What is the purpose of "? extends A"?

Question

If I have a list like this:

static ArrayList<? extends A> list = new ArrayList<A>();

What is the point of ? extends A? Does it mean that I am making a list of only subclasses that MUST inherit from A?

Normally I do it like this:

static ArrayList<A> list = new ArrayList<A>();

[Best answer ever](http://docs.oracle.com/javase/tutorial/java/generics/). — Maroun, May 25 '14 at 06:59
@MarounMaroun That is a link to generics. Don't just throw links at me man. — J_Strauton, May 25 '14 at 07:01
@J_Strauton a direct link [here](http://docs.oracle.com/javase/tutorial/java/generics/upperBounded.html). — Christian Tapia, May 25 '14 at 07:01
@J_Strauton Read the tutorial and you'll understand what it's about. — Maroun, May 25 '14 at 07:01
Ref. http://stackoverflow.com/questions/6279331/main-intention-or-purpose-of-wildcard-notation-extends-t?rq=1 , http://stackoverflow.com/questions/12894335/generic-lower-unbound-vs-upper-bounded-wildcards?lq=1 , http://stackoverflow.com/questions/7541849/wildcards-in-generics-super-t-works-while-extends-t-does-not?rq=1 — user2864740, May 25 '14 at 07:05
http://www.youtube.com/watch?v=V1vQf4qyMXg#t=1323 -- best resource to start learning about co/contra-variance in Java. A talk from Joshua Bloch about some chapters on his Effective Java. Flex your PECS. — Bruno Reis, May 25 '14 at 07:22

Afzaal Ahmad Zeeshan · Accepted Answer · 2014-05-25T07:09:29.440

3

extends keyword means that the class on the left of this keyword would be using all the methods and properties from the class that is one the right side. ? wouldn't be there. Some name would be. A name of a class.

static ArrayList<? extends A> list = new ArrayList<A>();

In the above code, you're making a new ArrayList object which uses (implements/extends) the Class A for its methods and properties but is not in real A.

static ArrayList<A> list = new ArrayList<A>();

Whereas in the second code that you usually use, you're actually creating an Object which uses class A in real.

In Java we call it inheritance where one class inherits all the properties and functions of the other class.

http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html

edited May 25 '14 at 07:09

answered May 25 '14 at 07:00

Afzaal Ahmad Zeeshan

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Let me double check. Does that mean that I am casting any future class that goes in that list as a child class of A? – J_Strauton May 25 '14 at 07:04
No not child, but some sort of a clone of it. Because it would have all the methods of Class `A` in it. – Afzaal Ahmad Zeeshan May 25 '14 at 07:05
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Considering that `class B extends A {}; List list = new ArrayList (); list.add(new B())` compiles fine, how do the first and second declarations differ? – user2864740 May 25 '14 at 07:11
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@user2864740: They differ in that, by contrast with your version, `List extends A> list = ...; list.add(new B())` does *not* compile fine (because the compiler can't tell *which* subtype of `A` is the list's component-type). – ruakh May 25 '14 at 07:24
@ruakh Then I'm left confused about "In the above code, you're making a new ArrayList object which uses (implements/extends) the Class A for its methods and properties but is not in real A" and "Whereas in the second code that you usually use, you're actually creating an Object which uses class A in real". – user2864740 May 25 '14 at 07:39
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@user2864740: The reason you're confused is that this answer is total gibberish. I can't believe that six people upvoted it. (Personally, I'm one of the two downvoters.) – ruakh May 25 '14 at 19:25

Christian Tapia · Answer 2 · 2014-05-25T07:14:43.503

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From Java Docs:

The upper bounded wildcard, <? extends Foo>, where Foo is any type, matches Foo and any subtype of Foo.

So, answering the question "Does it mean that I am making a list of only subclasses that MUST inherit from A?"

It means that you are making a list of objects which types are A or inherit from A.

Edit:

One difference is that in the second code

ArrayList<A> list = new ArrayList<A>();

you will be able to refer to the type A anywhere in the code.

edited May 25 '14 at 07:14

answered May 25 '14 at 07:05

Christian Tapia

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So how does this differ from the second code without type-bounds? When is one appropriate over the other? – user2864740 May 25 '14 at 07:08
@user2864740 bring up an excellent point! – J_Strauton May 25 '14 at 07:11

What is the purpose of "? extends A"?

2 Answers2