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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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Visual, hands-on lessons for every pattern in this topic. Do the lesson, then come back and the questions click.
100 questions, most asked first
Ordered so the most useful questions for learning each pattern come first.
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🔒