What does the question mark in Java generics type parameter mean

Question

This is a small snippet of code taken from some of the examples that accompany the Stanford Parser   I ve been developing in Java for about 4 years  but have never had a very strong understanding of what this style of code is supposed to indicate   List lt   extends HasWord gt  wordList   toke tokenize      I m not worried about the details of the code   What I m confused about is what exactly the generic expression is supposed to convey  in English   Can someone explain this to me

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Perhaps a contrived  real world  example would help   At my place of work we have rubbish bins that come in different flavours  All bins contain rubbish  but some bins are specialist and do not take all types of rubbish  So we have Bin lt CupRubbish gt  and Bin lt RecylcableRubbish gt   The type system needs to make sure I can t put my HalfEatenSandwichRubbish into either of these types  but it can go into a general rubbish bin Bin lt Rubbish gt   If I wanted to talk about a Bin of Rubbish which may be specialised so I can t put in incompatible rubbish  then that would be Bin lt   extends Rubbish gt     Note    extends does not mean read-only  For instance  I can with proper precautions take out a piece of rubbish from a bin of unknown speciality and later put it back in a different place    Not sure how much that helps  Pointer-to-pointer in presence of polymorphism isn t entirely obvious

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In English      It s a List of some type that extends the class HasWord  including HasWord   In general the   in generics means any class   And the extends SomeClass specifies that that object must extend SomeClass  or be that class

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A question mark is a signifier for  any type      alone means      Any type extending Object  including Object    while your example above means      Any type extending or implementing   HasWord  including HasWord if   HasWord is a non-abstract class

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extends HasWord   means  A class interface that extends HasWord    In other words  HasWord itself or any of its children    basically anything that would work with instanceof HasWord plus null   In more technical terms    extends HasWord is a bounded wildcard  covered in Item 31 of Effective Java 3rd Edition  starting on page 139   The same chapter from the 2nd Edition is available online as a PDF  the part on bounded wildcards is Item 28 starting on page 134   Update  PDF link was updated since Oracle removed it a while back   It now points to the copy hosted by the Queen Mary University of London s School of Electronic Engineering and Computer Science   Update 2  Lets go into a bit more detail as to why you d want to use wildcards   If you declare a method whose signature expect you to pass in List lt HasWord gt   then the only thing you can pass in is a List lt HasWord gt    However  if said signature was List lt   extends HasWord gt  then you could pass in a List lt ChildOfHasWord gt  instead   Note that there is a subtle difference between List lt   extends HasWord gt  and List lt   super HasWord gt    As Joshua Bloch put it  PECS   producer-extends  consumer-super   What this means is that if you are passing in a collection that your method pulls data out from  i e  the collection is producing elements for your method to use   you should use extends   If you re passing in a collection that your method adds data to  i e  the collection is consuming elements your method creates   it should use super   This may sound confusing   However  you can see it in List s sort command  which is just a shortcut to the two-arg version of Collections sort    Instead of taking a Comparator lt T gt   it actually takes a Comparator lt   super T gt    In this case  the Comparator is consuming the elements of the List in order to reorder the List itself

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The question mark is used to define wildcards  Checkout the Oracle documentation about them  http   docs oracle com javase tutorial java generics wildcards html

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List lt   extends HasWord gt  accepts any concrete classes that extends HasWord  If you have the following classes     public class A extends HasWord        public class B extends HasWord        public class C        public class D extends SomeOtherWord              the wordList can ONLY contain a list of either As or Bs or mixture of both because both classes extend the same parent or null  which fails instanceof checks for HasWorld

[java] What does the question mark in Java generics' type parameter mean?

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