Encapsulation vs Abstraction Interview Questions

Encapsulation is bundling data with the methods that change it, and hiding the fields. Abstraction is hiding how the work is done behind a simpler interface. They are related, not the same word. BankAccount with a private balance and deposit/withdraw is encapsulation — I protect an invariant. The caller not knowing whether transactions sit in SQL or in memory is abstraction. I can have encapsulation without a rich abstraction, and I can have an abstract API that still leaks fields. Campus interviews like this distinction, so I always give both examples.

A repository that returns a JDBC ResultSet, or a cache API that forces callers to catch RedisConnectionException. I would wrap that in a DTO and a domain exception. A public getter that returns the live internal list is an encapsulation leak — I return a copy.

No. That is a public struct with extra typing. I expose what the domain needs. If age is derived from date of birth I do not set age. If items should only grow through addItem I do not expose the list setter.

If mutation lives inside the object I can synchronize in one place. If every caller mutates a public list I cannot reason about threads. I can swap HashMap for ConcurrentHashMap without touching callers.

Yes. A function that hides HTTP, or a REST API over a database. OOP classes are one vehicle. I would not say abstraction equals abstract class. In an OOP interview I still show an interface, then I say the idea is broader.