Java add list of specific class to list of java.lang.Object works with java 8 streams - why?

Question

public class Test {

    static List<Object> listA = new ArrayList<>();

    public static void main(final String[] args) {
        final List<TestClass> listB = new ArrayList<>();
        listB.add(new TestClass());

        // not working
        setListA(listB);

        // working
        setListA(listB.stream().collect(Collectors.toList()));

        System.out.println();
    }

    private static void setListA(final List<Object> list) {
        listA = list;
    }

}

why does it work with streams and does not work for the simple set?

Answer 1

For the first case, it fails because List<TestClass> is not a subtype of List<Object>.¹

For the second case, we have the following method declarations:

interface Stream<T> {
    // ...
    <R, A> R collect(Collector<? super T, A, R> collector)
}

and:

class Collectors {
    // ...
    public static <T> Collector<T, ?, List<T>> toList()
}

This allows Java to infer the generic type parameters from the context.² In this case List<Object> is inferred for R, and Object for T.

Thus your code is equivalent to this:

Collector<Object, ?, List<Object>> tmpCollector = Collectors.toList();
List<Object> tmpList = listB.stream().collect(tmpCollector);
setListA(tmpList);

---

_{1. See e.g. here.}

_{2. See e.g. here or here.}

Answer 2

This line

setListA(listB);

doesn't work because List in Java is invariant, meaning List<TestClass> doesn't extends List<Object> when TestClass extends Object. More details here

This line

setListA(listB.stream().collect(Collectors.toList()));

works because Java infer Object for Collector's generic type from this method signature setListA(final List<Object> list) and so you actually pass List<Object> there