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;

Answer 1

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.

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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:

• Angelika Langer - Generics FAQs
• Is List<Dog> a subclass of List<Animal>? Why aren't Java's generics implicitly polymorphic?