Static variable not initialized when class is loaded

Question

I have an interesting question on initialization. I have the following code:

public class ErrorLookupProvider {  

private static final ErrorLookupProvider INSTANCE = new ErrorLookupProvider();

private  static Map<Long, List<String>> map = new HashMap<Long, List<String>>();

    private ErrorLookupProvider() {
        init();
    }

    private void init() {
        map.put(123L, ImmutableList.of("abc", "def"));
    }

    public static ErrorLookupProvider getInstance() {
        return INSTANCE;
    }
}

Now when I call ErrorLookupProvider.getInstance(), I hit an NPE. The map inside init() is not initialized with the new HashMap.

If I change the declaration of map to be final, then I see that it is initialized. Or, even if I remove static and make it a private class variable as private Map<.....> that works too.

I haven't been able to figure out why this happens. Can someone explain what is happening here?

Will that even compile with the non-long map key?

Dave Newton
– Dave Newton

2012-06-15 23:12:10 +00:00
Commented Jun 15, 2012 at 23:12 — Dave Newton
– Dave Newton, Commented Jun 15, 2012 at 23:12
My mistake, I missed out the 123L

Sudoer
– Sudoer

2012-06-15 23:17:42 +00:00
Commented Jun 15, 2012 at 23:17 — Sudoer
– Sudoer, Commented Jun 15, 2012 at 23:17

Dave Newton · Accepted Answer · 2012-06-15 23:27:32Z

5

Switch the order of the map and singleton instance initializations.

Static initialization occurs in the order it's encountered in the source.

See JLS 12.4.2 Detailed Initialization Procedure, steps 6 (the final part) and 9 (the "order" part).

(The singleton implementation and mucking with statics in a ctor, separate issue.)

edited Jun 15, 2012 at 23:27

answered Jun 15, 2012 at 23:22

Dave Newton

161k27 gold badges264 silver badges311 bronze badges

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

2 Comments

Sudoer Over a year ago

Oh! That's because of the sequencing of statics. If I put the map first, it would be ok I think. Which leads me to the question - Is this even allowed? private static final ErrorLookupProvider INSTANCE = new ErrorLookupProvider(); Going by the sequencing, I'm initializing my static variable by calling the constructor. This means the constructor is called even before the variable is initialized. Is this usage incorrect then? Usualy, the init happens as follows: 1. Static members 2. Static blocks 3. Member vars 4. Constructor Now, I'm trying to jump from 1 to 4. Should this be disallowed?

Dave Newton Over a year ago

@Sudoer Yep, that's what we said--order matters :) Of course it's allowed--it works, doesn't it?

Dave Newton · Accepted Answer · 2012-06-15 23:13:40Z

2

quoting from http://javapapers.com/core-java/explain-the-final-keyword-in-java/

A variable that is declared as final and not initialized is called a blank final variable. A blank final variable forces the constructors to initialise it.

That is why when declared final it is initialised

edited Jun 15, 2012 at 23:13

Dave Newton

161k27 gold badges264 silver badges311 bronze badges

answered Jun 15, 2012 at 23:12

MaVRoSCy

17.9k15 gold badges87 silver badges129 bronze badges

4 Comments

Sudoer Over a year ago

Is it even valid to call private static final ErrorLookupProvider INSTANCE = new ErrorLookupProvider(); The class initialization is first the static vars, static blocks, member vars and then the constructor. However, in this case, is it even valid to call the constructor to initialize the static variable?

MaVRoSCy Over a year ago

yes that would be fine. Just a minor comment to your code, you could use as a declaration of your HashMap this private static Map<Long, List<String>> map = new HashMap<>();

MaVRoSCy Over a year ago

NO NO NO, when declared final then if the variable is not initialised by you, the constructor will force it to be initialised

user949300 Over a year ago

@MaVRoSCy that new shorter syntax is nice, but only works in Java 7.

user949300 · Accepted Answer · 2012-06-15 23:27:49Z

2

ADDED: Order matters. Put the declaration for your static map before the declaration of INSTANCE. The Java compiler is a bit stupid about ordering...

Since map is static, it is shared among all instances of ErrorLookupProvider. Therefore, it is probably a mistake to play with it in the constructor. If you create multiple ErrorLookupProviders, you will redundantly add to the map many times. Instead, initilize it in a static initializer block. Or, if it is really meant to be independent between instances of ErrorLookupProvider, don't make it static.

edited Jun 15, 2012 at 23:27

answered Jun 15, 2012 at 23:19

user949300

15.7k7 gold badges39 silver badges69 bronze badges

Collectives™ on Stack Overflow

Static variable not initialized when class is loaded

3 Answers 3

2 Comments

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related