Earlier we mentioned that English is ambiguous. The phrase “animal cage” can reasonably mean “all-animal cage”, but it also suggests an entirely different concept: a cage designed not for any<\/I> kind of animal, but rather for some<\/I> kind of animal whose type is unknown. In generics, an unknown type is represented by the wildcard<\/I> character “?<\/CODE>“.
\nTo specify a cage capable of holding some<\/I> kind of animal: <\/P>\tCage<? extends Animal> someCage = …;
\n<\/PRE>Read “? extends Animal<\/CODE>” as “an unknown type that is a subtype of Animal<\/CODE>, possibly Animal<\/CODE> itself”, which boils down to “some kind of animal”. This is an example of a bounded wildcard<\/I>, where Animal<\/CODE> forms the upper bound<\/I> of the expected type. If you’re asked for a cage that simply holds some<\/I> kind of animal, you’re free to provide a lion cage or<\/I> a butterfly cage.
\n<\/P>
\n
\n
\nNote:<\/B> It’s also possible to specify a lower bound<\/I> by using the super<\/CODE> keyword instead of extends<\/CODE>. The code <? super Animal><\/CODE>, therefore, would be read as “an unknown type that is a supertype of Animal<\/CODE>, possibly Animal<\/CODE> itself”. You can also specify an unknown type with an unbounded wilcard<\/I>, which simply looks like <?><\/CODE>. An unbounded wildcard is essentially the same as saying <? extends Object><\/CODE>.
\n
\n<\/BLOCKQUOTE>
\nWhile Cage<Lion><\/CODE> and Cage<Butterfly><\/CODE> are not subtypes of Cage<Animal><\/CODE>, they are in fact subtypes of Cage<? extends Animal><\/CODE>: <\/P>\tsomeCage = lionCage; \/\/ OK
\n\tsomeCage = butterflyCage; \/\/ OK
\n<\/PRE>So now the question becomes, “Can you add butterflies and lions directly to someCage<\/CODE>?”. As you can probably guess, the answer to this question is “no”. \tsomeCage.add(king);\t\/\/ compiler-time error
\n\tsomeCage.add(monarch);\t\/\/ compiler-time error
\n<\/PRE>If someCage<\/CODE> is a butterfly cage, it would hold butterflies just fine, but the lions would be able to break free. If it’s a lion cage, then all would be well with the lions, but the butterflies would fly away. So if you can’t put anything at all into someCage<\/CODE>, is it useless? No, because you can still read its contents: \tvoid feedAnimals(Cage<? extends Animal> someCage) {
\n\t for (Animal a : someCage)
\n\t\ta.feedMe();
\n\t}
\n<\/PRE>Therefore, you could house your animals in their individual cages, as shown earlier, and invoke this method first for the lions and then for the butterflies: \tfeedAnimals(lionCage);
\n\tfeedAnimals(butterflyCage);
\n<\/PRE>Or, you could choose to combine your animals in the all-animal cage instead: \tfeedAnimals(animalCage);
\n<\/PRE><\/BLOCKQUOTE>
\n<\/DIV><\/p>\n","protected":false},"excerpt":{"rendered":"Wildcards Earlier we mentioned that English is ambiguous. The phrase “animal cage” can reasonably mean “all-animal cage”, but it also suggests an entirely different concept: a cage designed not for any kind of animal, but rather for some kind of animal whose type is unknown. In generics, an unknown type is represented by the wildcard … Continue reading Wildcards<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[],"tags":[],"class_list":["post-415","post","type-post","status-publish","format-standard","hentry"],"_links":{"self":[{"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/posts\/415","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.strongd.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=415"}],"version-history":[{"count":0,"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/posts\/415\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.strongd.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=415"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.strongd.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=415"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.strongd.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=415"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}