Generic class to array of generics of same generic type

Question

I have a method signature like

public <T extends SomeClass> T[] findAll(Class<T> oClass, String condition)

But I'm not sure how I can create that T[] type from Class<T>. I've tried using a Collection<T> but I cannot cast (T[]) collection.toArray() and Array.newInstance(Class<?>, int) asks for an array class (i.e. T[]) already, which is of no use and redundant.

Also, obvisously, new T[n] does not work.

I would presume it's something easy, but I'm numb for the solution right now :)

Thanks.

Mark Peters · Accepted Answer · 2011-04-11 20:08:18Z

5

newInstance does not ask for an array class, it asks for the component type. It will work fine, albeit with the usual problems with unchecked casts and generic arrays.

T[]  myArr = (T[]) Array.newInstance(oClass, length);

But why mix arrays and Generics at all? It'll just lead to headaches. Use something from the Collections API, like List.

answered Apr 11, 2011 at 20:08

Mark Peters

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4 Comments

Yanick Rochon Over a year ago

Yes, perhaps a Collection like a List should be fine, but since I don't want the returned value to be modifiable, I prefer to have an array instead of Collections.unmodifiableList()

Mark Peters Over a year ago

@Yanick: But you can always modify an array! Collections.unmodifiableList() does a much better job of keeping your collection unmodifiable.

Yanick Rochon Over a year ago

yes, of course, I was referring more about the add and remove methods; you can't add or remove items in an array. Replacing a value if of no concern, but point taken.

Paŭlo Ebermann Over a year ago

@Yanick: If you have an array (of any reference type), you can use Arrays.asList(array) to convert it into a fixed-lenght list. So you could use an empty Object[], wrap it into a list, cast the list to the right generic type, and then fill it (or first fill, then make list). This needs a SuppressWarning, but if the array stays in your method, it should be safe.

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