Converting Object[] to ArrayList[] in Java

Question

The following code throws an ClassCastException:

ArrayList<V>[] table = (ArrayList<V>[]) new Object[tableSize];

How can I create an Array of ArrayList if this doesn't work?

EDIT:

To be shure everybody understands the problem. I need an Array that contains multiple ArrayLists.

EDIT2:

tableSize is an int.

possible duplicate of [How to create ArrayList (ArrayList) from array (T\[\])](http://stackoverflow.com/questions/157944/how-to-create-arraylist-arraylistt-from-array-t) — sertsedat, Jul 08 '14 at 14:07
Of course it throws an exception. An array is NOT a list, and an Object list is NOT a V class. In fact, what is `V`? Is it a template parameter? — EpicPandaForce, Jul 08 '14 at 14:07

kajacx · Accepted Answer · 2014-07-08T14:19:29.537

-1

EDIT: OK, here is what you can do:

ArrayList<String>[] table = new ArrayList[tableSize];

It compiles and works on Java 8 i'm using.

edited Jul 08 '14 at 14:19

answered Jul 08 '14 at 14:14

kajacx

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Ah, last version you had <> between ArrayList and tablesize. This works. Thanks. – Raandom Jul 08 '14 at 14:22
And it works with `List[] table = new List[tableSize];` too. This is truly interesting. – EpicPandaForce Jul 08 '14 at 14:25
Interestion point, `List` is an interface, but can make an array of interfaces, no problem. – kajacx Jul 09 '14 at 06:44

score -1 · Answer 2 · answered Jul 08 '14 at 14:27

In case of Java you can not create new instance of array that type is generic.

T[] array = new T[size]; - cause compile error Cannot create a generic array of T.

The same error will be present when you will try to use it

ArrayList<T>[] array = new ArrayList<T>[size];

With the same reason. The Java runtime does not know what is T after compilation therefor will not be able to allocate valid memory space.

To pass this issue you have following options:

Remove the generic type and use a raw.

ArrayList[] array =new ArrayList[size];

Use list of a list

List<List<T>> list = new ArrayList<List<T>>();

score -1 · Answer 3 · answered Jul 08 '14 at 14:37

Not the smoothest way to do it But if I got your question correctly something like this should do the trick:

int tableSize = 2;
Collection<String> listWithItems = new ArrayList<>();
listWithItems.add("Foo");
listWithItems.add("Bar");    

Collection<Collection<String>> listOfList = new ArrayList<>();
listOfList.add(listWithItems);    

Object[] array = listOfList.toArray(new Object[tableSize]);    

System.out.println("Col: " + listOfList);
System.out.println("Array: " + Arrays.toString(array));

Print out is:

Col: [[Foo, Bar]]
Array: [[Foo, Bar], null]

Short example without extra "fluff" to get the inital array:

Object[] theArrayOfLists = new ArrayList<List<SomeClass>>().toArray(new Object[tableSize]);

Converting Object[] to ArrayList[] in Java

3 Answers3