What does the question mark in Java generics’ type parameter mean? [duplicate]

by 
? 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<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.

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<T>, it actually takes a Comparator<? super T>. In this case, the Comparator is consuming the elements of the List in order to reorder the List itself.

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