Convert Collection to List

Question

I would like to ask: how do you convert a Collection to a List in Java?

It seems that you've no idea on how to use general Collections in Java. What about reading a Tutorial? http://java.sun.com/docs/books/tutorial/collections/intro/index.html — mickthompson, Mar 18 '10 at 15:45

score 70 · Answer 1 · answered Mar 18 '10 at 15:09

70

Collection<MyObjectType> myCollection = ...;
List<MyObjectType> list = new ArrayList<MyObjectType>(myCollection);

See the Collections trail in the Java tutorials.

answered Mar 18 '10 at 15:09

Michael Myers

188,989
46
291
292

score 55 · Accepted Answer · answered Mar 18 '10 at 15:10

55

If you have already created an instance of your List subtype (e.g., ArrayList, LinkedList), you could use the addAll method.

e.g.,

l.addAll(myCollection)

Many list subtypes can also take the source collection in their constructor.

answered Mar 18 '10 at 15:10

Uri

88,451
51
221
321

score 8 · Answer 3 · edited Apr 29 '14 at 18:52

8

List list;
if (collection instanceof List)
{
  list = (List)collection;
}
else
{
  list = new ArrayList(collection);
 }

edited Apr 29 '14 at 18:52

Brad Larson

170,088
45
397
571

answered Apr 19 '13 at 15:33

Sandeep Bhardwaj

1,310
1
18
24

score 2 · Answer 4 · answered Mar 06 '14 at 07:19

Thanks for Sandeep putting it- Just added a null check to avoid NullPointerException in else statement.

if(collection==null){
  return Collections.emptyList();
}
List list;
if (collection instanceof List){
  list = (List)collection;
}else{
  list = new ArrayList(collection);
}

score 2 · Answer 5 · answered Mar 18 '10 at 15:09

2

Make a new list, and call addAll with the Collection.

answered Mar 18 '10 at 15:09

bmargulies

97,814
39
186
310

score 0 · Answer 6 · answered Mar 18 '10 at 15:25

0

you can use either of the 2 solutions .. but think about whether it is necessary to clone your collections, since both the collections will contain the same object references

answered Mar 18 '10 at 15:25

Cshah

5,612
10
33
37

Omnipresent · Answer 7 · 2010-03-18T15:41:59.637

-3

Collection and List are interfaces. You can take any Implementation of the List interface: ArrayList LinkedList and just cast it back to a Collection because it is at the Top

Example below shows casting from ArrayList

public static void main (String args[]) {
    Collection c = getCollection();
    List myList = (ArrayList) c;
}

public static Collection getCollection()
{
    Collection c = new ArrayList();
    c.add("Apple");
    c.add("Oranges");
    return c;
}

edited Mar 18 '10 at 15:41

answered Mar 18 '10 at 15:34

Omnipresent

29,434
47
142
186

4

I suspect you were downvoted because _this won't always work_. Sure, when the `Collection`'s implementing class implements `List` (e.g. `ArrayList`, `LinkedList`...), you're fine. But as soon as you try to do something like `Map myMap = new HashMap(); List fromMap = (List) myMap.values();` everything looks fine at compile time - but when you run it, you get `java.lang.ClassCastException: java.util.HashMap$Values cannot be cast to java.util.List`. – Matt Ball Sep 29 '10 at 14:34
1

This is just conceptually totally wrong. You cannot "force" an arbitrary Collection to become a List by casting. – Dirk Hillbrecht Jul 18 '17 at 13:57

Convert Collection to List

7 Answers7