Java 9–21 Features — Interview Questions (95+)

See `pkg2versions` and core16–core17, core24–core28.

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Detailed Questions

1. Java release cadence since Java 9?

• Short: 6-month releases; LTS every 2 years (11, 17, 21).
• Detailed: Feature releases (9,10,12…) get updates for ~6 months; LTS gets long-term support. Production typically targets LTS. Preview/incubator features need --enable-preview.
• Example: This project targets Java 21 LTS.

2. Java 9 — modules (JPMS)?

• Short: module-info.java defines exports/requires; strong encapsulation.
• Detailed: Modules control visibility at compile and runtime. requires transitive passes dependency to consumers. opens for reflection (Hibernate). Classpath JARs become unnamed module.
• Example: `pkg15modules`.

3. Java 9 — collection factories?

• Short: List.of, Set.of, Map.of — immutable, null-hostile.
• Detailed: Compact, thread-safe, no setters. Map.of max 10 pairs; use Map.ofEntries beyond. Duplicate keys throw IllegalArgumentException.
• Example: List.of(1,2,3) vs Arrays.asList (mutable size).

4. Java 9 — Stream takeWhile/dropWhile?

• Short: Short-circuit on sorted/predicate prefix; not same as filter.
• Detailed: takeWhile stops at first false; on infinite stream behaves like limit while true. dropWhile skips while true, then takes rest.
• Example: Stream.of(1,2,3,4,1).takeWhile(x->x<4) → [1,2,3].

5. Java 10 — `var`?

• Short: Local variable type inference; still statically typed.
• Detailed: Compiler infers type from initializer. Not for fields, method params, or without initializer. Use when type obvious (var list = new ArrayList<String>()). Avoid obscuring important types.
• Example: var map = Map.of("a", 1);

6. Java 11 — HttpClient?

• Short: Modern async/sync HTTP in java.net.http; replaces Apache for many cases.
• Detailed: HTTP/2, WebSocket, CompletableFuture async API. Immutable request/response objects.
• Example: `pkg10networking/networking5HttpClient.java`.

7. Java 14 — switch expressions?

• Short: Switch as expression with -> and yield; exhaustiveness for enums.
• Detailed: No fall-through with arrows. Blocks use yield value. Compiler checks enum/sealed coverage when exhaustive.
• Example: String r = switch (d) { case MON -> "weekday"; default -> "other"; };

8. Java 15 — text blocks?

• Short: Multi-line string literals with """; auto-indent strip.
• Detailed: Escape sequences still work; concatenation with + allowed. .formatted() for interpolation-style formatting.
• Example: JSON/SQL in core9StringsDemo.

9. Java 16 — records?

• Short: Immutable data carriers: canonical ctor, equals/hashCode/toString, accessors.
• Detailed: Compiler generates boilerplate. Can implement interfaces, define compact ctor validation. Not JPA entities without care (no no-arg ctor by default).
• Example: record Point(int x, int y) {}

10. Java 16 — pattern matching for instanceof?

• Short: if (o instanceof String s) binds variable in scope.
• Detailed: Eliminates cast after instanceof. Works with null (false, no binding).
• Example: if (obj instanceof Integer n) sum += n;

11. Java 17 — sealed classes?

• Short: Restrict which classes can extend/implement; enables exhaustive switches.
• Detailed: sealed class X permits A, B. Subclasses must be final, sealed, or non-sealed. Works with pattern matching switch.
• Example: core17SealedClassesDemo.

12. Java 21 — virtual threads?

• Short: Lightweight threads for massive blocking I/O concurrency.
• Detailed: JVM-scheduled; cheap to create millions. Don't pool them like platform threads. Pinning issue: synchronized block may pin carrier thread.
• Example: Thread.startVirtualThread(() -> ...) or Executors.newVirtualThreadPerTaskExecutor().

13. Java 21 — record patterns?

• Short: Deconstruct records in switch/instanceof patterns.
• Detailed: case Point(int x, int y) extracts components. Guards with when. Null case explicit in switch.
• Example: versions6Java21Features.describe().

14. Java 21 — sequenced collections?

• Short: SequencedCollection, SequencedSet, SequencedMap — uniform first/last/reversed.
• Detailed: getFirst(), getLast(), reversed(). Implemented by LinkedHashMap, ArrayList, etc.
• Example: list.getFirst() instead of list.get(0) with clearer intent.

15. Preview features — how to use?

• Short: --enable-preview on compile and run; API may change.
• Detailed: String templates, structured concurrency were previews. Don't use preview in production without acceptance of churn.
• Example: java --enable-preview MyApp.java

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Rapid-Fire (Q → A)

1. Java 8 headline? → Lambdas, streams, java.time.
2. Java 9 headline? → Modules, JShell, factory methods.
3. Java 10 headline? → var.
4. Java 11 LTS headline? → HttpClient, String methods, run single-file.
5. Java 14 headline? → Records preview, switch expr, helpful NPEs.
6. Java 15 headline? → Text blocks, sealed classes preview.
7. Java 16 headline? → Records, instanceof patterns.
8. Java 17 LTS headline? → Sealed classes, pattern switch preview.
9. Java 21 LTS headline? → Virtual threads, record patterns, sequenced collections.
10. LTS releases? → 11, 17, 21 (also 8 before cadence change).
11. module-info exports? → Public API of module.
12. requires transitive? → Implicit dependency for consumers.
13. opens package? → Deep reflection for framework.
14. unnamed module? → Classpath JARs on module path.
15. List.of mutable? → No.
16. List.of null element? → NPE.
17. Map.of max pairs? → 10.
18. copyOf collections? → Immutable copy.
19. var for field? → Not allowed.
20. var without init? → Not allowed.
21. HttpClient in which module? → java.net.http.
22. run-java source? → java File.java (11+).
23. switch arrow no fall-through? → Correct.
24. switch yield? → Return from block case.
25. record can extend class? → No (implicit extends Record).
26. record can implement interface? → Yes.
27. record accessor name? → name() not getName() unless override.
28. compact record ctor? → Validates before field assign.
29. sealed permits required? → Yes on sealed type.
30. non-sealed subclass? → Open to further extension.
31. pattern switch exhaustiveness? → Compiler checks sealed/enums.
32. virtual thread carrier? → Platform thread pool underneath.
33. pin virtual thread? → synchronized/native on carrier.
34. use platform threads when? → CPU-bound, many cores, short tasks.
35. structured concurrency preview? → Scope for child task lifetime.
36. String templates preview? → STR."Hello {name}".
37. foreign function API? → Panama; call native code.
38. vector API incubating? → SIMD operations.
39. ZGC generational? → Java 21 improvement.
40. Shenandoah? → Low-pause GC alternative.
41. deprecate finalize? → Yes; use Cleaner.
42. strong encapsulation JDK internals? → Illegal access warnings/errors.
43. jlink? → Custom runtime image.
44. reactive streams in JDK? → Flow API (Java 9).
45. Optional.orElseThrow? → Java 10 (was get()).
46. Collectors.toUnmodifiableList? → Java 10.
47. teeing collector? → Combine two collectors.
48. takeWhile on unordered stream? → Still short-circuits first false.
49. dropWhile difference from skip? → Predicate-based prefix skip.
50. Predicate.not? → Java 11 negation.
51. Files.readString? → Java 11.
52. isIdentical for strings? → Reference equality helper.
53. record serialization? → Serial form defined; consider stability.
54. record reflection? → Record components API.
55. hidden classes? → Framework-generated (lambda impls).
56. nestmates? → Inner/outer access control.
57. compact source? → Smaller source file feature (preview).
58. multi-file source? → Single class per file loosened (preview).
59. deprecate SecurityManager? → Removed/disabled modern JDKs.
60. javax to jakarta? → EE namespace change (not JDK but ecosystem).
61. microprofile? → Small EE specs on Jakarta.
62. enable-preview compile? → javac --enable-preview --release 21.
63. migration 8→11 pain? → JAXB removed, illegal reflective access.
64. migration 11→17? → Fewer breaking; sealed/records adoption.
65. migration 17→21? → Virtual threads opt-in.
66. jpackage? → Native installer (14+).
67. instanceof pattern null? → false, no binding.
68. switch null case? → Java 21 explicit case null.
69. sequenced map reversed? → Reverse-order view.
70. LinkedHashMap getFirst? → Java 21 SequencedMap.
71. ArrayList reversed view? → SequencedCollection.
72. ListIterator vs reversed? → reversed() is view.
73. record generic? → record Box<T>(T value) {}.
74. local record? → Allowed in methods.
75. anonymous to private instance? → Less boilerplate with records.
76. pattern for guarded case? → case Integer i when i > 0.
77. exhaustiveness default needed? → For non-sealed/non-enum.
78. java.time in which version? → Java 8.
79. var with lambda? → Must infer functional interface type.
80. var with diamond? → var list = new ArrayList<>() infers raw-ish; specify generic.