How to cast List<? extends Foo> to List<Foo>

Spring Batch's ItemWriter开发者_Python百科 interface is this:

write(List<? extends T> items); 

I'd like the ItemWriter to call a Service but my service has this:

process(List<T> items); 

AFAIK, Java Generics are strict about casting types within collections.

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Just go ahead and cast it. For reading, List<? extends Foo> is certainly a List<Foo>, the cast is absolutely safe. You can wrap it with Collections.unmodifiableList() if you are paranoid.

List<? extends Foo> foos1 = ...;

@SuppressWarnings("unchecked")
List<Foo> foos2 = (List<Foo>)(List<?>)foos1;    

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Ensure that your method receives an object that inherits from T, and then perform a cast.

Or change the signature of your method to be equal to the write method.

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List<? extends Foo> list1 = ...
List<Foo> list2 = Collections.unmodifiableList(list1);

Reason why list2 has to be read-only view of list1 is nicely explained in an answer of Generics : List is same as List?

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If your service allows ? extends T, then that's what the type should be. Currently, it says what's passed in must be exactly T.

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Gennerics in Java helps you only in compilation time. You can cast anything, make sure that the value is castable for the new type.

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Can't you just cast it since you know that if you've got a type that extends Foo that it is a Foo?
Something like this:

write(List<? extends Foo> items) {
    //noinspection unchecked
    process( (List<Foo>) items );
}

process(List<Foo> items) {
    // ...
}