Do generic classes share static members?

Question

I would like to ask about generic classes. What happens when I create two object instance from a generic class. Do they share every static members, or both have it's own static members?

So for example:

public A<?>(){
    public static Integer member = 0;
}

A<Integer> integer = new A<Integer>();
A<String> string = new A<String>();

Do both Integer and String have the same reference behind member?

Your question is valid, but the code you posted is bad: It's neither a valid constructor, nor a class declaration. The answer to your question is: Yes, they share every static member. Also, you could answer the question yourself: `System.out.println(integer.member == string.member);` Which is the same as `System.out.println(A.member == A.member);` because of being `static`) — jlordo, Jun 05 '13 at 21:06
What happens if member itself is generic? Like `public class MyClass { public static T member; }`?? — Ravi K Thapliyal, Jun 05 '13 at 21:12
This is what happens when you're too lazy to just try the code... — Luiggi Mendoza, Jun 05 '13 at 21:21
Sorry guys, it maybe seems to be laziness, but I would like to get some hints and confirmation about the topic. — Quirin, Jun 05 '13 at 21:27
@Lucia - It is best not to "fix" the code within a *question* (even when it is obviously wrong), because it may obscure the problem that brought that person here in the first place. If you spot an error, leave a comment or post an answer. — Leigh, Jun 05 '13 at 21:30

score 11 · Accepted Answer · answered Jun 05 '13 at 21:12

11

public class A<T>{
    public static Integer member = 0;

    public static void main(String[] args)
    {
      A<Integer> integer = new A<Integer>();
      A<String> string = new A<String>();

      integer.member++;
      System.out.println(string.member);
    }
}

Output

So, yes the two instances share the same member variable.

answered Jun 05 '13 at 21:12

Petros Tsialiamanis

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+1 Nice example for the question. Also noteworthy that `integer.member++;` is bad style, and stylecheckers discourage code like this. `static` members should always be accessed in a static manner, i.e.: `A.member++;`. – jlordo Jun 05 '13 at 21:16
When you're in doubt test it! :) +1 – Pragmateek Jun 05 '13 at 21:19

score 3 · Answer 2 · answered Jun 05 '13 at 21:10

3

Good question but the anwser is: no you can't for a very simple reason: once compiled there is no such thing as A<Integer> or A<String>, only A because for backward compatibility reasons Java uses generics type-erasure.

In other languages like C# things would work as you expect because C# was created from start with generics.

answered Jun 05 '13 at 21:10

Pragmateek

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Thanks for mentioning C#. The original of my thought came from a C# example. – Quirin Jun 05 '13 at 21:14
I'm in a similar situation: hard to go back 10 years in the past when using Java but I'm sure it will fill the gap. :) – Pragmateek Jun 05 '13 at 21:16

Bhesh Gurung · Answer 3 · 2013-06-05T21:24:56.630

There is no class literal (A<Integer>.class or Class instance) that represent any of parameterized type (like A<Integer>). There will always be only one for the raw version A (i.e. A.class), and that's the one with whom all the static members belong.

That's why you won't do something like A<Integer>.member, it should always be the raw version A.member (and that is the one (and only) shared by all the instances).

You can read an article on the topic here - Is there one instances of a static field per instantiation of a generic type?.

score 0 · Answer 4 · answered Jun 05 '13 at 21:07

0

Yes, they would share the variable. The rules about static class variables don't change based on the use of Generics. A static class variable is shared by all instances.

answered Jun 05 '13 at 21:07

Simon Curd

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This should not be taken for granted. In C++ `A` and `A` would be different classes with distinct static data members. – Nicola Musatti Jun 05 '13 at 21:11
@NicolaMusatti: This is a question under the `java` tag.... – jlordo Jun 05 '13 at 21:11
Indeed. Yet Java Generics were inspired by C++ templates and I think it's important to point out the differences. – Nicola Musatti Jun 05 '13 at 21:12
@NicolaMusatti Then why is this missing in your own answer? :) – Ravi K Thapliyal Jun 05 '13 at 21:17
Because Simon seemed to take for granted that there was only one A class. I just wanted to point out that this might not be self-evident for the OP – Nicola Musatti Jun 05 '13 at 21:19
...and it didn't occur to me until after I read Simon's answer :-) – Nicola Musatti Jun 05 '13 at 21:20

score 0 · Answer 5 · answered Jun 05 '13 at 21:08

0

They all share their static attributes because at runtime A<Integer> and A<String> are not distinct classes.

answered Jun 05 '13 at 21:08

Nicola Musatti

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score 0 · Answer 6 · answered Jun 05 '13 at 21:08

You just have to think that generics are for compilation purpose. At runtime, there is no difference between A<Integer> and A<String>: they are all instances of A. So static members are common to any instances you can create, without any regard to the type you are using.

Do generic classes share static members?

6 Answers6