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Java
equals and hashCode, collections internals, strings, exceptions, generics, streams, concurrency, the JVM and modern Java: what Java and Spring teams test.
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100 questions, most asked first
Ranked by how often each question comes up in real interviews. #1 is the most common.
All patternsCore Java & OOPCollectionsExceptionsJVM & memoryConcurrencyStreams & lambdasGenericsModern JavaTestingSpring & ecosystem
#1💬The equals() and hashCode() contractCore Java & OOPMediumFree#2💬How does HashMap work internally?CollectionsMediumFree#3💬String immutability, the string pool, and == vs equalsCore Java & OOPEasyFree#4💬Abstract class vs interfaceCore Java & OOPEasyFree#5💬ArrayList vs LinkedListCollectionsEasyFree#6💬final vs finally vs finalizeCore Java & OOPEasyFree#7💬Checked vs unchecked exceptionsExceptionsMediumFree#8💬JDK vs JRE vs JVMJVM & memoryEasyFree#9💬Stack vs heap, and how garbage collection worksJVM & memoryMediumFree#10💬Overloading vs overridingCore Java & OOPEasyFree🔓Unlock 90 more Java questions · all 1,200 questions for $39/year (about 11¢ a day)See full access#11💬String vs StringBuilder vs StringBufferCore Java & OOPEasy🔒#12💬What does static mean?Core Java & OOPEasy🔒#13💬Comparable vs ComparatorCollectionsEasy🔒#14💬HashMap vs Hashtable vs ConcurrentHashMapConcurrencyMedium🔒#15💬synchronized vs volatileConcurrencyHard🔒#16💬Threads, Runnable and ExecutorServiceConcurrencyMedium🔒#17💬Java Streams: map, filter, reduce, collectStreams & lambdasMedium🔒#18💬Lambdas and functional interfacesStreams & lambdasMedium🔒#19💬Using Optional correctlyStreams & lambdasMedium🔒#20💬How to write an immutable classCore Java & OOPMedium🔒#21💬Is Java pass-by-value or pass-by-reference?Core Java & OOPEasy🔒#22💬Generics and type erasureGenericsMedium🔒#23💬try-with-resources and AutoCloseableExceptionsEasy🔒#24💬HashSet vs LinkedHashSet vs TreeSetCollectionsEasy🔒#25💬ConcurrentModificationException and fail-fast iteratorsCollectionsMedium🔒#26💬public, protected, package-private and privateCore Java & OOPEasy🔒#27💬What is a deadlock, and how do you prevent it?ConcurrencyHard🔒#28💬CompletableFuture and async compositionConcurrencyMedium🔒#29💬What are records, and when should you use them?Modern JavaEasy🔒#30💬Writing a thread-safe singletonCore Java & OOPMedium🔒#31💬Autoboxing and the Integer cacheCore Java & OOPMedium🔒#32💬HashMap vs LinkedHashMap vs TreeMapCollectionsEasy🔒#33💬Memory leaks in a garbage-collected languageJVM & memoryMedium🔒#34💬How class loading worksJVM & memoryHard🔒#35💬AtomicInteger and compare-and-swapConcurrencyMedium🔒#36💬Thread lifecycle and statesConcurrencyEasy🔒#37💬wait/notify and the producer-consumer problemConcurrencyHard🔒#38💬Default methods and the diamond problemCore Java & OOPMedium🔒#39💬Switch expressions and pattern matchingModern JavaMedium🔒#40💬Sealed classes and interfacesModern JavaMedium🔒#41💬Polymorphism and dynamic dispatchCore Java & OOPEasy🔒#42💬Static nested, inner, local and anonymous classesCore Java & OOPMedium🔒#43💬Exception handling best practicesExceptionsMedium🔒#44💬PriorityQueue and heapsCollectionsMedium🔒#45💬Arrays.asList vs List.of vs unmodifiable listsCollectionsMedium🔒#46💬Virtual threads (Java 21)ConcurrencyMedium🔒#47💬ThreadLocal and its pitfallsConcurrencyMedium🔒#48💬Choosing a garbage collectorJVM & memoryHard🔒#49💬Bytecode, the interpreter and the JITJVM & memoryMedium🔒#50💬groupingBy, flatMap and other collectorsStreams & lambdasMedium🔒#51💬Wildcards, bounded types and PECSGenericsHard🔒#52💬Serialization and the transient keywordCore Java & OOPMedium🔒#53💬Reflection and annotationsCore Java & OOPMedium🔒#54💬Enums are full classesCore Java & OOPEasy🔒#55💬Shallow vs deep copy, and clone()Core Java & OOPMedium🔒#56💬When are parallel streams a good idea?Streams & lambdasHard🔒#57💬Intermediate vs terminal operations (streams are lazy)Streams & lambdasMedium🔒#58💬ReentrantLock and ReadWriteLock vs synchronizedConcurrencyMedium🔒#59💬CountDownLatch, CyclicBarrier and SemaphoreConcurrencyMedium🔒#60💬Unit testing with JUnit 5 and MockitoTestingMedium🔒#61💬The Java Memory Model and happens-beforeConcurrencyHard🔒#62💬Integer overflow and numeric typesCore Java & OOPEasy🔒#63💬Why not use double for money?Core Java & OOPEasy🔒#64💬Local variable type inference with varModern JavaEasy🔒#65💬What changed from Java 8 to Java 21?Modern JavaMedium🔒#66💬Making your own class work in a for-each loopCollectionsMedium🔒#67💬Strong, soft, weak and phantom referencesJVM & memoryHard🔒#68💬OutOfMemoryError vs StackOverflowErrorJVM & memoryMedium🔒#69💬Dependency injection and inversion of controlSpring & ecosystemMedium🔒#70💬The Builder patternCore Java & OOPEasy🔒#71💬How does Spring Boot auto-configuration work?Spring & ecosystemMedium🔒#72💬Spring bean scopes and lifecycleSpring & ecosystemMedium🔒#73💬@Transactional and its pitfallsSpring & ecosystemHard🔒#74💬The N+1 query problem in JPA / HibernateSpring & ecosystemHard🔒#75💬Lazy vs eager loading and LazyInitializationExceptionSpring & ecosystemMedium🔒#76💬Building a clean REST endpoint in SpringSpring & ecosystemMedium🔒#77💬Maven vs Gradle and dependency managementSpring & ecosystemEasy🔒#78💬Mutable objects as HashMap keysCollectionsMedium🔒#79💬SOLID principles in JavaCore Java & OOPMedium🔒#80💬Configuration and profiles in Spring BootSpring & ecosystemEasy🔒#81💬The Fork/Join framework and work stealingConcurrencyHard🔒#82💬Sizing and shutting down thread poolsConcurrencyMedium🔒#83💬Check-then-act races and atomic map operationsConcurrencyMedium🔒#84💬String.split surprisesCore Java & OOPEasy🔒#85💬Initialization order of static and instance membersCore Java & OOPMedium🔒#86💬Checked exceptions inside lambdas and streamsStreams & lambdasMedium🔒#87💬Method referencesStreams & lambdasEasy🔒#88💬How does Java sort, and is it stable?CollectionsMedium🔒#89💬Why use ArrayDeque instead of Stack?CollectionsEasy🔒#90💬Stopping a thread and handling InterruptedExceptionConcurrencyMedium🔒#91💬Running the JVM in containersJVM & memoryMedium🔒#92💬Diagnosing high CPU or a hung Java serviceJVM & memoryHard🔒#93💬The Java Platform Module SystemModern JavaMedium🔒#94💬equals() with inheritance (getClass vs instanceof)Core Java & OOPHard🔒#95💬Integration tests with Spring and TestcontainersTestingMedium🔒#96💬Testing code that depends on timeTestingMedium🔒#97💬Logging well in Java servicesSpring & ecosystemEasy🔒#98💬Handling null and NullPointerExceptionCore Java & OOPEasy🔒#99💬EnumMap and EnumSetCollectionsEasy🔒#100💬Why naive Java benchmarks lie (JMH)JVM & memoryHard🔒