Don't use inner classes

We read that inner classes can be accessed only by the outer classes that enclose them. But this isn't true. Java bytecode has no concept of inner classes, so inner classes are translated by the compiler into ordinary classes that happen to be accessible to any code in the same package.

But wait, it gets worse. An inner class gets access to the fields of the enclosing outer class, even if these fields are declared private. And the inner class is translated into a separate class.

To let this separate class access the fields of the outer class, the compiler silently changes these fields from private to package scope! It's bad enough that the inner class is exposed; but it's even worse that the compiler is silently overruling your decision to make some fields private. Don't use inner classes if you can help it.