java.util.ArrayList class, the object array for the list’s elements is defined as package-private:
transient Object elementData; // non-private to simplify nested class access
The comment states that the reason why this field is not private is easier access in nested classes. However, nested classes can access private data of the enclosing class just fine. So why is
elementData not private? Is there something happening in the background (e.g., at compilation time)?
When you access a private field from a nested class, the compiler actually generates a synthetic accessor method that is package-visible, and then uses that for the access. It can’t access the private member directly, so to avoid that indirection you can make the member package-visible instead.
Here’s an answer with more details.