How to properly cast nested generic types in Java

Question

In Java I can cast:

List<?> j = null;
List<Integer> j2 = (List<Integer>)j;

So why does the following fail?

List<List<?>> i = null;
List<List<Integer>> i2 = (List<List<Integer>>)i;

This explains it so so well : http://stackoverflow.com/questions/3546745/multiple-wildcards-on-a-generic-methods-makes-java-compiler-and-me-very-confu — Sajal Dutta, Aug 06 '13 at 19:24
Both casts are a pretty bad idea, it's just that the first one is dubious (not provably correct), the latter one is flat out wrong. — millimoose, Aug 06 '13 at 19:30
What you *can* cast, though, is `List>` into `List>`---because `?` can stand for anything, including `List`. — Marko Topolnik, Aug 06 '13 at 20:12

score 6 · Answer 1 · edited May 23 '17 at 12:18

In your 1^st snippet:

List<?> j = null;
List<Integer> j2 = (List<Integer>)j;

The compiler won't give you error, because List<?> is a super type of List<Integer>, because the family of types denoted by the wildcard "?" is a superset of Integer. So you can perform the cast from List<?> to List<Integer> (A downcast, you can say), but the compiler will show you an Unchecked Warning, to save you against a cast from say - List<Date> to List<Integer>. The warning is shown, because the cast would otherwise succeed at runtime, due to type erasure.

In the 2^nd case:

List<List<?>> i = null;
List<List<Integer>> i2 = (List<List<Integer>>)i;

Here you are casting from List<List<?>> (referred to by FIRST from hereon) to List<List<Integer>>(referred to by SECOND from hereon).

Since, FIRST is not a super type of SECOND, clearly because the family of types denoted by List<?> (it can be List<Long>, List<Date>, List<String> or anything) is not a super set of List<Integer>. Hence a Compiler Error.

Suggested Reading:

*"[...] the family of types denoted by `List>` [...] is not a super set of `List`.* It actually is! Any object of type `List` matches the more general type `List>`. — Cedric Reichenbach, Jul 25 '18 at 13:41

score 4 · Answer 2 · edited May 23 '17 at 11:47

4

Try:

List<? extends List<?>> i = null;

List<List<Integer>> i2 = (List<List<Integer>>)i;

Source (this source will reference to other great sources):

https://stackoverflow.com/a/3575895/2498729

edited May 23 '17 at 11:47

Community

1
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answered Aug 06 '13 at 19:14

But I'm Not A Wrapper Class

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score 1 · Answer 3 · edited May 23 '17 at 12:26

1

I'm guessing you need to do a wildcard cast to cast it to what you want.

List<?> j = null;
List<Integer> j2 = (List<Integer>)j;

List<List<?>> i = null;
List<List<Integer>> i2 = (List<List<Integer>>) (List<?>) i;

That compiles fine. You just needed to do an additional cast just to add a little buffer.

See here: http://ideone.com/xh88lX

If you want to know why, check here

Basically, here's the relevant info Here's another way to look at it:

Java generics is type invariant
There's a conversion from Integer to Number, but a List<Integer> is not a List<Number>
Similarly, a List<Integer> can be capture-converted by a List<?>, but a List<List<Integer>> is not a List<List<?>>
Using bounded wildcard, a List<? extends Number> can capture-convert a List<Integer>
Similarly, a List<? extends List<?>> can capture-convert a List<List<Integer>>

edited May 23 '17 at 12:26

Community

1
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answered Aug 06 '13 at 19:12

JGibbers

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I just looked it up a little more, and found another stackoverflow post that explains why, check my edit. – JGibbers Aug 06 '13 at 19:16
I didn't see that till I posted, I'll still leave it up here since it provides another solution – JGibbers Aug 06 '13 at 19:20
The only answer that actually answers the question of *how* to do it has the least upvotes :/ – Earthcomputer Dec 06 '19 at 14:49

How to properly cast nested generic types in Java

3 Answers3