Method has interface type as parameter, but doesn't take objects which implement this interface?

Asked Nov 25 '21 at 18:31

Active Nov 25 '21 at 18:41

Viewed 23 times

In short, I have a method which takes a list of an interface, but when I provide such a list via generic constraint <C extends FooInterface>, it doesn't work:

public class Utils {
    public void testMethod(List<FooInterface> list){}
}

public class TestClass<C extends FooInterface> {
    public final List<C> lists;
    public TestClass(){
        Utils.testMethod(lists); //<< this does not work because "C" is not "FooInterface"
    }
}

I think this should work as I defined C as implements FooInterface.

edited Nov 25 '21 at 18:41

Mark Rotteveel

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asked Nov 25 '21 at 18:31

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is FooInterface class or interface? If interface it should C implements FooInterface. – Digsb Nov 25 '21 at 18:33
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It's poker night, 'cos you're going to learn about [wildcards](https://docs.oracle.com/javase/tutorial/extra/generics/wildcards.html). – Kayaman Nov 25 '21 at 18:34
1

This is perfectly normal. It's probably simplest to change `testMethod` to accept a `List extends FooInterface>`. – Louis Wasserman Nov 25 '21 at 18:35
@LouisWasserman this also does not work: Required typ as ? extends FooInterface, but provided is "List" – Loading Nov 25 '21 at 18:37
Post your complete code as a new question. – Louis Wasserman Nov 25 '21 at 18:47
the provided answer which flags my question as duplicate answered my question. Its just not possible. – Loading Nov 25 '21 at 18:53
This is almost certainly a solvable problem, depending on what exactly `testMethod` does. – Louis Wasserman Nov 25 '21 at 19:56

Method has interface type as parameter, but doesn't take objects which implement this interface?

0 Answers0