☕ 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.

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

Unlock 15 more Java questions · all 1,200 questions for $39/year (about 11¢ a day)See full access #15synchronized vs volatileConcurrencyHard🔒#27What is a deadlock, and how do you prevent it?ConcurrencyHard🔒#34How class loading worksJVM & memoryHard🔒#37wait/notify and the producer-consumer problemConcurrencyHard🔒#48Choosing a garbage collectorJVM & memoryHard🔒#51Wildcards, bounded types and PECSGenericsHard🔒#56When are parallel streams a good idea?Streams & lambdasHard🔒#61The Java Memory Model and happens-beforeConcurrencyHard🔒#67Strong, soft, weak and phantom referencesJVM & memoryHard🔒#73@Transactional and its pitfallsSpring & ecosystemHard🔒#74The N+1 query problem in JPA / HibernateSpring & ecosystemHard🔒#81The Fork/Join framework and work stealingConcurrencyHard🔒#92Diagnosing high CPU or a hung Java serviceJVM & memoryHard🔒#94equals() with inheritance (getClass vs instanceof)Core Java & OOPHard🔒#100Why naive Java benchmarks lie (JMH)JVM & memoryHard🔒