Initialize an Array of List<String> Objects

Question

I want to store 4 Lists of Strings to an array within index 0-3 and be able to check wether the index is filled (!=null) or not.

For that reason I need to initialize an Array of Type List, which fails in eclipse with message "Cannot create a generic array of List":

// Does not work
List<String>[] myArray = new List<String>[4];

// Does not work
List<String>[] myArray = new ArrayList<String>[4];

Doing it like promoted at Convert an ArrayList to an object array :

ArrayList<List<String>> myArrayList = new ArrayList<List<String>>();
myArrayList.add(new ArrayList<String>());
myArrayList.add(new ArrayList<String>());
myArrayList.add(new ArrayList<String>());
myArrayList.add(new ArrayList<String>());

// Does not work
List<String>[] myArray = myArrayList.toArray(new List<String>[myArrayList.size()]);

// Does not work
List<String>[] myArray = myArrayList.toArray(new ArrayList<String [myArrayList.size()]);

But why is this not working?

Because "you cannot create a generic array" in Java. This was a design choice which has to do with the mismatch between the reifiability of array types vs. non-reifiability of generic type parameters. — Marko Topolnik
– Marko Topolnik, Commented Jan 21, 2014 at 13:47
only Cuck Norris can initiate Interfaces: new List<String>[myArrayList.size()]!!! :-) — nano_nano
– nano_nano, Commented Jan 21, 2014 at 13:47
@StefanBeike There's nothing wrong with an array of interface type: List[]. — Marko Topolnik
– Marko Topolnik, Commented Jan 21, 2014 at 13:48
@Marko Topolnik: with "new List" I am able to create a List object??? — nano_nano
– nano_nano, Commented Jan 21, 2014 at 13:51

Keerthivasan · Accepted Answer · 2014-01-21 13:49:13Z

0

ArrayList is generic class in Java. You cannot create an array of generic type. It is not allowed. What you can do is create a list of generic type

ArrayList<ArrayList<String>> listOfStringList = new ArrayList<ArrayList<String>>();

answered Jan 21, 2014 at 13:49

Keerthivasan

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3 Comments

Martin Figel Over a year ago

Thanks for the answer! Despite of this it does not come in my head, why Java is disallowing that.

Keerthivasan Over a year ago

That is going to be design choice, it causes problems with backward compatibility, type casting issues etc.

Keerthivasan Over a year ago

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