Java Full Stack with React Last-Minute Revision

30 quick revision notes for Java Full Stack with React

1. String is immutable, so use StringBuilder when building strings inside loops to avoid creating many temporary objects.
2. Always override equals and hashCode together, using the same fields, or hash-based collections will fail to find objects.
3. HashMap allows one null key, is not thread-safe, and converts large buckets to red-black trees since Java 8.
4. ConcurrentHashMap gives thread-safe reads and writes without locking the whole map and does not allow null keys or values.
5. Checked exceptions must be handled or declared, while unchecked exceptions extend RuntimeException and signal programming errors.
6. G1 is the default garbage collector since Java 9, and ZGC offers very low pause times for large heaps.
7. Java 21 is a long-term support release with virtual threads, pattern matching for switch and record patterns.
8. Stream intermediate operations are lazy and nothing runs until a terminal operation like collect or count is called.
9. Spring Boot 3 requires Java 17 or later and uses jakarta packages instead of javax packages.
10. Constructor injection is preferred because dependencies become explicit, fields can be final and tests need no Spring context.
11. Singleton is the default bean scope, so Spring beans must be stateless or thread-safe.
12. @Transactional works through a proxy, so self-invocation and private methods bypass it.
13. By default Spring rolls back transactions only for unchecked exceptions and errors, not checked exceptions.
14. @OneToMany and @ManyToMany are lazy by default, while @ManyToOne and @OneToOne are eager by default.
15. Fix the N+1 query problem with JOIN FETCH, @EntityGraph, batch fetching or DTO projections.
16. Use ddl-auto validate or none in production and manage schema changes with Flyway or Liquibase.
17. Optimistic locking uses a @Version column and throws an exception when another transaction updated the row first.
18. GET, PUT and DELETE are idempotent, while POST is not, so payment APIs use idempotency keys for safe retries.
19. 401 means the user is not authenticated, while 403 means the user is authenticated but not allowed.
20. JWTs are signed, not encrypted, so never put passwords or sensitive personal data in the payload.
21. Store passwords with BCrypt or Argon2, which add a salt and are deliberately slow to resist brute force.
22. CORS is enforced by the browser, and the server must allow the frontend origin in its response headers.
23. WHERE filters individual rows before grouping, while HAVING filters groups after aggregation such as COUNT or SUM.
24. A composite index follows the leftmost prefix rule, so column order should match the most common filters.
25. Use parameterised queries to prevent SQL injection and never concatenate user input into SQL strings.
26. Keyset pagination stays fast on deep pages, while large OFFSET values force the database to skip many rows.
27. React state updates are asynchronous snapshots, so use the updater form when the next state depends on the previous one.
28. Use stable unique IDs as React list keys, not array indexes, when items can be reordered or removed.
29. Promise microtasks run before setTimeout callbacks once the call stack is empty.
30. Create React App is deprecated, so new single-page apps commonly use Vite, and SEO-heavy apps use a framework like Next.js.

Night-before checklist

• Revise your saved questions on this page once, aloud.
• Read the quick notes and mark any you cannot explain in one line.
• Rehearse a two-minute self-introduction and two project stories.
• Keep CV, ID, laptop charger and interview link ready; sleep on time.