Top 10 Java Interview Questions and Answers for 2025

Preparing for a Java interview in 2025? The Java ecosystem continues to evolve, and companies seek developers with strong problem-solving skills and up-to-date knowledge. Here are the top 10 Java interview questions with answers to help you ace your next interview.

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1. What are the main features introduced in Java 17?

Answer: Java 17 (LTS) introduced several key features, including:

• Sealed Classes

• Pattern Matching for switch

• Strongly Encapsulated JDK Internals

• New macOS Rendering Pipeline

• Deprecation of Security Manager

Example:

sealed class Vehicle permits Car, Bike {}
final class Car extends Vehicle {}
final class Bike extends Vehicle {}

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2. What is the difference between var, final var, and record in Java?

Answer:

• var is used for local variable type inference.

• final var ensures that the variable cannot be reassigned.

• record is a new type for immutable data classes.

Example:

record Person(String name, int age) {}
Person p = new Person("Alice", 30);
System.out.println(p.name()); // Alice

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3. How does Java handle memory management?

Answer: Java uses Automatic Garbage Collection (GC) with different collectors like:

• G1 GC (Default since Java 9)

• ZGC (Low-latency GC introduced in Java 11)

• Shenandoah GC (Highly concurrent GC)

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4. Explain the difference between ExecutorService and ForkJoinPool.

Answer:

• ExecutorService is for managing a fixed pool of threads.

• ForkJoinPool is for parallel processing and divide-and-conquer algorithms.

Example:

ForkJoinPool pool = new ForkJoinPool();

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5. What are Virtual Threads in Java?

Answer: Virtual Threads (Project Loom) provide lightweight, scalable concurrency.

Example:

Thread.startVirtualThread(() -> System.out.println("Hello Virtual Threads!"));

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6. What is the difference between CompletableFuture and Reactive Streams?

Answer:

• CompletableFuture handles asynchronous programming.

• Reactive Streams (Project Reactor) provides backpressure support.

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7. How to implement a Kafka consumer in Java?

Answer:

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
Consumer<String, String> consumer = new KafkaConsumer<>(props);
consumer.subscribe(Collections.singletonList("my-topic"));

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8. What is the difference between HashMap and ConcurrentHashMap?

Answer:

• HashMap is not thread-safe.

• ConcurrentHashMap is optimized for concurrent read/write operations.

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9. How to optimize performance in Spring Boot applications?

Answer:

• Use @Transactional to manage DB transactions efficiently.

• Enable caching with @Cacheable.

• Use Connection Pooling (HikariCP).

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10. What are the best practices for writing clean Java code?

Answer:

• Follow SOLID principles.

• Use meaningful variable names.

• Avoid redundant code (DRY principle).

• Use logging frameworks (Log4j, SLF4J).

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Happy learning....