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☕ Interview Prep

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.

Inheritance vs CompositionHash Maps & SetsMemory: The Stack, the Heap & ReferencesProcesses, Threads & Doing Things at OnceUnit Testing & Test-Driven Development

100 questions, most asked first

Ranked by how often each question comes up in real interviews. #1 is the most common.

Any difficulty Easy Medium Hard

All patterns Core Java & OOP Collections Exceptions JVM & memory Concurrency Streams & lambdas Generics Modern Java Testing Spring & ecosystem

#1The equals() and hashCode() contractCore Java & OOPMediumFree #2How does HashMap work internally?CollectionsMediumFree #7Checked vs unchecked exceptionsExceptionsMediumFree #9Stack vs heap, and how garbage collection worksJVM & memoryMediumFree Unlock 51 more Java questions · all 1,200 questions for $39/year (about 11¢ a day)See full access #14HashMap vs Hashtable vs ConcurrentHashMapConcurrencyMedium🔒#16Threads, Runnable and ExecutorServiceConcurrencyMedium🔒#17Java Streams: map, filter, reduce, collectStreams & lambdasMedium🔒#18Lambdas and functional interfacesStreams & lambdasMedium🔒#19Using Optional correctlyStreams & lambdasMedium🔒#20How to write an immutable classCore Java & OOPMedium🔒#22Generics and type erasureGenericsMedium🔒#25ConcurrentModificationException and fail-fast iteratorsCollectionsMedium🔒#28CompletableFuture and async compositionConcurrencyMedium🔒#30Writing a thread-safe singletonCore Java & OOPMedium🔒#31Autoboxing and the Integer cacheCore Java & OOPMedium🔒#33Memory leaks in a garbage-collected languageJVM & memoryMedium🔒#35AtomicInteger and compare-and-swapConcurrencyMedium🔒#38Default methods and the diamond problemCore Java & OOPMedium🔒#39Switch expressions and pattern matchingModern JavaMedium🔒#40Sealed classes and interfacesModern JavaMedium🔒#42Static nested, inner, local and anonymous classesCore Java & OOPMedium🔒#43Exception handling best practicesExceptionsMedium🔒#44PriorityQueue and heapsCollectionsMedium🔒#45Arrays.asList vs List.of vs unmodifiable listsCollectionsMedium🔒#46Virtual threads (Java 21)ConcurrencyMedium🔒#47ThreadLocal and its pitfallsConcurrencyMedium🔒#49Bytecode, the interpreter and the JITJVM & memoryMedium🔒#50groupingBy, flatMap and other collectorsStreams & lambdasMedium🔒#52Serialization and the transient keywordCore Java & OOPMedium🔒#53Reflection and annotationsCore Java & OOPMedium🔒#55Shallow vs deep copy, and clone()Core Java & OOPMedium🔒#57Intermediate vs terminal operations (streams are lazy)Streams & lambdasMedium🔒#58ReentrantLock and ReadWriteLock vs synchronizedConcurrencyMedium🔒#59CountDownLatch, CyclicBarrier and SemaphoreConcurrencyMedium🔒#60Unit testing with JUnit 5 and MockitoTestingMedium🔒#65What changed from Java 8 to Java 21?Modern JavaMedium🔒#66Making your own class work in a for-each loopCollectionsMedium🔒#68OutOfMemoryError vs StackOverflowErrorJVM & memoryMedium🔒#69Dependency injection and inversion of controlSpring & ecosystemMedium🔒#71How does Spring Boot auto-configuration work?Spring & ecosystemMedium🔒#72Spring bean scopes and lifecycleSpring & ecosystemMedium🔒#75Lazy vs eager loading and LazyInitializationExceptionSpring & ecosystemMedium🔒#76Building a clean REST endpoint in SpringSpring & ecosystemMedium🔒#78Mutable objects as HashMap keysCollectionsMedium🔒#79SOLID principles in JavaCore Java & OOPMedium🔒#82Sizing and shutting down thread poolsConcurrencyMedium🔒#83Check-then-act races and atomic map operationsConcurrencyMedium🔒#85Initialization order of static and instance membersCore Java & OOPMedium🔒#86Checked exceptions inside lambdas and streamsStreams & lambdasMedium🔒#88How does Java sort, and is it stable?CollectionsMedium🔒#90Stopping a thread and handling InterruptedExceptionConcurrencyMedium🔒#91Running the JVM in containersJVM & memoryMedium🔒#93The Java Platform Module SystemModern JavaMedium🔒#95Integration tests with Spring and TestcontainersTestingMedium🔒#96Testing code that depends on timeTestingMedium🔒