Review enums, records, sealed classes, nested classes, pattern matching, and how to choose language features deliberately.
Foundations · modern Java
Use the strong answer as your reference, explain it aloud in your own words, then choose how soon you want to review it again.
Interview prompt
Strong answer
Modern Java features reduce ceremony while preserving explicit types. Records model transparent data carriers, sealed types restrict an inheritance hierarchy, and enums model a finite set of domain values. Use them when the domain is genuinely closed and the simpler representation improves correctness.
Key points
Interviewer looks for
Mention the trade-off: concise syntax is valuable, but domain behavior and mutability still need deliberate design.
Java example
1record UserId(long value) {
2 UserId {
3 if (value <= 0) throw new IllegalArgumentException("positive id required");
4 }
5}Topic interview questions
These questions are tied to this lesson. Read each question aloud, then compare your explanation with the strong answer.
Strong answer
A record concisely models a transparent data carrier with final components, accessors, equals, hashCode, and toString generated from those components. It is shallowly immutable, so referenced objects can still be mutable.
Strong answer
A sealed class or interface restricts which types may extend or implement it. This makes a domain hierarchy explicit and allows exhaustive reasoning in pattern matching and switch expressions.
Strong answer
No. A record prevents reassignment of its component references, but a component such as a List can still be mutated unless the constructor makes a defensive copy or uses an immutable representation.
Strong answer
An enum models a closed set of values with type safety and can hold fields and behavior. It prevents unrelated integers or strings from being passed accidentally as domain values.
Strong answer
var removes repeated local variable type syntax while retaining static typing and compile-time checking. It should improve readability when the initializer makes the type obvious, not hide an important API or domain type.
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