Java List Initialization in One Line

1. Introduction

In this quick tutorial, we’ll investigate how can we initialize a List using one-liners.

Further reading:

Collections.emptyList() vs. New List Instance

Learn the differences between the Collections.emptyList() and a new list instance.

Read more →

Guide to the Java ArrayList

Quick and practical guide to ArrayList in Java

Read more →

2. Create From an Array

We can create a List from an array and thanks to array literals we can initialize them in one line:

List<String> list = Arrays.asList(new String[]{"foo", "bar"});

We can trust the varargs mechanism to handle the array creation. By that, we can write more concise and readable code:

@Test
public void givenArraysAsList_thenInitialiseList() {
    List<String> list = Arrays.asList("foo", "bar");

    assertTrue(list.contains("foo"));
}

The result instance of this code implements the List interface but it isn’t a java.util.ArrayList nor a LinkedList. Instead, it’s a List backed by the original array which has two implications.

Although, the class’s name happens to be ArrayList but in the java.util.Arrays package.

2.1. Fixed Size

The result instance from Arrays.asList will have a fixed size:

@Test(expected = UnsupportedOperationException.class)
public void givenArraysAsList_whenAdd_thenUnsupportedException() {
    List<String> list = Arrays.asList("foo", "bar");

    list.add("baz");
}

2.2. Shared Reference

The original array and the list share the same references to the objects:

@Test
public void givenArraysAsList_whenCreated_thenShareReference(){
    String[] array = {"foo", "bar"};
    List<String> list = Arrays.asList(array);
    array[0] = "baz";

    assertEquals("baz", list.get(0));
}

3. Create From a Stream (Java 8)

We can easily convert a Stream into any kind of Collection.

Therefore with the factory methods for Streams, we can create and initialize lists in one line:

@Test
public void givenStream_thenInitializeList(){
    List<String> list = Stream.of("foo", "bar")
      .collect(Collectors.toList());

    assertTrue(list.contains("foo"));
}

We should mark here that Collectors.toList() doesn’t guarantee the exact implementation of the returned List.

There’s no general contract about the mutability, serializability or thread-safety of the returned instance. Therefore our code shouldn’t rely on any of these properties.

Some sources highlight that Stream.of(…).collect(…) may have larger memory and performance footprint than Arrays.asList() but in almost all cases, it’s such a micro-optimization that there is little difference.

4. Factory Methods (Java 9)

In JDK 9, several convenient factory methods have been introduced for collections:

List<String> list = List.of("foo", "bar", "baz");
Set<String> set = Set.of("foo", "bar", "baz");

One important detail is the returned instances are immutable. Beyond that, the factory methods have several advantages in space efficiency and thread safety.

This topic is explored more in this article.

5. Double-Brace Initialization

In several places, we can find a method called ‘double brace initialization’ which looks like:

@Test
public void givenAnonymousInnerClass_thenInitialiseList() {
    List<String> cities = new ArrayList() {{
        add("New York");
        add("Rio");
        add("Tokyo");
    }};

    assertTrue(cities.contains("New York"));
}

The name ‘double brace initialization’ is quite misleading. The syntax may look compact and elegant but it dangerously hides what is going under the hood.

There isn’t actually a ‘double brace’ syntax element in Java, those are two blocks formatted intentionally this way.

With the outer braces, we declare an anonymous inner class which will be a subclass of the ArrayList. Inside these braces, we can declare the details of our subclass.

As usual, we can use instance initializer blocks and that is where the inner pair of braces come from.

The brevity of this syntax is tempting however it’s considered an anti-pattern.

To read more about double-brace initialization, have a look at our article here.

6. Conclusion

Modern Java offers several options to create a Collection in one line. The method we chose is almost entirely down to personal preference, rather than technical reasoning.

An important takeaway is that, although it looks graceful, the anti-pattern of anonymous inner class initialization (a.k.a. ‘double brace’) has many negative side-effects.

As always, the code is available over on GitHub.

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