Explain Encapsulation through an Object Invariant
Company: Apple
Role: Software Engineer
Category: Software Engineering Fundamentals
Difficulty: medium
Interview Round: Technical Screen
What is data encapsulation in Java, and how does it help keep an object valid? Give a concrete example rather than only listing private fields, getters, and setters.
### What a Strong Answer Covers
Identify an invariant, keep representation private, and expose operations that preserve it. Explain why unrestricted setters or returning mutable internal collections can defeat encapsulation.
### Follow-up Questions
- Does a private collection remain protected if a getter returns the same mutable instance?
- Why might an operation such as reserve be better than exposing a setAvailable method?
Overview: Explain encapsulation using a reservation invariant and controlled operations. Show why private fields alone are insufficient when setters, mutable references, or concurrent updates can bypass validation.
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