Java Developer interview questions and answers.

Questions

Technical questions to practise out loud

1. 01

   How does Java HashMap work internally?

   HashMap hashes a key into a bucket, resolves collisions with a linked list or tree, and resizes when its load threshold is exceeded; correct equals and hashCode implementations are mandatory.

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2. 02

   How does Spring @Transactional work?

   @Transactional is normally applied by a Spring proxy that opens, commits, or rolls back a transaction around an intercepted public call; self-invocation bypasses the proxy and rollback defaults matter.

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3. 03

   How do you make Java code thread-safe?

   Thread safety requires protecting shared invariants through immutability, confinement, atomic classes, locks, or concurrent collections; visibility and compound operations matter, not only individual reads and writes.

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4. 04

   How does Java garbage collection affect an application?

   Java GC reclaims objects no longer reachable from roots; allocation rate, heap sizing, object lifetime and collector choice affect throughput and pause time, so tune from GC logs and latency goals.

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5. 05

   When should you use Java streams?

   Streams express transformations over data with lazy intermediate operations and a terminal operation; use them for clear stateless pipelines, not side-effect-heavy logic or automatic parallelism.

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6. 06

   How do you secure a Spring application?

   Secure Spring applications with explicit authentication, least-privilege authorization at request and method boundaries, CSRF protection where cookies authenticate, safe password hashing, validation, and audited security events.

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7. 07

   How does Spring Boot auto-configuration work?

   Spring Boot auto-configuration conditionally registers beans based on the classpath, properties, and existing beans; user-defined beans normally override defaults and the conditions report explains decisions.

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8. 08

   How do you avoid JPA N+1 queries?

   N+1 occurs when one query loads parents and subsequent lazy access issues one query per parent; solve it with a fetch join, entity graph, batch fetching, or a projection chosen for the use case.

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9. 09

   How do you diagnose Java performance problems?

   Diagnose Java performance from evidence: correlate service metrics and traces, inspect GC and thread states, capture JFR or async-profiler data, fix the measured bottleneck, and rerun the same workload.

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10. 10

    How do you design exception handling in Java?

    Use exceptions for exceptional failures, preserve causes, expose domain-specific errors at boundaries, avoid catch-all swallowing, and translate once into an API or retry decision where enough context exists.

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