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A nested class is any class whose declaration occurs within the body of another class or interface [JLS 20052015]. The use of a nested class is error prone unless the semantics are well understood. A common notion is that only the nested class may access the contents of the outer class. Not only does the nested class have access to the private fields of the outer class, but the same fields can be accessed by any other class within the package when the nested class is declared public or if it contains public methods or constructors. As a result, the nested class must not expose the private members of the outer class to external classes or packages.

According to The Java Language Specification (JLS), §8.3, "Field Declarations" [JLS 20052015]:

Note that a private field of a superclass might be accessible to a subclass (for example, if both classes are members of the same class). Nevertheless, a private field is never inherited by a subclass.

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MITRE CWE

CWE-492, Use of Inner Class Containing Sensitive Data

Bibliography

[JLS 20052015]

§8.1.3, Inner Classes and Enclosing Instances
§8.3, "Field Declarations"

[Long 2005]

Section 2.3, "Inner Classes"

[McGraw 1999]

Securing Java: Getting Down to Business with Mobile Code

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