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Java generics super keyword

I went through these topics

However, I still seem to be kind of lost with super keyword:

  1. When we declare a collection like that:

    List<? super Number> list = null;
list.add(new Integer(0)); // this compiles
list.add(new Object()); // this doesn't compile

shouldn’t it be the opposite – we have a list that contains some objects (of unknown type) which are parents of Number. So Object should fit (since it is the parent of Number), and Integer shouldn’t. The opposite is the case for some reason.

  1. Provided we have the following code

    static void test(List<? super Number> param) {
  param.add(new Integer(2));
}

public static void main(String[] args) {
  List<String> sList = new ArrayList<String>();
  test(sList);            // will never compile, however...
}

It is impossible to compile the above code (and my sanity suggests that this is the right behaviour), but the basic logic could prove the opposite:

String is Object, Object is superclass of Number. So String should work.

I know this is crazy but isn’t this the reason why they didn’t allow <S super T> constructs? If yes, then why <? super T> is allowed?

Could someone help me restore the missing part of this logic chain?

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Answer

The bounded wildcard in List<? super Number> can capture Number and any of its supertypes. Since Number extends Object implements Serializable, this means that the only types that are currently capture-convertible by List<? super Number> are:

  • List<Number>
  • List<Object>
  • List<Serializable>

Note that you can add(Integer.valueOf(0)) to any of the above types. however, you CAN’T add(new Object()) to a List<Number> or a List<Serializable>, since that violates the generic type safety rule.

Hence it is NOT true that you can add any supertype of Number to a List<? super Number>; that’s simply not how bounded wildcard and capture conversion work. You don’t declare a List<? super Number> because you may want to add an Object to it (you can’t!); you do because you want to add Number objects to it (i.e. it’s a “consumer” of Number), and simply a List<Number> is too restrictive.

References

See also

  • Effective Java 2nd Edition, Item 28: Use bounded wildcards to increase API flexibility
    • “PECS stands for producer-extends, consumer-super

Related questions

  • Too many to list, PECS, new Integer(0) vs valueOf, etc
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