PECS - wildcards - generics

PECS - Pruducer extends, Consumer super

Only applies to input parameters.
Do not use wildcard type for return value - doesn't make it more flexible!

List<String> is a subtype of List<? extends Object>
List<Object> is a subtype of List<? super String>

eg.
pushAll(Collection<? extends E> src);
 - src is an E producer; you can push Long to Collection<Number>
popAll(Collection<? super E> dst);
 - dst is an E consumer; you can pop Numbers to Collection<Object>

Both produces and consumes use T.
Neither produces or consumes use ?.