Question

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What is a difference between <? super E> and <? extends E>?

Some Java programmers on a team I’m on are writing functions that return objects of type List<? extends T> to make read-only lists and return objects of type List<? super T> to make write-only lists.

In Java, what makes List<? extends T> read-only and List<? super T> write-only?

Answer 1

please read up on “producer extends, consumer super” (PECS) – I may need to do the same 🙂

Read only:

In case you would like to ensure that a method takes as a parameter a collection of items ( using generics) – when you use List<? extends T> – the list can contain any subtype of T but cannot add to the collection since it does not know at runtime the specific type of T that the List contains.

Write only:

For List<? super T>, the list can contain T regardless of the actual parameterized type (using super will allow that to happen).

Hope it helps.