Java inner and nested classes

Question

I've started preparing myself for the OCJP7 exam and I found this chapter that seems to be very complicated.

Let's say I have this code:

class Outer1{
    interface InnerInterface{
        String x = "test";
    }
    class InnerClass{
        String x = "test";
    }
}
class Outer2{
    static interface NestedInterface{
        String x = "test";
    }
    static class NestedClass{
        String x = "test";
    }
}
class Main{
    public static void main(String [] args){
        String s1 = Outer1.InnerInterface.x;
        String s2 = new Outer1().new InnerClass().x;
        String s3 = Outer2.NestedInterface.x;
        String s4 = new Outer2.NestedClass().x;
    }
}

Could you tell me why we can access Outer1.InnerInterface.x and Outer2.NestedInterface.x in the same manner? Inner interfaces are static by default? I'm trying to make some connections to make them more clearly.

possible duplicate of [Why would a static inner interface be used in Java?](http://stackoverflow.com/questions/71625/why-would-a-static-inner-interface-be-used-in-java) — Sam Redway, Jul 29 '15 at 21:05

score 11 · Accepted Answer · edited Jun 20 '20 at 09:12

From Oracle's Java Tutorial :

A nested class is a member of its enclosing class. Non-static nested classes (inner classes) have access to other members of the enclosing class, even if they are declared private. Static nested classes do not have access to other members of the enclosing class. As a member of the OuterClass, a nested class can be declared private, public, protected, or package private. (Recall that outer classes can only be declared public or package private.)

Interfaces cannot be instantiated. Thus they only make sense as static. Declaring a nested interface as static is redundant.

Also, the exercise uses confusing names for the interfaces.
Both InnerClass and NestedClass are nested classes. But only InnerClass is an inner class since "inner class" means "non-static nested class".
Similarly one might expect InnerInterface to be an "inner interface" meaning "non-static nested interface"; but such a thing does not exist. Both InnerInterface and NestedInterface are nested interfaces and none of them are inner interfaces.

Not sure why you quote this part from the Java Tutorial, as it talks about classes, and the question is about interfaces. The naming of this exercise is not only confusing but plain wrong, because there is no such thing as an Inner Interface. The only type of nested interface that exists is the member interface. https://docs.oracle.com/javase/specs/jls/se7/html/jls-9.html — Erwin Bolwidt, Jul 30 '15 at 04:04
The tittle of the question is "Java inner and nested classes". And the code mentions both interfaces and classes. I consider the question to be abouth both nested/inner clases and nested/inner interfaces. And about what difference there are between them. The difference (besides those between classes and interfaces in general) being that inner classes do exist and inner interfaces don't, as you say in your commend and I do in my answer. — Anonymous Coward, Jul 30 '15 at 16:55

score 7 · Answer 2 · answered Jul 29 '15 at 20:49

Yes, nested interfaces are implicitly static. Section 8.5.1 of the JLS states:

A member interface is implicitly static (§9.1.1). It is permitted for the declaration of a member interface to redundantly specify the static modifier.

Because of this, you can access x through the nested interface in the same manner through outer classes Outer1 and Outer2 -- both nested interfaces, InnerInterface and NestedInterface are static.

Adam Stelmaszczyk · Answer 3 · 2015-07-29T21:03:42.903

3

Could you tell me why we can access Outer1.InnerInterface.x and Outer2.NestedInterface.x in the same manner?

Because they're both static fields.

From Java Language Specification, Chapter 9. Interfaces, point 9.3 Field (Constant) Declarations:

Every field declaration in the body of an interface is implicitly public, static, and final.

Inner interfaces are static by default?

Yes.

From 8.1.3 Inner Classes and Enclosing Instances:

Member interfaces (§8.5) are implicitly static so they are never considered to be inner classes.

edited Jul 29 '15 at 21:03

answered Jul 29 '15 at 20:50

Adam Stelmaszczyk

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Yes, I know that fields of an interface are static, but I didn't know that an interface declared within a class is static. I've finished the chapter and I didn't find an answer... Thank you – Mike Jul 29 '15 at 21:02

score 1 · Answer 4 · answered Jul 29 '15 at 20:53

You are right. The keyword static for nested interfaces is unnecessary and redundant. A nested interface is always static.

From a logical point of view it also makes no sense that a interface definition is bound to a specific instance of the outer class.

Java inner and nested classes

4 Answers4