Explain PECS (Producer Extends, Consumer Super) with a concrete example of choosing the right wildcard.

Use <? extends T> when a structure produces T values you only read (you can get a T out but can't add, since the exact subtype is unknown), and <? super T> when it consumes T values you write into (you can add a T but reads come back as Object).

Collections.copy(dest, src) is the canonical example: src is <? extends T> (producer) and dest is <? super T> (consumer). Getting this right maximizes API flexibility — callers can pass List<Integer> where List<? extends Number> is expected.