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According to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\]:

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Nested classes are a broad set of classes that are classified as static member and inner classes. "An inner class is a nested class that is not explicitly or implicitly declared {{static}}." \[[JLS 05 Section 8.1.3, Inner Classes and Enclosing Instances|http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.1.3|AA. Java References#JLS 05]\]. An inner class may be local, anonymous or non-static.

The use of nested class is error-prone to error unless the semantics are well understood. A common notion is that only the outer class can access the contents of the nested inner class(es). 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/constructors.

Also, according to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\]:

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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The code in this noncompliant example illegally exposes the (x,y) coordinates through the getPoint() method of the inner class. The As a result, the AnotherClass class can as a result illegally access the coordinates which is clearly not desired. Note that the fields x and y cannot be directly accessed (Coordinates.Point.x, for instance, is inaccessible) through the class Point at compile time. however, at runtime, these fields have package-private access.

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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