Can’t add value to the Java collection with wildcard generic type

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It’s doing that for the sake of safety. Imagine if it worked:

List<Child> childList = new ArrayList<Child>();
childList.add(new Child());

List<? extends Parent> parentList = childList;
parentList.set(0, new Parent());

Child child = childList.get(0); // No! It's not a child! Type safety is broken...

The meaning of List<? extends Parent> is “The is a list of some type which extends Parent. We don’t know which type – it could be a List<Parent>, a List<Child>, or a List<GrandChild>.” That makes it safe to fetch any items out of the List<T> API and convert from T to Parent, but it’s not safe to call in to the List<T> API converting from Parent to T… because that conversion may be invalid.

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