How to cast List to List

Question

Spring Batch's ItemWriter 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.

Why does your service have `process(List items)` and not `process(List extends T> items)`? — ILMTitan, Oct 14 '11 at 21:30
Because that's what I WANT. I don't have to adhere to what Spring says. — pri, Oct 15 '11 at 15:15
Why do you want that? Are you inserting elements into items (the only functionality you would lose by making it a wildcard)? — ILMTitan, Oct 17 '11 at 15:35
As a Service, the signature is clean and declarative with List than a wildcard, which the service doesn't need. (Yes, useful for other cases though). — pri, Oct 28 '11 at 15:59

score 13 · Accepted Answer · edited Dec 20 '17 at 19:42

13

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;

edited Dec 20 '17 at 19:42

Bax

4,070
3
41
65

answered Oct 14 '11 at 22:04

irreputable

43,835
9
63
92

2

Brilliant. You are GOD> PS: Note why Java Generics SUCK. – pri Oct 15 '11 at 15:23
4

Why the extra (List>) cast? – David Bosschaert Jan 31 '14 at 08:32

score 7 · Answer 2 · answered Oct 14 '11 at 20:35

7

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.

answered Oct 14 '11 at 20:35

Tarcísio Júnior

1,229
7
15

Why should I CHANGE my signature for Spring? NO. – pri Oct 15 '11 at 15:16
1

I said I was not to change, just gave a suggestion. He could use the given cast option. – Tarcísio Júnior Oct 15 '11 at 17:53

score 5 · Answer 3 · edited May 23 '17 at 12:00

5

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?

edited May 23 '17 at 12:00

Community

1
1

answered Sep 29 '15 at 20:25

czerny

13,045
13
64
86

score 2 · Answer 4 · answered Oct 14 '11 at 20:34

2

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.

answered Oct 14 '11 at 20:34

Ryan Stewart

120,981
20
172
194

1

He explicitly says that the service accepts List, not List extends T>. – Yngvar Kristiansen Dec 12 '14 at 07:24

score 1 · Answer 5 · answered Oct 15 '11 at 18:31

1

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

answered Oct 15 '11 at 18:31

user997135

29
3

score 0 · Answer 6 · answered Oct 14 '11 at 20:35

0

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) {
    // ...
}

answered Oct 14 '11 at 20:35

Nate W.

8,891
6
41
65

Be sure to catch the potential exception if you are not 100% certain that the items are `List` type. – Jesse Webb Oct 14 '11 at 20:45

How to cast List to List

6 Answers6