Creating generic arrays

[stijn1989]

Hello

I followed a conference about java generics at Devoxx. The person who gave the presentation was a prof. at the university of Leuven, Belgium. He told that you can't create an array of generics. Let me give an example.

T [] names = new T[10];

The new T[10] part gives the error. That's something I don't understand. The T in this piece of code is replaced by the name of the class during the compiling. So this should change to this for example:

String [] names = new String[10];

So during runtime this creates a array holding Strings. I understand that you can't do the follow thing because you don't know the constructor of T.

T obj = new T;

Can someone explain why I can't create a generic array. Thanks!

[xcallmejudasx]

I'm assuming because there is no generics constructor which when you create an array you need. Does T[] names = null work?

[stijn1989]

I'm assuming because there is no generics constructor which when you create an array you need. Does T[] names = null work?

T[] names = null; works. But if you create an array you create an array with null references and no references to T. That's why I don't understand it. You can't initialize the array, so the follow should not work.

T [] names = new T{new T(...) , ....};

Does the compiler needs to know the size of the array during compile time? I don't think so because during runtime the array can grow and shrink...

[xcallmejudasx]

Wrong. An array cannot change sizes. An ArrayList however can. Try creating a generic ArrayList that might give you better results.

[Tolls]

It's all, apparently, to do with type erasure.
There's a brief explanation here in the reply by gafter (first reply).

[stijn1989]

So it's because array's are dynamic and generic's are static? What does that mean "array's are dynamic"? Is it because you can change an array to another array?

[Tolls]

His example was pretty clear I thought.
By allowing generic arrays it opens up some potential errors which generics were there to solve when handling Lists. In other words that casting a generic array to an Object array, assigning an object of a different generic type to it, then accessing that object through the original generic array...and bang.

Since, as I say, this is the sort of thing generics were brought in to solve (remember being able to put any old rubbish into a List?)...