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A nested class is any class whose declaration occurs within the body of another class or interface \[[JLS 2005|AA. Bibliography#JLS 05]\]. Nested classes are a broad set of classes that are classified as {{static}} classes and inner classes. "An inner class is a nested class that is not explicitly or implicitly declared {{static}}" \[[JLS 2005|AA. Bibliography#JLS 05]\]. An inner class may be local, anonymous, or non-static.

The use of a nested class is error-prone unless the semantics are well understood. A common notion is that only the outer class can access the contents of the nested class. Not only does the nested class have access to the private fields of the outer class, the same fields can be accessed by another class in the package depending on whether the nested class is declared public or if it contains public methods or constructors. By default, the javac compiler converts the accessibility of private methods of a nested class to package-private.

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Use the private access specifier for hiding the inner class and all contained methods and constructors. The compiler will refuse to compile AnotherClass because of its attempt to access a private nested class.

class Coordinates {
  private int x;
  private int y;

  private class Point {
    private void getPoint() {
      System.out.println("(" + x + "," + y + ")");    
    }
  }
}

class AnotherClass {
  public static void main(String[] args) {
    Coordinates c = new Coordinates();
    Coordinates.Point p = c.new Point();    // fails to compile
    p.getPoint();
  }        
}

Compilation of AnotherClass now results in a compilation error because the class attempts to access a private nested class.

Risk Assessment

The Java Language System language system weakens the accessibility of sensitive, private entities in inner classes which can result in a security weakness.

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