Java bounded wildcards

From Effective Java, 2nd edition

For maximum flexibility, use wildcard types on input parameters that represent producers or consumers.
Mnemonic:
PECS stands for producer-extends, consumer-super.

E.g., method that takes a sequence of elements and pushes them all onto the stack:

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// pushAll method without wildcard type - deficient! public void pushAll(Iterable<E> src) {     for (E e : src) {         push(e);     } }

should be re-written:

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// Wildcard type for parameter that serves as an E producer public void pushAll(Iterable<? extends E> src) {     for (E e : src) {         push(e);     } }

Similarly, a method that pops each element off the stack and adds the elements to the given collection:

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// popAll method without wildcard type - deficient! public void popAll(Collection<E> dst) {     while (!isEmpty()) {         dst.add(pop());     } }

should be re-written:

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// Wildcard type for parameter that serves as an E consumer public void popAll(Collection<? super E> dst) {     while (!isEmpty()) {         dst.add(pop());     } }