Package Name from Calls

Question

My problem is this. I have the following 3 packages.
layer1.layer2a
layer1.layer2a.layer3
layer1.layer2b
I have a interface in layer2a that is either public to everyone or private just to that package. I need for layer2b NOT to have access to it but for all packages that start with layer1.layer2a to have access to it. So package layer1.layer2a.layer3 would have access to it and package layer1.layer2b would not.
I thought that if I could get the calling classes package name I could return a null value if the package name does not start with layer1.layer2a, however the below code always returns null.
this.getClass().getPackage(); //returns null
this.getClass().getDeclaringClass() //returns null
All of my classes definetly have package names so I'm not sure why the first line returns null. And yes I know that the first line will not return the calling classes package. I was just testing it.
Can anyone help me with this? If you have a better solution to my problem please post.
Below is my code example.
package layer1.layer2a;
public class DataFile {
     private String userId="test";
     public String getUserId(){
//System.out.println("package name = "+this.getClass().getPackage());
//System.out.println("calling class is = "+this.getClass().getDeclaringClass());
          return userId;
-----------------------calling class---------------------------
package layer1.layer2a;
public class MyTest{
     public void callDataFile(){
          DataFile df = new DataFile();
          String s = df.getUserId();          
System.out.println("s = "+s);          
}

Answer 1

So package layer1.layer2a.layer3 would have access to it and package layer1.layer2b would not.Not possible.
There is no hierarchial relationship between packages. Given the following packages:
A.B
A.B.C
A.Z
The package A.B.C has the same relationship to A.B as it does to A.Z. There is no way to give expose something to one package without exposing it to all other packages.
Solutions:
-Look at the design again. Maybe it can be refactored to provide a different solution.
-Just expose it, and document that the interface should only be used in a certain way.