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Inner classes can access even the private members of the outer class.. doesn't it violates the privacy?

My interviewer asked me about inner classes.. After explaining him everything he stopped me on my one sentence- if inner classes can access private members of outer class then doesn't it violate privacy?
I was unable to answer it.

4 Answers
4

Answer is No as inner class is part of the outer class, just like other variable and methods are

No

All private variable/method of a class can be accessed inside all methods of the same class. An inner class is a special case where an instance of InnerClass can exist only within an instance of OuterClass. Hence it has direct access to the methods and fields of its enclosing instance.

private

From a JVM perspective, yes, an inner class accessing a private member of the outer class violates privacy.

But, from a Java perspective, no, it does not violate privacy.

The Java Virtual Machine Specification, section 5.4.4. Access Control says:

A field or method R is accessible to a class or interface D if and only if any of the following is true:

[...]

R is private and is declared in D.

private

So, the JVM will only allow private members to be accessed from code in the same class, i.e. a nested class cannot access private members of the outer class.

private

private

The Java Language Specification, section 6.6.1. Determining Accessibility says:

A member (class, interface, field, or method) of a reference type, or a constructor of a class type, is accessible only if the type is accessible and the member or constructor is declared to permit access:

[...]

Otherwise, the member or constructor is declared private, and 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.

private

So, a private member in a top-level class and/or nested class is accessible from code anywhere within that top-level class. Since nested classes by definition occur within the body of the enclosing top-level class, code in nested classes can access private members of the outer class.

private

private

To solve the discrepancy, the Java compiler creates hidden (synthetic) methods for allowing "private" access between closely related classes, i.e. between a top-level class and all its nested classes.

This is an internal trick of the compiler and is not really documented in the specifications. JVMS, section 4.7.8. The Synthetic Attribute says:

[...] A class member that does not appear in the source code must be marked using a Synthetic attribute, or else it must have its ACC_SYNTHETIC flag set. [...]

Synthetic

ACC_SYNTHETIC

The Synthetic attribute was introduced in JDK 1.1 to support nested classes and interfaces.

Synthetic

For more information, do a web search for java synthetic accessor.

java synthetic accessor

See also: Synthetic accessor method warning

Answer : No inner class does not voilate the privacy of outer class.

Explanation : All instance methods defined in a class are able to access the private or not private fields and methods defined in the class. This happens as all the instance methods and fields belong to the current object of the class.
Same is true for any inner (non static) class, it has an implicit reference of outerclass current object.
This is the reason as why you can only create the object of inner (non static) class with the help of an object of outer class. If you create the object of inner class inside any instance method of outer class then it is created with the help of implicit current object reference of the outer class.

If you have inner class which is static, then it does not has implicit reference to current object of outer class. Any instance field or method belong to an Object of the class. Hence static inner class can not access any private or non private instance field or method of outer class. You can set it explicitly and then it can acess. Now with the help of this explicitly set reference of outer class you can access the private Fields and methods.

So now lets modify the question as why inner static class wuth an explicit reference of outer class set can acess private methods and fields ?

Answer is related to our decision for having such design. The intention of defining any entity within the scope of a class is belongingness. If belongingness is missing then you should reconsider your decision lf making the class as inner (static or non static). Inner classes should be made when we wish to encapsulate a sub responsibility to an entity. This makes the related responsibility still cohesive.
Iterator is a part of any Collection and hence it is inner class. Custom AsyncTask class defined in custom Activity class in android is often made as private static (with weak reference of outer class activity) to prevwnt activity leak as the intention is to modify the fields which are private.

P.S : Afer compiler compiles the code it generates separate files for inner class and you can refer the link to understand as how the interaction of fields of one class being accessible to other class happens when other class is defined as inner class
https://stackoverflow.com/a/24312109/504133 . But still this is backend task done by langauge tools.

The answer is NO, because inner class has internal link to the outer class and inner class does not exists without concrecte instance of outer class.

But if you add static to the inner class declaration, it means the it does not have link to the outer class and this is the same, when you declare class in it's own file.

That is all, clear and simple.

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