Why can outer Java classes access inner class private members?

Question

The inner class is just a way to cleanly separate some functionality that really belongs to the original outer class. They are intended to be used when you have 2 requirements:

1. Some piece of functionality in your outer class would be most clear if it was implemented in a separate class.
2. Even though it's in a separate class, the functionality is very closely tied to way that the outer class works.

Given these requirements, inner classes have full access to their outer class. Since they're basically a member of the outer class, it makes sense that they have access to methods and attributes of the outer class -- including privates.

If you like to hide the private members of your inner class, you may define an Interface with the public members and create an anonymous inner class that implements this interface. Example bellow:

class ABC{
    private interface MyInterface{
         void printInt();
    }

    private static MyInterface mMember = new MyInterface(){
        private int x=10;

        public void printInt(){
            System.out.println(String.valueOf(x));
        }
    };

    public static void main(String... args){
        System.out.println("Hello :: "+mMember.x); ///not allowed
        mMember.printInt(); // allowed
    }
}

The inner class is (for purposes of access control) considered to be part of the containing class. This means full access to all privates.

The way this is implemented is using synthetic package-protected methods: The inner class will be compiled to a separate class in the same package (ABC$XYZ). The JVM does not support this level of isolation directly, so that at the bytecode-level ABC$XYZ will have package-protected methods that the outer class uses to get to the private methods/fields.

There's a correct answer appearing on another question similar to this: Why can the private member of an nested class be accessed by the methods of the enclosing class?

It says there's a definition of private scoping on JLS - Determining Accessibility:

Otherwise, if the member or constructor is declared private, then access is permitted if and only if it occurs within the body of the top level class (§7.6) that encloses the declaration of the member or constructor.

Thilo added a good answer for your first question "How is this possible?". I wish to elaborate a bit on the second asked question: Why is this behavior allowed?

For starters, let's just be perfectly clear that this behavior is not limited to inner classes, which by definition are non-static nested types. This behavior is allowed for all nested types, including nested enums and interfaces which must be static and cannot have an enclosing instance. Basically, the model is a simplification down to the following statement: Nested code have full access to enclosing code - and vice versa.

So, why then? I think an example illustrate the point better.

Think of your body and your brain. If you inject heroin into your arm, your brain gets high. If the amygdala region of your brain see what he believe is a threat to your personally safety, say a wasp for example, he'll make your body turn the other way around and run for the hills without You "thinking" twice about it.

So, the brain is an intrinsic part of the body - and strangely enough, the other way around too. Using access control between such closely related entities forfeit their claim of relationship. If you do need access control, then you need to separate the classes more into truly distinct units. Until then, they are the same unit. A driving example for further studies would be to look at how a Java Iterator usually is implemented.

Unlimited access from enclosing code to nested code makes it, for the most part, rather useless to add access modifiers to fields and methods of a nested type. Doing so is adding clutter and might provide a false sense of safety for new comers of the Java programming language.

An IMHO important use case for inner classes is the factory pattern. The enclosing class may prepare an instance of the inner class w/o access restrictions and pass the instance to the outside world, where private access will be honored.

In contradiction to abyx declaring the class static doesn't change access restrictions to the enclosing class, as shown below. Also the access restrictions between static classes in the same enclosing class are working. I was surprised ...

class MyPrivates {
    static class Inner1 { private int test1 = 2; }
    static class Inner2 { private int test2 = new Inner1().test1; }

    public static void main(String[] args) {
        System.out.println("Inner : "+new Inner2().test2);
    }
}

Access restrictions are done on a per class basis. There is no way for a method declared in a class to not be able to access all of the instance/class members. It this stands to reason that inner classes also have unfettered access to the members of the outer class, and the outer class has unfettered access to the members of the inner class.

By putting a class inside another class you are making it tightly tied to the implementation, and anything that is part of the implementation should have access to the other parts.