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

Asked 2010-06-09T20:38:22.003

273

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<? extends HasWord> 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?

java generics

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? extends HasWord

means "A class or 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: Let's go into a bit more detail as to why you'd want to use wildcards.

If you declare a method whose signature expects you to pass in List<HasWord>, then the only thing you can pass in is a List<HasWord>.

However, if said signature was List<? extends HasWord> then you could pass in a List<ChildOfHasWord> instead.

Note that there is a subtle difference between List<? extends HasWord> and List<? super HasWord>. 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


                            answered 2010-06-09T20:42:50.177


            
            
                
                    
                        
                            
                            
                        
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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).

                        
                        
                            answered 2010-06-09T20:43:02.057



        
        
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