How am I able to insert an ArrayList<String> into a constructor that accepts List<Integer>? [duplicate]

Question

I have this class:

public class TestSubject {
    public TestSubject(List<Integer> list) {
    }
}

I'm instantiating it like this and somehow it's working, even though I'm inserting an ArrayList<String> into a constructor that accepts List<Integer>:

List<String> strings = new ArrayList<>();
strings.add("foo");
Constructor<TestSubject> constructor = TestSubject.class.getConstructor(List.class);
TestSubject test = constructor.newInstance(strings);

This is what I see after instantiation:

How can this be possible?

Also, how can I make sure from the instantiation code that the correct type of list is being used?

Answer 1

This is happening due to type erasure. As <Integer> will be erased and it will be List only. However you are creating the instance with reflection and at runtime it will not check the type of the List.

Here type of the list is checked during compile time if you create it with new but in this case you skipped the compile time check and during runtime it is valid because of type erasure.

In your case there is no direct way other than checking the type of elements manually in the constructor.