Cracking the Java Collections Interview: Tips and Must-Know Questions
Cracking the Java Collections Interview: Tips and Must-Know Questions 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.
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...
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 +...
IdentityHashMap vs HashMap — when would you use IdentityHashMap?
Use when identity matters not value — tracking object visitation in deep graph walk. Business keys (userId, email) belong in HashMap with proper equals/hashCode....
What is the N+1 query problem and how do you fix it?
Loading 100 orders then accessing each order's items triggers 101 queries. Fix with JOIN FETCH in JPQL, @EntityGraph on repository method, or a DTO query selecting only...
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...
Explain garbage collection basics in the JVM.
New objects allocate in Eden; minor GC copies survivors to survivor spaces, then old gen. When old gen fills, major/full GC runs — longer pauses. Tune -Xmx, choose G1 for...
When should you use CopyOnWriteArrayList?
Use for subscriber lists in pub/sub where adds are rare and iteration is constant. Avoid for high-frequency add/remove — use ConcurrentLinkedQueue or synchronized collections....
What are weak references and WeakHashMap use cases?
Attach metadata to objects without extending their lifetime — canonical mapping while object lives. When key GCs, entry drops. Do not rely on timing. For API response caching...
Lazy vs eager fetching in JPA — trade-offs?
Default associations LAZY. Service method needing orders+items uses JOIN FETCH in repository query. Disable spring.jpa.open-in-view=false — forces explicit fetch in service...
Load shedding and backpressure in microservices?
Tomcat max queue + reject policy returns 503 with Retry-After. WebClient timeout + circuit breaker stops calling slow dependency. Kafka consumer pause partitions when...
Explain the difference between == and .equals() in Java.
For primitives, == compares values. For objects, == asks if two references point to the same instance. equals() compares content when overridden — String overrides it for...
What is the difference between ArrayList and LinkedList?
ArrayList: dynamic array, great for get(i) and iteration, amortized append. LinkedList: node chain, costly random access, only wins for frequent insert/remove in middle with...
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