Instantiating static nested classes

Question

This class

public class Main {

    public static void main(String[] args) {
        Main m = new Main();
        m.new A();
        m.new B();    //1 - compilation error

        new Main.B();
    }

    class A{}
    static class B{}  //2
}

will result in a compile time error at line 1:

Illegal enclosing instance specification for type Main.B

But I cannot understand why, I find it a bit counterintuitive: at line 2 we have a static class definition, shouldn'it be accessible from object m as well?

Edit

If Main had a static variable i, m.i wouldn't result in a compilation error. Why is the behaviour different with class definition?

Accessibility is not relevant. The class StringBuilder is also accessible from the object `m`. But you don't use `m.new StringBuilder()` to instantiate it. The syntax `m.new` is used to instantiate an inner class of Main referring to the enclosing object `m`. B is not an inner class, and doesn't have any implicit reference to any Main object. — JB Nizet, Dec 20 '15 at 14:35
It's not an inner class? Only non static classes are defined "inner", while static ones are "nested"? — Luigi Cortese, Dec 20 '15 at 14:42
Yes. Read the Terminology section in https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html — JB Nizet, Dec 20 '15 at 14:43
The fast that they're both **nested** class doesn't make B an **inner** class. — JB Nizet, Dec 20 '15 at 14:49

score 2 · Accepted Answer · answered Dec 20 '15 at 14:32

2

No.

The whole point of a static inner class is that it doesn't have an instance of the enclosing class.

answered Dec 20 '15 at 14:32

SLaks

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I understand that. It's like any other static member. But if `Main` had a static variable `i`, `m.i` wouldn't result in a compilation error – Luigi Cortese Dec 20 '15 at 14:34
1

@LuigiCortese: That feature was a mistake. The language designers did not carry on that mistake to new language features. – SLaks Dec 20 '15 at 14:38
Seriously?? Any reference? – Luigi Cortese Dec 20 '15 at 14:39
It's always cool when you cannot distinguish a bug from a feature – Luigi Cortese Dec 20 '15 at 14:47
1

It's not a bug, but a design flaw. – SLaks Dec 20 '15 at 14:53
This may also explain why static method of an interface can only be accessed by using the name of that interface, no references allowed: being static method new to java 8 (or 7...) they didn't carry on that mistake neither, what do you think? – Luigi Cortese Dec 20 '15 at 16:41

score 1 · Answer 2 · answered Dec 20 '15 at 14:39

1

m.new B(); is incorrect way of instantiating a nested static class as B is not an instance variable of class Main - so does not need instance of Main to create it. Rather you can do

new Main.B();

Quoting from docs for clarity

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.

answered Dec 20 '15 at 14:39

Aniket Thakur

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Only non static nested classes need outer class for its instantiation. So saying `m.new B();` is not correct as `m` does not own `B` it is like any other top level class. So what you mean is why can't I access the class as `m.B` like any other static variable. I may not be the correct person to answer that :) this is how it was designed. – Aniket Thakur Dec 20 '15 at 14:53

Instantiating static nested classes

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