Static nested class does not initialize outer class

Question

When i initialize the static inner class i am expecting that outer class is also initialized and will print I should see this as well. however this is not happening and i am getting only class Main as a output

 class AA {
    static {
        System.out.println("I should see this as well.");
    }

    public static class BB {
        BB() {
            Object o = Main.class; 
            System.out.println(o.toString());
        }
    };
}

public class Test {
    public static void main(String[] args) {
        new AA.BB();
    }
}

Can some one help me , explaining this behavior.

Static inner classes are not as closely related to their outer classes as non-static ones. In fact you can see the outer class of static inner classes as some kind of namespace (or package). — Thomas
– Thomas, Commented May 5, 2017 at 9:11
A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience. source — user6073886
– user6073886, Commented May 5, 2017 at 9:11
Once they're compiled, they're sufficiently separated that you can load one class without the other. — khelwood
– khelwood, Commented May 5, 2017 at 9:17
One of those questions that make your head spin for a second ;-) ... funny thing. Have my vote for that! — GhostCat
– GhostCat, Commented May 5, 2017 at 9:18
Wow - so AA and AA.BB implemented as different classes? Never thought of that! — Ulysses
– Ulysses, Commented May 5, 2017 at 9:29

GhostCat · Accepted Answer · 2017-05-05 09:41:23Z

3

Thing is: that static initializer block gets executed lazily. Meaning: this code gets executed the first time that the AA class is really "required". But AA is not required to instantiate AA$BB.

If you change

BB() {
  Object o = Main.class; 
  System.out.println(o.toString());
}

to really require class AA to be loaded:

BB() {
  Object o = Main.class; 
  System.out.println(o.toString());
  new AA();
}

then that other string gets printed, too.

Keep in mind: it is only within your source code that BB is "inside" AA. From a class loader point of view, AA and BB are (somehow) two independent classes coming from two different class files!

Edit, given the question "how to see" that:

A) I replaced Main.class with Test.class and compiled, and I find in my file system AA$BB.class AA.class Test.class afterwards.

B) now you can run [javap][1] -c "AA$BB.class" to see more about the content of that class

edited May 5, 2017 at 9:41

answered May 5, 2017 at 9:14

GhostCat

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2 Comments

Joker Over a year ago

You are right when you say "From a class loader point of view, AA and BB are (somehow) two independent classes coming from two different class files!"--- I want to check this behavior any tool or other way you recommend to see this behavior.

Lew Bloch Over a year ago

@Show Stopper, ??? Your question includes code that shows this behavior already! It's the basis of your question, that it shows the behavior!

Lew Bloch · Accepted Answer · 2017-05-05 17:28:37Z

3

A nested class creates a distinct class.

Here you would have two classes :

AA.class
AA$BB.class

Using the second one doesn't require to load the first one as the second is a public static class.

edited May 5, 2017 at 17:28

Lew Bloch

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answered May 5, 2017 at 9:14

davidxxx

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5 Comments

Lew Bloch Over a year ago

There's no inner class in the question.

Lew Bloch Over a year ago

BB is static. It therefore is not an inner class. Read the Fine Manual at docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.1.3.

Lew Bloch Over a year ago

"An inner class is a nested class that is not explicitly or implicitly declared static."

davidxxx Over a year ago

@Lew Bloch You learn me something. To complete : "Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are called static nested classes. Non-static nested classes are called inner classes.". I edit consequently.

Lew Bloch Over a year ago

You haven't edited your answer to remove the incorrect reference to "inner class".

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