How to add elements in List when used Arrays.asList()

Question

We cannot perform <Collection>.add or <Collection>.addAll operation on collections we have obtained from Arrays.asList .. only remove operation is permitted.

So What if I come across a scenario where I require to add new Element in List without deleting previous elements in List?. How can I achieve this?

Why can't you add? `asList` returns a `new ArrayList`. Oh, it's not the same `ArrayList` — Sotirios Delimanolis, Aug 22 '13 at 19:14
@SotiriosDelimanolis Lists returned by Arrays.asList are backed by the array passed in, and are unmodifiable. From [the documentation](http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#asList(T...)) (emphasis added), "Returns a ***fixed-size list*** backed by the specified array. (Changes to the returned list "write through" to the 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." — Joshua Taylor, Aug 22 '13 at 19:15
@JoshuaTaylor Had not noticed it was `java.util.Arrays.ArrayList` — Sotirios Delimanolis, Aug 22 '13 at 19:16
@SotiriosDelimanolis Well, the javadoc only guarantees that's it's a fixed size list, though yes, it does seem that's it's an `java.util.Arrays$ArrayList`. `java.util.ArrayLists`, of course, are, in general, modifiable. — Joshua Taylor, Aug 22 '13 at 19:19
@JoshuaTaylor `java.util.Arrays.ArrayList` extends `AbstractList` which throws `UnsupportedOperationException()` for some of the methods. — Sotirios Delimanolis, Aug 22 '13 at 19:19
@SotiriosDelimanolis Right; my point is that _the javadoc_ doesn't guarantee that you'll get a `java.util.Arrays.ArrayList` back, just that you'll get a fixed size `List` (so it makes sense that it throws an `UnsupportedOperationException` when attempts are made to change its size). However, `java.util.ArrayList` (not the `java.util.Arrays$ArrayList`) is a "resizable-array implementation of the List interface", according to [the documentation](http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html). — Joshua Taylor, Aug 22 '13 at 19:23
@prafulsurve The remove operation isn't permitted either; the list returned by Arrays.asList is fixed in size. A simple test will confirm that you can neither add elements to nor remove elements from the list. — Joshua Taylor, Aug 22 '13 at 19:27

score 85 · Accepted Answer · answered Aug 22 '13 at 19:14

85

Create a new ArrayList using the constructor:

List<String> list = new ArrayList<String>(Arrays.asList("a", "b"));

answered Aug 22 '13 at 19:14

Rohit Jain

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or Stream.of("a", "b", "c").collect(Collectors.toList()) – B. Stackhouse May 11 '20 at 23:57

score 15 · Answer 2 · answered Aug 22 '13 at 19:14

15

One way is to construct a new ArrayList:

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

Having done that, you can modify list as you please.

answered Aug 22 '13 at 19:14

NPE

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score 6 · Answer 3 · answered Jul 07 '16 at 17:10

Arrays.asList(),generates a list which is actually backed by an array and it is an array which is morphed as a list. You can use it as a list but you can't do certain operations on it such as adding new elements. So the best option is to pass it to a constructor of another list obj like this:

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

score 4 · Answer 4 · answered Nov 23 '17 at 15:01

You can get around the intermediate ArrayList with Java8 streams:

    Integer[] array = {1, 2, 3};
    List<Integer> list = Streams.concat(Arrays.stream(array),
                                        Stream.of(4)).collect(Collectors.toList());

This should be pretty efficient as it can just iterate over the array and also pre-allocate the target list. It may or may not be better for large arrays. As always, if it matters you have to measure.

score 3 · Answer 5 · answered Aug 22 '13 at 19:20

The Constructor for a Collection, such as the ArrayList, in the following example, will take the array as a list and construct a new instance with the elements of that list.

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

http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#ArrayList(java.util.Collection)

score 3 · Answer 6 · answered Apr 26 '21 at 11:30

These days the streams API can easily get you an ArrayList in a concise and functional manner:

Stream.of("str1", "str2").collect(Collectors.toList()));

Of course this also has the flexibility to transform using mappings. For example, while writing unit tests for Spring security code it was convenient to write the following:

Stream.of("ROLE_1", "ROLE_2").map(SimpleGrantedAuthority::new).collect(Collectors.toList()));

The list returned by Collectors.toList is an ArrayList and may be modified as required by your code.

score 0 · Answer 7 · answered Jul 21 '20 at 16:13

0

Arrays.asList()

generates an unmodifiable list on object creation. You can use the below code.

List list = Collections.synchronizedList(new ArrayList(...));

This convert allows the list to add and remove objects. I have only tested in java 8.

answered Jul 21 '20 at 16:13

Fatih

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score 0 · Answer 8 · answered Sep 08 '20 at 20:55

0

ArrayList<Object> MyObjectList = new ArrayList<>();
Arrays.asList(params[1]).forEach((item)-> {
    MyObjectList.add(item);
});

answered Sep 08 '20 at 20:55

barryleerobinson

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Thanks for your contribution! However, you should add a few comments to your answer to explain what advantage your solution has over the accepted answer, or what other new circumstances make your solution more attractive. – Patrick Sep 09 '20 at 02:25

How to add elements in List when used Arrays.asList()

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