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Praful Surve
Praful Surve

Reputation: 788

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

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?

Upvotes: 50

Views: 54339

Answers (8)

Andy Brown
Andy Brown

Reputation: 12999

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.

Upvotes: 3

barryleerobinson
barryleerobinson

Reputation: 1

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

Upvotes: 0

Fatih
Fatih

Reputation: 41

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.

Upvotes: 0

Jens
Jens

Reputation: 9406

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.

Upvotes: 4

Mr.Q
Mr.Q

Reputation: 4524

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(...));

Upvotes: 6

blackpanther
blackpanther

Reputation: 11486

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)

Upvotes: 3

NPE
NPE

Reputation: 500367

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.

Upvotes: 15

Rohit Jain
Rohit Jain

Reputation: 213261

Create a new ArrayList using the constructor:

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

Upvotes: 87

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