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Java Collections

List/Set/Map internals, HashMap, ConcurrentHashMap, performance.

23 questions
List vs Set vs MapHashMap InternalsConcurrentHashMapIteratorsTreeMapPerformance

Sample Questions (23 live)

List vs Set vs MapEasy

How do you choose between List, Set and Map for a new feature?

InfosysTCSCognizantRead →
ListEasy

ArrayList vs LinkedList — which would you actually pick in production, and why?

AmazonInfosysTCSRead →
SetEasy

HashSet vs LinkedHashSet vs TreeSet — when do you use each?

InfosysCognizantCapgeminiRead →
IteratorsEasy

Iterator vs ListIterator — how do you safely remove elements while iterating?

TCSInfosysAccentureRead →
HashMap InternalsMedium

Walk me through what happens inside HashMap.put() in Java 8.

AmazonMicrosoftDeloitteRead →
PerformanceMedium

How do load factor and resizing affect HashMap performance, and how do you tune it?

AmazonGoogleWiproRead →
IteratorsMedium

What causes ConcurrentModificationException, and what are the correct fixes?

InfosysCognizantAccentureRead →
HashMap InternalsMedium

How do you build an LRU cache using LinkedHashMap?

AmazonMicrosoftDeloitteRead →
TreeMapMedium

When do you reach for TreeMap/NavigableMap instead of HashMap?

AmazonGoogleDeloitteRead →
PerformanceMedium

List.of vs Collections.unmodifiableList vs Arrays.asList — what's the difference?

AmazonMicrosoftCognizantRead →
ListMedium

What are the pitfalls of Arrays.asList(), and how do they bite in production?

TCSInfosysAccentureRead →
PerformanceMedium

How do you find the top-K elements from a large stream efficiently?

AmazonGoogleMicrosoftRead →
ConcurrentHashMapMedium

When is CopyOnWriteArrayList the right choice, and when is it a trap?

AmazonDeloitteMicrosoftRead →
HashMap InternalsMedium

What happens if you mutate an object after using it as a HashMap key?

AmazonDeloitteCognizantRead →
ConcurrentHashMapHard

How does ConcurrentHashMap stay thread-safe without locking the whole map (Java 8)?

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ConcurrentHashMapHard

Why can a plain HashMap corrupt data — or even spin the CPU — under concurrent access?

AmazonMicrosoftDeloitteRead →
IteratorsHard

Fail-fast vs fail-safe iterators — how do they work internally?

AmazonGoogleMicrosoftRead →
PerformanceHard

What's the best collection for high-contention counters — ConcurrentHashMap, AtomicLong, or LongAdder?

AmazonGoogleMicrosoftRead →
PerformanceHard

How do you reduce the memory overhead of very large Java collections?

AmazonGoogleDeloitteRead →
ConcurrentHashMapHard

How do you implement a bounded producer-consumer pipeline with BlockingQueue?

AmazonMicrosoftGoogleRead →
Sequenced CollectionsMedium

What do SequencedCollection, SequencedSet and SequencedMap (Java 21) fix that List and LinkedHashMap couldn't?

AmazonMicrosoftRead →
ConcurrentHashMapMedium

ConcurrentHashMap.size() vs mappingCount() — why does the Javadoc specifically call one of them 'an estimate'?

AmazonGoogleOracleRead →
ConcurrentHashMapMedium

ConcurrentHashMap is thread-safe — so why can a get-then-put sequence still race, and how do computeIfAbsent/merge fix it?

AmazonMicrosoftGoogleRead →

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