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The fundamental problem is that there is no inheritance relationship between List<ChildA> and List<Parent>. You mistakenly believe that ChildA inherits from Parent, so List<ChildA> inherits from List<Parent>. This is wrong.

List<ChildA> inherits from List<? extends Parent>.

If you understand list as "house", maybe you can figure this problem out：

public interface Parent { }

public interface Boys extends Parent{ }
public interface Girls extends Parent{ }

List<? extends Parent> aHouseForChildrenWhoExtendsParent = new List<Boys>(); // boys house.
List<? extends Parent> otherHouseForChildrenWhoExtendsParent = new List<Grils>(); // grils house.

List<Parent> parentHouse = new List<Boys>(); // ??? this is parent house, boys and girls can not go in here.

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