https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

## Access Modifiers

+-------------------------+---------+---------+-----------+--------+
|                         | default | private | protected | public |
+-------------------------+---------+---------+-----------+--------+
|same class               |    Y    |    Y    |     Y     |    Y   |
+-------------------------+---------+---------+-----------+--------+
|same package subclass    |    Y    |    N    |     Y     |    Y   |
+-------------------------+---------+---------+-----------+--------+
|same package non-subclass|    Y    |    N    |     Y     |    Y   |
+-------------------------+---------+---------+-----------+--------+
|diff package subclass    |    N    |    N    |     Y     |    Y   |
+-------------------------+---------+---------+-----------+--------+
|diff package non-subclass|    N    |    N    |     N     |    Y   |
+-------------------------+---------+---------+-----------+--------+