java method is not applicable for the arguments

Question

I can't see why size(test); won't compile. Can someone help me understand?

public class TestContainer<T extends Object> {
}

.

public class Main {
    public static int size(List<TestContainer<?>> list) {
        return list.size(); 
    }

    public static void main(String[] args) {
        List<TestContainer<Object>> test = new ArrayList<TestContainer<Object>>();
        size(test); // this does not compile
        test.size(); // of course this works fine
    }
}

Possible duplicate? [Java nested generic type mismatch](https://stackoverflow.com/questions/13229979/java-nested-generic-type-mismatch) — rgettman, Feb 08 '18 at 19:57

davidxxx · Answer 1 · 2018-02-08T20:00:12.963

A method with as parameter a generic List can accept subclasses of the generic type only if the List parameter declaration is parameterized with an upper bounded wildcard.

You don't declare any upper bounded wildcard (<? extend T>) for List<TestContainer> here :

public static int size(List<TestContainer<?>> list) {
    return list.size(); 
}

So you can only pass this exact type as parameter :

List<TestContainer<?>>

To achieve you want, you should so write :

public static int size(List<? extends TestContainer<?>> list) {
    return list.size(); 
}

Note that the upper bounded wildcard is designed to ensure the type safety of the passed List.
As a consequence, inside the method, you could not add anything in but null.
But for your use case (returning the size of the list), it is not a problem.

score 0 · Answer 2 · answered Feb 08 '18 at 19:56

List<TestContainer<Object>> is not a List<TestContainer<?>>.

A TestContainer<Integer> is a TestContainer<?>. Therefore, you can add a TC<Integer> to a List<TC<?>>. Similarly, you can add a TC<String> to a List<TC<?>>.

However, a TC<Integer> is not a TC<Object> (because generics are invariant in Java), so you mustn't be allowed to add a TC<Integer> to a List<TC<Object>>. As such, a List<TC<Object>> isn't a List<TC<?>>.

If you make it so that you are unable to add anything (except literal null) to the List - by making the method parameter List<? extends TC<?>> (or List<?>, if you are really uninterested in the elements) - it is safe, and thus allowed.

java method is not applicable for the arguments

2 Answers2