Most Asked Java 8 Interview Questions in 2026

Most Asked Java 8 Interview Questions in 2026 Work through these 12 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.

  1. What is Metaspace and how do class loading issues cause memory leaks?

    Production set MaxMetaspaceSize 256m with monitoring. Investigate Metaspace OOM: heap dump + classloader histogram. Avoid generating unlimited dynamic proxies. OSGi/script...

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  2. HashMap vs ConcurrentHashMap — when and why?

    HashMap for single-threaded or external synchronization. ConcurrentHashMap for shared concurrent maps — finer-grained locking than Hashtable. Cannot store null keys/values in...

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  3. What is the difference between Comparable and Comparator?

    Comparable built into the type — Employee implements compareTo by id. Comparator external — sort by name or salary without changing Employee. Use...

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  4. What is Optional and when should you use it?

    Use Optional as return type when absence is expected — findById returns Optional<User>. Chain with map/flatMap/orElseThrow. Do not use for fields, method parameters, or Jackson...

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  5. How does Stream API parallel stream work?

    parallelStream() splits work across common pool threads. Good for CPU-heavy map on large in-memory collections. Bad for small lists, IO inside stream, or ordered stateful...

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  6. Compare BlockingQueue implementations for producer-consumer workloads.

    For order processing, use ArrayBlockingQueue with capacity matching SLA — slow consumer blocks producer instead of OOM. ThreadPoolExecutor with SynchronousQueue +...

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  7. What is ForkJoinPool common pool starvation and how do you avoid it?

    Never block on common pool — no DB/HTTP in parallelStream without custom pool. Use Executors.newFixedThreadPool for blocking batch jobs. CompletableFuture.supplyAsync(task,...

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  8. How does spring.config.import work for externalized configuration?

    In K8s: spring.config.import=optional:configtree:/etc/secrets/,optional:configtree:/etc/config/ mounts volumes as properties. Vault:...

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  9. How do Spring Boot Actuator health probes work with Kubernetes?

    Enable probes, configure K8s readinessProbe on /actuator/health/readiness, liveness on /actuator/health/liveness. Custom HealthIndicator for critical deps on readiness group...

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  10. How do JPA Specifications (Criteria API) help dynamic queries?

    UserSearchSpec.withStatus(status).and(withNameLike(name)) — repository.findAll(spec, pageable). Criteria API verbose — wrap in static factory methods. Alternative: Querydsl or...

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  11. SecurityFilterChain vs deprecated WebSecurityConfigurerAdapter?

    @Bean SecurityFilterChain filterChain(HttpSecurity http) throws Exception { http.csrf(csrf -> csrf.disable()).authorizeHttpRequests(auth ->...

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  12. When and why use CQRS (Command Query Responsibility Segregation)?

    E-commerce: write Order aggregate to PostgreSQL; project OrderSummaryView to Elasticsearch for search/filter dashboard. Accept projection lag. Not for simple User CRUD. Combine...

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