Initialization of an ArrayList in one line example

This tutorials demonstrates how to initialize a list or arraylist in one line in different ways.

List<String> planets = new ArrayList<String>();
planets.add("Earth");
planets.add("Mars");
planets.add("Venus");

The following examples demonstrate how to create and initialize a List or ArrayList in a single line of code.

Method 1: Creating a singleton list

Creates an immutable list containing only the specified object. The returned list is serializable.

List<String> planets = Collections.singletonList("Earth");

Method 2a: Creating and initialize a list in one line

Creates a fixed-size list backed by the specified array. This method acts as bridge between array-based and collection-based APIs, in combination with Collection.toArray(). The returned list is serializable and implements RandomAccess. This method also provides a convenient way to create a fixed-size list initialized to contain several elements:

List<String> planets = Arrays.asList("Earth", "Mars", "Venus");

Method 2b: Creating and initialize a list in one line and static import

We can statically import the asList()

import static java.util.Arrays.asList;

List<String> planets = asList("Earth", "Mars", "Venus");

Method 3a: Create and initialize an arraylist in one line

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

List<String> planets = new ArrayList<String>(Arrays.asList("Earth", "Mars", "Venus"));

Method 3b: Create and initialize an arraylist in one line and static import

We can statically import the asList()

import static java.util.Arrays.asList;

List<String> planets = new ArrayList<String>(asList("Earth", "Mars", "Venus"));

Method 4: Create and initialize an arraylist using anonymous inner class

Using an anonymous inner class with an instance initializer (also known as an “double brace initialization”).

List<String> planets = new ArrayList<String>() {{
    add("Earth");
    add("Mars");
    add("Venus");
}};

Method 5a: Create and initialize a list using Java 8

Stream.of() returns a sequential ordered stream whose elements are the specified values. Collectors.toList() returns a Collector that accumulates the input elements into a new List. There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned; if more control over the returned List is required, use toCollection(Supplier).

List<String> planets = Stream
        .of("Earth", "Mars", "Venus")
        .collect(Collectors.toList());

Method 5b: Create and initialize an arraylist using Java 8

Stream.of() returns a sequential ordered stream whose elements are the specified values. Collectors.toCollection() returns a Collector that accumulates the input elements into a new Collection, in encounter order. The Collection is created by the provided factory.

ArrayList<String> planets = Stream
        .of("Earth", "Mars", "Venus")
        .collect(Collectors.toCollection(ArrayList::new));

Method 6a: Create and initialize a list using Java 8 static import

We can statically import the of() and toList() methods.

import static java.util.stream.Stream.of;
import static java.util.stream.Collectors.toList;

List<String> planets = of("Earth", "Mars", "Venus").collect(toList());

Method 7b: Create and initialize an arraylist using Java 8 static import

We can statically import the of() and toCollection() methods.

import static java.util.stream.Stream.of;
import static java.util.stream.Collectors.toCollection;

ArrayList<String> planets = of("Earth", "Mars", "Venus").collect(toCollection(ArrayList::new));

References

• ArrayList JavaDoc
• Collections JavaDoc
• Arrays JavaDoc
• Stream JavaDoc
• Collectors JavaDoc

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