or: -

List<String> list = Arrays.asList("this", "is", "an", "array", "of", "strings");

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If you want to increase the size of this list, you would have to pass it in ArrayList constructor.

List<String> list = new ArrayList<String>(Arrays.asList(arrayOfStrings));

How to initialize List<Short> in one line?

Use Integer.shortValue() method: Like below

  List<Short> numbers = Stream.of(1, 2, 3, 4, 5).map(value -> value.shortValue()).collect(Collectors.toList());

How to Create and Initialize (in Java) a List<String[]> in the same statement? (Editors please NOTE: this *not* the same as just a List<String>)

The correct answer was given in the comments (above) by @user16320675 , and this works well. It handles both the creation of the outer List and the internal array of strings. Other answers pointing to a simple Array asList are for simple strings, which is not the question that was asked.

to create a mutable list

final List<String[]> rolesCsv =
     new ArrayList<String[]>Collections.singletonList(new String[]{"Role Name","Description"}));

Otherwise (despite the array, its element, will not be immutable)

final List<String[]> rolesCsv =
      Collections.singletonList(new String[]{"Role Name","Description"})

Initialize new arraylist with (custom) items in one line in Kotlin

arrayListOf(items)

So you can just do

arrayListOf(MyCustomItem(1), MyCustomItem(2))

How to declare an ArrayList with values?

In Java 9+ you can do:

var x = List.of("xyz", "abc");
// 'var' works only for local variables

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Java 8 using Stream:

Stream.of("xyz", "abc").collect(Collectors.toList());

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And of course, you can create a new object using the constructor that accepts a Collection:

List<String> x = new ArrayList<>(Arrays.asList("xyz", "abc"));

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Tip: The docs contains very useful information that usually contains the answer you're looking for. For example, here are the constructors of the ArrayList class:

• ArrayList()

  Constructs an empty list with an initial capacity of ten.

• ArrayList(Collection<? extends E> c) (*)

  Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

• ArrayList(int initialCapacity)

  Constructs an empty list with the specified initial capacity.

How to initialize ArrayList<Double> in one line from a list of mixed integer and float?

You can do this with a DoubleStream:

public static ArrayList<Double> Foo() {
    return DoubleStream.of(1.0, 2, 3.1, 4, 5)
            .boxed()
            .collect(Collectors.toCollection(ArrayList::new));
}

How to initialize List Of List in one line

List<List<Integer>> list = Arrays.asList(Arrays.asList(1,2), Arrays.asList(3,4));

In Java 9+ you can replace Arrays.asList() with List.of():

List<List<Integer>> list = List.of(List.of(1,2), List.of(3,4));

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