Wildcard (Java): Difference between revisions

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== Bounded Wildcards ==

A bounded wildcard is one with either an upper or a lower [[Inheritance (object-oriented programming)|inheritance]] constraint. The Notbounds onlycan thebe formalboth typeclass parametersand ininterface types. Upper bounds are expressed using the generic'''extends''' unit,keyword butand alsolower bounds using the '''super''' kkeyword. An upper bound on a wildcard canmust be (further)a constrainedsubtype ifof onethe doesn'tupper wantbound toon bethe compatibletype withparameter all instantiations:it is assigned.

<code>Generic<? '''extends''' SubtypeOfUpperBound> referenceConstrainedFromAbove;</code>

This reference can hold any instantiation of <code>Generic</code> with an actual type parameter of <code>SubtypeOfUpperBound</code>'s subtype. A wildcard that does not have aan constraintupper bound is effectively the same as one that has the constraint <code>extends Object</code>, since all types implicitly extend Object. A constraintwildcard with a lower bound

<code>Generic<? '''super''' SubtypeOfUpperBound> referenceConstrainedFromBelow;</code>