Initialization of an ArrayList in one line

Question

I want to create a list of options for testing purposes. At first, I did this:

ArrayList<String> places = new ArrayList<String>();
places.add("Buenos Aires");
places.add("Córdoba");
places.add("La Plata");

Then I refactored the code as follows:

ArrayList<String> places = new ArrayList<String>(
    Arrays.asList("Buenos Aires", "Córdoba", "La Plata"));

Is there a better way to do this?

Answer 1

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.

Answer 2

It would be simpler if you were to just declare it as a List - does it have to be an ArrayList?

List<String> places = Arrays.asList("Buenos Aires", "Córdoba", "La Plata");

Or if you have only one element:

List<String> places = Collections.singletonList("Buenos Aires");

This would mean that places is immutable (trying to change it will cause an UnsupportedOperationException exception to be thrown).

To make a mutable list that is a concrete ArrayList you can create an ArrayList from the immutable list:

ArrayList<String> places = new ArrayList<>(Arrays.asList("Buenos Aires", "Córdoba", "La Plata"));

Answer 3

The simple answer

In Java 10, 11, 12, or later:

var strings = List.of("foo", "bar", "baz");

In Java 9 or later:

List<String> strings = List.of("foo", "bar", "baz");

This will give you an immutable List, so it cannot be changed.
Which is what you want in most cases where you're prepopulating it.

---

Java 8 or earlier:

List<String> strings = Arrays.asList("foo", "bar", "baz");

This will give you a List backed by the array, so it cannot change length.
But you can call List.set, so it's still mutable.

---

You can make Arrays.asList even shorter with a static import:

List<String> strings = asList("foo", "bar", "baz");

The static import:

import static java.util.Arrays.asList;  

Which any modern IDE will suggest and automatically do for you.
For example in IntelliJ IDEA you press Alt+Enter and select Static import method....

---

However, i don't recommend shortening the Java 9 List.of method, because having just of becomes confusing.
List.of is already short enough and reads well.

---

Using Streams

Why does it have to be a List?
With Java 8 or later you can use a Stream which is more flexible:

Stream<String> strings = Stream.of("foo", "bar", "baz");

You can concatenate Streams:

Stream<String> strings = Stream.concat(Stream.of("foo", "bar"),
                                       Stream.of("baz", "qux"));

Or you can go from a Stream to a List:

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

List<String> strings = Stream.of("foo", "bar", "baz").collect(toList());

But preferably, just use the Stream without collecting it to a List.

---

If you really specifically need a java.util.ArrayList

(You probably don't.)
To quote JEP 269 (emphasis mine):

There is a small set of use cases for initializing a mutable collection instance with a predefined set of values. It's usually preferable to have those predefined values be in an immutable collection, and then to initialize the mutable collection via a copy constructor.

---

If you want to both prepopulate an ArrayList and add to it afterwards (why?), use

ArrayList<String> strings = new ArrayList<>(List.of("foo", "bar"));
strings.add("baz");

or in Java 8 or earlier:

ArrayList<String> strings = new ArrayList<>(asList("foo", "bar"));
strings.add("baz");

or using Stream:

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

ArrayList<String> strings = Stream.of("foo", "bar")
                             .collect(toCollection(ArrayList::new));
strings.add("baz");

But again, it's better to just use the Stream directly instead of collecting it to a List.

---

Program to interfaces, not to implementations

You said you've declared the list as an ArrayList in your code, but you should only do that if you're using some member of ArrayList that's not in List.

Which you are most likely not doing.

Usually you should just declare variables by the most general interface that you are going to use (e.g. Iterable, Collection, or List), and initialize them with the specific implementation (e.g. ArrayList, LinkedList or Arrays.asList()).

Otherwise you're limiting your code to that specific type, and it'll be harder to change when you want to.

For example, if you're passing an ArrayList to a void method(...):

// Iterable if you just need iteration, for (String s : strings):
void method(Iterable<String> strings) { 
    for (String s : strings) { ... } 
}

// Collection if you also need .size(), .isEmpty(), or .stream():
void method(Collection<String> strings) {
    if (!strings.isEmpty()) { strings.stream()... }
}

// List if you also need .get(index):
void method(List<String> strings) {
    strings.get(...)
}

// Don't declare a specific list implementation
// unless you're sure you need it:
void method(ArrayList<String> strings) {
    ??? // You don't want to limit yourself to just ArrayList
}

---

Another example would be always declaring variable an InputStream even though it is usually a FileInputStream or a BufferedInputStream, because one day soon you or somebody else will want to use some other kind of InputStream.

Answer 4

If you need a simple list of size 1:

List<String> strings = new ArrayList<String>(Collections.singletonList("A"));

If you need a list of several objects:

List<String> strings = new ArrayList<String>();
Collections.addAll(strings,"A","B","C","D");

Answer 5

With Guava you can write:

ArrayList<String> places = Lists.newArrayList("Buenos Aires", "Córdoba", "La Plata");

In Guava there are also other useful static constructors. You can read about them here.

Answer 6

Collection literals didn't make it into Java 8, but it is possible to use the Stream API to initialize a list in one rather long line:

List<String> places = Stream.of("Buenos Aires", "Córdoba", "La Plata").collect(Collectors.toList());

If you need to ensure that your List is an ArrayList:

ArrayList<String> places = Stream.of("Buenos Aires", "Córdoba", "La Plata").collect(Collectors.toCollection(ArrayList::new));

Answer 7

import com.google.common.collect.ImmutableList;

....

List<String> places = ImmutableList.of("Buenos Aires", "Córdoba", "La Plata");

Answer 8

With Java 9, as suggested in JDK Enhancement Proposal - 269, this could be achieved using collection literals now as -

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

Set<String> set = Set.of("A", "B", "C");

Also a similar approach would apply to Map as well -

Map<String, String> map = Map.of("k1", "v1", "k2", "v2", "k3", "v3")

which is similar to Collection Literals proposal as stated by @coobird as well. Further clarified in the JEP doc as well -

---

Alternatives

Language changes have been considered several times, and rejected:

Project Coin Proposal, 29 March 2009

Project Coin Proposal, 30 March 2009

JEP 186 discussion on lambda-dev, January-March 2014

The language proposals were set aside in preference to a library-based proposal as summarized in this message.

Related => What is the point of overloaded Convenience Factory Methods for Collections in Java 9

Answer 9

You could create a factory method:

public static ArrayList<String> createArrayList(String ... elements) {
  ArrayList<String> list = new ArrayList<String>();
  for (String element : elements) {
    list.add(element);
  }
  return list;
}

....

ArrayList<String> places = createArrayList(
  "São Paulo", "Rio de Janeiro", "Brasília");

But it's not much better than your first refactoring.

For greater flexibility, it can be generic:

public static <T> ArrayList<T> createArrayList(T ... elements) {
  ArrayList<T> list = new ArrayList<T>();
  for (T element : elements) {
    list.add(element);
  }
  return list;
}

Answer 10

In Java 9 we can easily initialize an ArrayList in a single line:

List<String> places = List.of("Buenos Aires", "Córdoba", "La Plata");

or

List<String> places = new ArrayList<>(List.of("Buenos Aires", "Córdoba", "La Plata"));

This new approach of Java 9 has many advantages over the previous ones:

1. Space Efficiency
2. Immutability
3. Thread Safe

See this post for more details -> What is the difference between List.of and Arrays.asList?

Answer 11

With Eclipse Collections you can write the following:

List<String> list = Lists.mutable.with("Buenos Aires", "Córdoba", "La Plata");

You can also be more specific about the types and whether they are Mutable or Immutable.

MutableList<String> mList = Lists.mutable.with("Buenos Aires", "Córdoba", "La Plata");
ImmutableList<String> iList = Lists.immutable.with("Buenos Aires", "Córdoba", "La Plata");

You can also do the same with Sets and Bags:

Set<String> set = Sets.mutable.with("Buenos Aires", "Córdoba", "La Plata");
MutableSet<String> mSet = Sets.mutable.with("Buenos Aires", "Córdoba", "La Plata");
ImmutableSet<String> iSet = Sets.immutable.with("Buenos Aires", "Córdoba", "La Plata");

Bag<String> bag = Bags.mutable.with("Buenos Aires", "Córdoba", "La Plata");
MutableBag<String> mBag = Bags.mutable.with("Buenos Aires", "Córdoba", "La Plata");
ImmutableBag<String> iBag = Bags.immutable.with("Buenos Aires", "Córdoba", "La Plata");

Note: I am a committer for Eclipse Collections.

Answer 12

About the most compact way to do this is:

Double array[] = { 1.0, 2.0, 3.0};
List<Double> list = Arrays.asList(array);

Answer 13

Here is another way:

List<String> values = Stream.of("One", "Two").collect(Collectors.toList());

Answer 14

Simply use below code as follows.

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

Answer 15

You can use the below statements:

Code Snippet:

String [] arr = {"Sharlock", "Homes", "Watson"};

List<String> names = Arrays.asList(arr);

Answer 16

(Should be a comment, but too long, so new reply). As others have mentioned, the Arrays.asList method is fixed size, but that's not the only issue with it. It also doesn't handle inheritance very well. For instance, suppose you have the following:

class A{}
class B extends A{}

public List<A> getAList(){
    return Arrays.asList(new B());
}

The above results in a compiler error, because List<B>(which is what is returned by Arrays.asList) is not a subclass of List<A>, even though you can add Objects of type B to a List<A> object. To get around this, you need to do something like:

new ArrayList<A>(Arrays.<A>asList(b1, b2, b3))

This is probably the best way to go about doing this, esp. if you need an unbounded list or need to use inheritance.

Answer 17

Like Tom said:

List<String> places = Arrays.asList("Buenos Aires", "Córdoba", "La Plata");

But since you complained of wanting an ArrayList, you should firstly know that ArrayList is a subclass of List and you could simply add this line:

ArrayList<String> myPlaces = new ArrayList(places);

Although, that might make you complain of 'performance'.

In that case it doesn't make sense to me, why, since your list is predefined it wasn't defined as an array (since the size is known at time of initialisation). And if that's an option for you:

String[] places = {"Buenos Aires", "Córdoba", "La Plata"};

In case you don't care of the minor performance differences then you can also copy an array to an ArrayList very simply:

ArrayList<String> myPlaces = new ArrayList(Arrays.asList(places));

Okay, but in future you need a bit more than just the place name, you need a country code too. Assuming this is still a predefined list which will never change during run-time, then it's fitting to use an enum set, which would require re-compilation if the list needed to be changed in the future.

enum Places {BUENOS_AIRES, CORDOBA, LA_PLATA}

would become:

enum Places {
    BUENOS_AIRES("Buenos Aires",123),
    CORDOBA("Córdoba",456),
    LA_PLATA("La Plata",789);

    String name;
    int code;
    Places(String name, int code) {
      this.name=name;
      this.code=code;
    }
}

Enum's have a static values method that returns an array containing all of the values of the enum in the order they are declared, e.g.:

for (Places p:Places.values()) {
    System.out.printf("The place %s has code %d%n",
                  p.name, p.code);
}

In that case I guess you wouldn't need your ArrayList.

P.S. Randyaa demonstrated another nice way using the static utility method Collections.addAll.

Answer 18

Java 9 has the following method to create an immutable list:

List<String> places = List.of("Buenos Aires", "Córdoba", "La Plata");

which is easily adapted to create a mutable list, if required:

List<String> places = new ArrayList<>(List.of("Buenos Aires", "Córdoba", "La Plata"));

Similar methods are available for Set and Map.

Answer 19

List<String> names = Arrays.asList("2","@2234","21","11");

Answer 20

You can use StickyList from Cactoos:

List<String> names = new StickyList<>(
  "Scott Fitzgerald", "Fyodor Dostoyevsky"
);

Answer 21

Try with this code line:

Collections.singletonList(provider)

Answer 22

Yes with the help of Arrays you can initialize array list in one line,

List<String> strlist= Arrays.asList("aaa", "bbb", "ccc");

Answer 23

In Java, you can't do

ArrayList<String> places = new ArrayList<String>( Arrays.asList("Buenos Aires", "Córdoba", "La Plata"));

As was pointed out, you'd need to do a double brace initialization:

List<String> places = new ArrayList<String>() {{ add("x"); add("y"); }};

But this may force you into adding an annotation @SuppressWarnings("serial") or generate a serial UUID which is annoying. Also most code formatters will unwrap that into multiple statements/lines.

Alternatively you can do

List<String> places = Arrays.asList(new String[] {"x", "y" });

but then you may want to do a @SuppressWarnings("unchecked").

Also according to javadoc you should be able to do this:

List<String> stooges = Arrays.asList("Larry", "Moe", "Curly");

But I'm not able to get it to compile with JDK 1.6.

Answer 24

Collections.singletonList(messageBody)

If you'd need to have a list of one item!

Collections is from java.util package.

Answer 25

The best way to do it:

package main_package;

import java.util.ArrayList;


public class Stackkkk {
    public static void main(String[] args) {
        ArrayList<Object> list = new ArrayList<Object>();
        add(list, "1", "2", "3", "4", "5", "6");
        System.out.println("I added " + list.size() + " element in one line");
    }

    public static void add(ArrayList<Object> list,Object...objects){
        for(Object object:objects)
            list.add(object);
    }
}

Just create a function that can have as many elements as you want and call it to add them in one line.

Answer 26

Here is code by AbacusUtil

// ArrayList
List<String> list = N.asList("Buenos Aires", "Córdoba", "La Plata");
// HashSet
Set<String> set = N.asSet("Buenos Aires", "Córdoba", "La Plata");
// HashMap
Map<String, Integer> map = N.asMap("Buenos Aires", 1, "Córdoba", 2, "La Plata", 3);

// Or for Immutable List/Set/Map
ImmutableList.of("Buenos Aires", "Córdoba", "La Plata");
ImmutableSet.of("Buenos Aires", "Córdoba", "La Plata");
ImmutableSet.of("Buenos Aires", 1, "Córdoba", 2, "La Plata", 3);

// The most efficient way, which is similar with Arrays.asList(...) in JDK. 
// but returns a flexible-size list backed by the specified array.
List<String> set = Array.asList("Buenos Aires", "Córdoba", "La Plata");

Declaration： I'm the developer of AbacusUtil.

Answer 27

For me Arrays.asList() is the best and convenient one. I always like to initialize that way. If you are a beginner into Java Collections then I would like you to refer ArrayList initialization

Answer 28

Why not make a simple utility function that does this?

static <A> ArrayList<A> ll(A... a) {
  ArrayList l = new ArrayList(a.length);
  for (A x : a) l.add(x);
  return l;
}

"ll" stands for "literal list".

ArrayList<String> places = ll("Buenos Aires", "Córdoba", "La Plata");

Answer 29

I am creating a list of messageTemplate from an xml file stored in path prior in static context

public final class TemplateStore {

    private TemplateStore(){}

    private static final String [] VERSIONS = {"081"};
    private static final String TEMPLATE_FILE_MASK = "template/EdiTemplates_v%s.xml";
    private static final String ERROR = "Error In Building Edifact Message Template Store";

    public static final List<MessageTemplate> TEMPLATE_LIST = Arrays.stream(VERSIONS)
            .map(version -> TemplateStore.class
                    .getClassLoader().getResourceAsStream(String.format(TEMPLATE_FILE_MASK, version)))
            .map(inputStream -> {
                try {
                    return ((EdiTemplates) JAXBContext.newInstance(EdiTemplates.class).createUnmarshaller()
                            .unmarshal(inputStream)).getMessageTemplate();
                } catch (JAXBException e) {
                    throw new IllegalArgumentException(ERROR, e);
                }})
            .flatMap(Collection::stream)
            .collect(Collectors.toList());
}

Answer 30

Actually, it's possible to do it in one line:

Arrays.asList(new MyClass[] {new MyClass("arg1"), new MyClass("arg2")})