Understanding syntax in Java

Question

I would like your help in understanding what this syntax means:

class Node<K extends Comparable<? super K>, V>

What does the ? stands for?

And isn't there one < missing?

See also Can someone explain what does <? super T> mean and when should it be used and how this construction should cooperate with <T> and <? extends T>? — Péter Török
– Péter Török, Commented Apr 20, 2011 at 10:02
A very good resource about Java generics is Angelika Langer's Java Generics FAQ: angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html — Jesper
– Jesper, Commented Apr 20, 2011 at 11:16

Joachim Sauer · Accepted Answer · 2011-04-20 09:59:25Z

9

The ? stands for "some unknown type". In this specific case it's ? super K which means "some unknown type that's a base type of K (i.e. "super class of" or "interface implemented by") .

And no, there's no < missing: you have two < and two >, they match up.

Practically it means that Node has two type arguments: K which probably represents a key, which must be Comparable to itself and V which probably represents a value.

answered Apr 20, 2011 at 9:59

Joachim Sauer

309k59 gold badges568 silver badges624 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

bobah · Accepted Answer · 2011-04-20 14:34:25Z

0

the generics/wildcards tutorial explains this syntax

edited Apr 20, 2011 at 14:34

answered Apr 20, 2011 at 10:00

bobah

19k2 gold badges43 silver badges74 bronze badges

5 Comments

Joachim Sauer Over a year ago

I downvoted this because answers that are just a single link are not considered to be good answers.

bobah Over a year ago

@Joachim Sauer - right, cut-n-paste is better than the link to the exact answer on the technology owner's site ...

Joachim Sauer Over a year ago

Who's talking about cut-and-paste? Did you read the question I linked to? The issue has been discussed.

bobah Over a year ago

@Joachim Sauer - the author of the question is confused with the syntax of wildcards in generics that is precisely what the article behind the link I gave explains. I did read the question you gave a link to, and I do believe that being a robot is not the best attitude to develop in yourself. In this particular case a single link IS a good answer.

Joachim Sauer Over a year ago

@bobah: and it's great if you helped, my personal opinion is something different, but that's no problem. A down-vote is not a personal attack. Obviously at least 2 people think that your answer is good, so you got the upvotes. I don't see the problem.

Collectives™ on Stack Overflow

Understanding syntax in Java

2 Answers 2

Comments

5 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

5 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related