25 Java Interview Questions for 2026: From JVM to Spring Boot

Java Interview Questions 2026: Core Java to Spring Boot

Java remains the enterprise standard with 35% of backend services running Java. Interviews cover: core Java (OOP, collections, concurrency), JVM internals (garbage collection, memory model), and frameworks (Spring Boot, Hibernate). Java 21's virtual threads are now interview-relevant.

The three Java topics that appear in every senior interview: 1) Concurrency (synchronized, locks, CompletableFuture, virtual threads), 2) Memory management (GC algorithms, heap analysis, memory leaks), 3) Design patterns (Singleton, Factory, Observer, Strategy).

Core Topics

• Java 21 Features — Virtual threads, pattern matching, sealed classes, records
• Collections — HashMap internals, ConcurrentHashMap, TreeMap vs HashMap
• Concurrency — Thread lifecycle, ExecutorService, virtual threads, deadlock avoidance
• Spring Boot — Dependency injection, auto-configuration, REST controllers, testing
• JVM — Class loading, garbage collection (G1, ZGC), JIT compilation

See backend guide, OOD interview guide. Practice: AissenceAI.