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A nested class is any class whose declaration occurs within the body of another class or interface \[[JLS 2005|AA. Bibliography#JLS 05]\]. 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 within 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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Also, according to the _Java Language Specification_ \[[JLS 2005|AA. Bibliography#JLS 05]\], [§8.3|http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.3] "Field Declarations," |
Note that a
privatefield of a superclass might be accessible to a subclass (for example, if both classes are members of the same class). Nevertheless, aprivatefield is never inherited by a subclass.
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class Coordinates {
private int x;
private int y;
public class Point {
public 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();
p.getPoint();
}
}
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Compliant Solution
Use the private access specifier for hiding the inner class and all contained methods and constructors.
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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();
}
}
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Compilation of AnotherClass now results in a compilation error because the class attempts to access a private nested class.
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Automated detection of non-private nested classes that define non-private members and constructors is straight-forwardstraightforward. However, this guideline applies only when those classes could potentially expose sensitive data or operations from the outer class. Detection of sensitive data or operations requires programmer assistance.
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