Top 20 HashMap Interview Questions and Answers
Top 20 HashMap Interview Questions and Answers Work through these 20 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.
volatile vs synchronized — when to use each?
volatile for single-writer flags (shutdown, initialized) where no read-modify-write. synchronized or java.util.concurrent locks when multiple threads update shared state. For...
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...
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...
Explain method overloading vs overriding.
Overload print(int) and print(String) in one class — compiler picks at compile time. Override toString in subclass — JVM calls subclass version via virtual method table....
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...
What is an ExecutorService and why use it?
Instead of new Thread per task, submit Runnable to fixed pool sized to CPU/IO profile. Handle Future results and exceptions. On shutdown hook call shutdown() and...
What are virtual threads in Java 21 and when should you use them?
Virtual threads excel when code blocks on IO — HTTP clients, JDBC, file reads — allowing one platform thread to serve thousands of concurrent requests. For CPU-heavy parallel...
Explain sealed classes and their purpose in Java 17+.
sealed interface Shape permits Circle, Rectangle limits who implements Shape. Compiler verifies switch (Shape s) covers all cases — no default needed. Use for domain variants...
What is pattern matching for switch (Java 21)?
Instead of if (obj instanceof String s) { ... }, use switch (obj) { case String s when s.length() > 5 -> ...; case String s -> ...; case null -> ...; default -> ... }. Works...
Why is ThreadLocal problematic with virtual threads?
Libraries storing per-request state in ThreadLocal allocate one entry per virtual thread — catastrophic memory with 100k concurrent requests. Prefer ScopedValue for request...
How do CompletableFuture and virtual threads compare for concurrency?
Use virtual threads when business logic is naturally blocking — call DB, call HTTP, return. Use CompletableFuture when composing multiple independent async results with...
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...
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 immutability and why String is immutable.
Immutable objects cannot change after build — thread-safe without synchronization. String immutability allows intern pool, caching hashCode, and security when used as...
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...
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 do Java Records provide and their limitations?
DTO: record UserResponse(String id, String name) {} — JSON mapping works. Service layer immutable value objects. Not for entities needing setters/lazy proxies yet. Combine with...
What are sequenced collections in JDK 21, and which calls replace fragile index access?
I reach for getFirst/getLast on lists and deques instead of magic indexes. For a recent-activity map I use LinkedHashMap as a SequencedMap and pollFirstEntry to evict oldest....
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...
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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