PECS stands for producer-extends, consumer-super. In other words, if a parameterized type represents a T producer, use <? extends T>; if it represents a T consumer, use <? super T>. In our Stack example, pushAll’s src parameter produces E instances for use by the Stack, so the appropriate type for src is Iterable<? extends E>; popAll’s dst parameter consumes E instances from the Stack, so the appropriate type for dst is Collection<? super E>. The PECS mnemonic captures the fundamental principle that guides the use of wild-card types. Naftalin and Wadler call it the Get and Put Principle
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