See how Private Class Data in Java helps preserve invariants, reduce accidental leakage, and narrow the public state surface.
Private Class Data is not a security boundary by itself, but it improves internal safety by narrowing what other code can observe or mutate.
This matters in Java because public getters over complex mutable state often become implicit coupling points. Once outside code starts depending on internal structure, refactoring becomes harder.
If callers only need outcomes, do not expose raw internal data structures just because getters are easy to generate.