Create ArrayList from array

Ron Tuffin Source

I have an array that is initialized like:

Element[] array = {new Element(1), new Element(2), new Element(3)};

I would like to convert this array into an object of the ArrayList class.

ArrayList<Element> arraylist = ???;
javaarraysarraylisttype-conversion

Answers

answered 10 years ago Tom #1

new ArrayList<>(Arrays.asList(array))

answered 10 years ago Bill the Lizard #2

new ArrayList<T>(Arrays.asList(myArray));

Make sure that myArray is the same type as T. You'll get a compiler error if you try to create a List<Integer> from an array of int, for example.

answered 10 years ago Kip #3

You probably just need a List, not an ArrayList. In that case you can just do:

List<Element> arraylist = Arrays.asList(array);

answered 10 years ago Alex Miller #4

Given:

Element[] array = new Element[] { new Element(1), new Element(2), new Element(3) };

The simplest answer is to do:

List<Element> list = Arrays.asList(array);

This will work fine. But some caveats:

  1. The list returned from asList has fixed size. So, if you want to be able to add or remove elements from the returned list in your code, you'll need to wrap it in a new ArrayList. Otherwise you'll get an UnsupportedOperationException.
  2. The list returned from asList() is backed by the original array. If you modify the original array, the list will be modified as well. This may be surprising.

answered 7 years ago Tim B├╝the #5

Since this question is pretty old, it surprises me that nobody suggested the simplest form yet:

List<Element> arraylist = Arrays.asList(new Element(1), new Element(2), new Element(3));

As of Java 5, Arrays.asList() takes a varargs parameter and you don't have to construct the array explicitly.

answered 6 years ago Peter Tseng #6

Another way (although essentially equivalent to the new ArrayList(Arrays.asList(array)) solution performance-wise:

Collections.addAll(arraylist, array);

answered 6 years ago haylem #7

(old thread, but just 2 cents as none mention Guava or other libs and some other details)

If You Can, Use Guava

It's worth pointing out the Guava way, which greatly simplifies these shenanigans:

Usage

For an Immutable List

Use the ImmutableList class and its of() and copyOf() factory methods (elements can't be null):

List<String> il = ImmutableList.of("string", "elements");  // from varargs
List<String> il = ImmutableList.copyOf(aStringArray);      // from array

For A Mutable List

Use the Lists class and its newArrayList() factory methods:

List<String> l1 = Lists.newArrayList(anotherListOrCollection);    // from collection
List<String> l2 = Lists.newArrayList(aStringArray);               // from array
List<String> l3 = Lists.newArrayList("or", "string", "elements"); // from varargs

Please also note the similar methods for other data structures in other classes, for instance in Sets.

Why Guava?

The main attraction could be to reduce the clutter due to generics for type-safety, as the use of the Guava factory methods allow the types to be inferred most of the time. However, this argument holds less water since Java 7 arrived with the new diamond operator.

But it's not the only reason (and Java 7 isn't everywhere yet): the shorthand syntax is also very handy, and the methods initializers, as seen above, allow to write more expressive code. You do in one Guava call what takes 2 with the current Java Collections.


If You Can't...

For an Immutable List

Use the JDK's Arrays class and its asList() factory method, wrapped with a Collections.unmodifiableList():

List<String> l1 = Collections.unmodifiableList(Arrays.asList(anArrayOfElements));
List<String> l2 = Collections.unmodifiableList(Arrays.asList("element1", "element2"));

Note that the returned type for asList() is a List using a concrete ArrayList implementation, but it is NOT java.util.ArrayList. It's an inner type, which emulates an ArrayList but actually directly references the passed array and makes it "write through" (modifications are reflected in the array).

It forbids modifications through some of the List API's methods by way of simply extending an AbstractList (so, adding or removing elements is unsupported), however it allows calls to set() to override elements. Thus this list isn't truly immutable and a call to asList() should be wrapped with Collections.unmodifiableList().

See the next step if you need a mutable list.

For a Mutable List

Same as above, but wrapped with an actual java.util.ArrayList:

List<String> l1  = new ArrayList<String>(Arrays.asList(array));    // Java 1.5 to 1.6
List<String> l1b = new ArrayList<>(Arrays.asList(array));          // Java 1.7+
List<String> l2  = new ArrayList<String>(Arrays.asList("a", "b")); // Java 1.5 to 1.6
List<String> l2b = new ArrayList<>(Arrays.asList("a", "b"));       // Java 1.7+

For Educational Purposes: The Good ol' Manual Way

// for Java 1.5+
static <T> List<T> arrayToList(final T[] array) {
  final List<T> l = new ArrayList<T>(array.length);

  for (final T s : array) {
    l.add(s);
  }
  return (l);
}

// for Java < 1.5 (no generics, no compile-time type-safety, boo!)
static List arrayToList(final Object[] array) {
  final List l = new ArrayList(array.length);

  for (int i = 0; i < array.length; i++) {
    l.add(array[i]);
  }
  return (l);
}

answered 5 years ago TwoThe #8

There is another option if your goal is to generate a fixed list at runtime, which is as simple as it is effective:

static final ArrayList<Element> myList = generateMyList();

private static ArrayList<Element> generateMyList() {
  final ArrayList<Element> result = new ArrayList<>();
  result.add(new Element(1));
  result.add(new Element(2));
  result.add(new Element(3));
  result.add(new Element(4));
  return result;
}


The benefit of using this pattern is, that the list is for once generated very intuitively and therefore is very easy to modify even with large lists or complex initialization, while on the other hand always contains the same Elements on every actual run of the program (unless you change it at a later point of course).

answered 4 years ago Bohdan #9

// Guava
import com.google.common.collect.ListsLists
...
List<String> list = Lists.newArrayList(aStringArray); 

answered 4 years ago Nicolas Zozol #10

If you use :

new ArrayList<T>(Arrays.asList(myArray));

you may create and fill two lists ! Filling twice a big list is exactly what you don't want to do because it will create another Object[] array each time the capacity needs to be extended.

Fortunately the JDK implementation is fast and Arrays.asList(a[]) is very well done. It create a kind of ArrayList named Arrays.ArrayList where the Object[] data points directly to the array.

// in Arrays
@SafeVarargs
public static <T> List<T> asList(T... a) {
    return new ArrayList<>(a);
}
//still in Arrays, creating a private unseen class
private static class ArrayList<E>

    private final E[] a;    
    ArrayList(E[] array) {
        a = array; // you point to the previous array
    }
    ....
}

The dangerous side is that if you change the initial array, you change the List ! Are you sure you want that ? Maybe yes, maybe not.

If not, the most understandable way is to do this :

ArrayList<Element> list = new ArrayList<Element>(myArray.length); // you know the initial capacity
for (Element element : myArray) {
    list.add(element);
}

Or as said @glglgl, you can create another independant ArrayList with :

new ArrayList<T>(Arrays.asList(myArray));

I love to use Collections, Arrays, or Guava. But if it don't fit, or you don't feel it, just write another inelegant line instead.

answered 4 years ago Nepster #11

To convert an array to an ArrayList, developers often do this:

List<String> list = Arrays.asList(arr);// this is wrong way..

Arrays.asList() will return an ArrayList which is a private static class inside Arrays, it is not the java.util.ArrayList class. The java.util.Arrays.ArrayList class has set(), get(), contains() methods, but does not have any methods for adding elements, so its size is fixed. To create a real ArrayList, you must do:

ArrayList<String> arrayList = new ArrayList<String>(Arrays.asList(arr));

The constructor of ArrayList can accept a Collection type, which is also a super type for java.util.Arrays.ArrayList

answered 4 years ago whyem #12

Another update, almost ending year 2014, you can do it with Java 8 too:

ArrayList<Element> arrayList = Stream.of(myArray).collect(Collectors.toCollection(ArrayList::new));

A few characters would be saved, if this could be just a List

List<Element> list = Stream.of(myArray).collect(Collectors.toList());

answered 3 years ago nekperu15739 #13

According with the question the answer using java 1.7 is:

ArrayList<Element> arraylist = new ArrayList<Element>(Arrays.<Element>asList(array));

However it's better always use the interface:

List<Element> arraylist = Arrays.<Element>asList(array);

answered 3 years ago spencer.sm #14

Another simple way is to add all elements from the array to a new ArrayList using a for-each loop.

ArrayList<Element> list = new ArrayList<>();

for(Element e : array)
    list.add(e);

answered 2 years ago Vaseph #15

You also can do it with stream in Java 8.

 List<Element> elements = Arrays.stream(array).collect(Collectors.toList()); 

answered 2 years ago Vikrant Kashyap #16

  1. If we see the definition of Arrays.asList() method you will get something like this:

     public static <T> List<T> asList(T... a) //varargs are of T type. 
    

    So, you might initialize arraylist like this:

     List<Element> arraylist = Arrays.asList(new Element(1),new Element(2),new Element(3));
    

    Note : each new Element(int args) will be treated as Individual Object and can be passed as a var-args.

  2. There might be another answer for this question too.
    If you see declaration for java.util.Collections.addAll() method you will get something like this:

    public static <T> boolean addAll(Collection<? super T> c, T... a);
    

    So, this code is also useful to do so

    Collections.addAll(arraylist, array);
    

answered 2 years ago Ali Dehghani #17

Java 9

In Java 9, you can use List.of static factory method in order to create a List literal. Something like the following:

List<Element> elements = List.of(new Element(1), new Element(2), new Element(3));

This would return an immutable list containing three elements. If you want a mutable list, pass that list to the ArrayList constructor:

new ArrayList<>(List.of(// elements vararg))

JEP 269: Convenience Factory Methods for Collections

JEP 269 provides some convenience factory methods for Collections API. This factory methods are not in current Java version, which is 8, but are planned for Java 9 release. You can try out this feature using JDK 9 Early Access.

answered 2 years ago oxy_js #18

You can convert using different methods

  1. List<Element> list = Arrays.asList(array);

  2. List<Element> list = new ArrayList();
    Collections.addAll(list, array);

  3. Arraylist list = new Arraylist();
    list.addAll(Arrays.asList(array));

For more detail you can refer to http://javarevisited.blogspot.in/2011/06/converting-array-to-arraylist-in-java.html

answered 1 year ago Andrii Abramov #19

Since Java 8 there is an easier way to transform:

public static <T> List<T> fromArray(T[] array) {
    return Arrays.stream(array).collect(toList());
}

answered 1 year ago Abojemyeg #20

as all said this will do so

 new ArrayList<>(Arrays.asList(array))

and the common newest way to create array is observableArrays

ObservableList: A list that allows listeners to track changes when they occur.

for Java SE you can try

FXCollections.observableArrayList(new Element(1), new Element(2), new Element(3));

that is according to Oracle Docs

observableArrayList() Creates a new empty observable list that is backed by an arraylist. observableArrayList(E... items) Creates a new observable array list with items added to it.

answered 1 year ago Devendra Lattu #21

Even though there are many perfectly written answers to this question, I will add my inputs.

Say you have Element[] array = { new Element(1), new Element(2), new Element(3) };

New ArrayList can be created in the following ways

ArrayList<Element> arraylist_1 = new ArrayList<>(Arrays.asList(array));
ArrayList<Element> arraylist_2 = new ArrayList<>(
    Arrays.asList(new Element[] { new Element(1), new Element(2), new Element(3) }));

// Add through a collection
ArrayList<Element> arraylist_3 = new ArrayList<>();
Collections.addAll(arraylist_3, array);

And they very well support all operations of ArrayList

arraylist_1.add(new Element(4)); // or remove(): Success
arraylist_2.add(new Element(4)); // or remove(): Success
arraylist_3.add(new Element(4)); // or remove(): Success

But the following operations returns just a List view of an ArrayList and not actual ArrayList.

// Returns a List view of array and not actual ArrayList
List<Element> listView_1 = (List<Element>) Arrays.asList(array);
List<Element> listView_2 = Arrays.asList(array);
List<Element> listView_3 = Arrays.asList(new Element(1), new Element(2), new Element(3));

Therefore, they will give error when trying to make some ArrayList operations

listView_1.add(new Element(4)); // Error
listView_2.add(new Element(4)); // Error
listView_3.add(new Element(4)); // Error

More on List representation of array link.

answered 1 year ago MarekM #22

In Java 9 you can use:

List<String> list = List.of("Hello", "World", "from", "Java");
List<Integer> list = List.of(1, 2, 3, 4, 5);

answered 1 year ago Lakshan #23

Given:

Element[] array = new Element[] { new Element(1), new Element(2), new Element(3) };

Use:

List<Element> listArray = Arrays.asList(array);

answered 1 year ago Hemin #24

Simplest way to do so is by adding following code. Tried and Tested.

String[] Array1={"one","two","three"};
ArrayList<String> s1= new ArrayList<String>(Arrays.asList(Array1));

answered 1 year ago Adit A. Pillai #25

You can do it in java 8 as follows

ArrayList<Element> list = (ArrayList<Element>)Arrays.stream(array).collect(Collectors.toList());

answered 1 year ago yousef #26

 Element[] array = {new Element(1), new Element(2), new Element(3)};
 List<Element> elements=Arrays.asList(array);

answered 1 year ago Toothless Seer #27

Another Java8 solution (I may have missed the answer among the large set. If so, my apologies). This creates an ArrayList (as opposed to a List) i.e. one can delete elements

package package org.something.util;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Junk {

    static <T> ArrayList<T>  arrToArrayList(T[] arr){
        return Arrays.asList(arr)
            .stream()
            .collect(Collectors.toCollection(ArrayList::new));
    }

    public static void main(String[] args) {
        String[] sArr = new String[]{"Hello", "cruel", "world"};
        List<String> ret = arrToArrayList(sArr);
        // Verify one can remove an item and print list to verify so
        ret.remove(1);
        ret.stream()
            .forEach(System.out::println);
    }
}

Output is...
Hello
world

answered 12 months ago yegor256 #28

You can create an ArrayList using Cactoos (I'm one of the developers):

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

There is no guarantee that the object will actually be of class ArrayList. If you need that guarantee, do this:

ArrayList<String> list = new ArrayList<>(
  new StickyList<>(
    "Scott Fitzgerald", "Fyodor Dostoyevsky"
  )
);

answered 12 months ago rashedcs #29

We can easily convert an array to ArrayList. We use Collection interface's addAll() method for the purpose of copying content from one list to another.

 Arraylist arr = new Arraylist();
 arr.addAll(Arrays.asList(asset));

answered 11 months ago A1m #30

If the array is of a primitive type, the given answers won't work. But since Java 8 you can use:

int[] array = new int[5];
Arrays.stream(array).boxed().collect(Collectors.toList());

answered 11 months ago Sumit Das #31

Already everyone has provided enough good answer for your problem. Now from the all suggestions, you need to decided which will fit your requirement. There are two types of collection which you need to know. One is unmodified collection and other one collection which will allow you to modify the object later.

So, Here I will give short example for two use cases.

  • Immutable collection creation :: When you don't want to modify the collection object after creation

    List<Element> elementList = Arrays.asList(array)

  • Mutable collection creation :: When you may want to modify the created collection object after creation.

    List<Element> elementList = new ArrayList<Element>(Arrays.asList(array));

answered 11 months ago Tamzid Babu #32

try this...

List<ObjectName> list = Arrays.asList(array) 

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