Java Interview Questions

Q: What are records and sealed classes?

Records are immutable data carriers with auto-generated methods. Sealed classes restrict which classes can extend them. record Point(int x, int y) auto-generates constructor, getters, equals, hashCode, toString. sealed class Shape permits Circle, Square restricts subclasses. Together they create algebraic data types for pattern matching.

Q: Java app has memory leak. How to find and fix?

Use heap dumps with JVisualVM or Eclipse MAT, identify retained objects, fix unclosed resources and growing collections. Steps: (1) Monitor heap with JVisualVM. (2) Take heap dump: jmap -dump. (3) Analyze with Eclipse MAT. (4) Check: static collections, unclosed streams (try-with-resources), event listeners, ThreadLocal. Use -XX:+HeapDumpOnOutOfMemoryError.

Q: Google: Design a thread-safe LRU cache.

Use ConcurrentHashMap with doubly-linked list, or LinkedHashMap with removeEldestEntry, or Caffeine library. Simple: synchronized LinkedHashMap with removeEldestEntry. Production: Caffeine library (near-optimal hit rate). Features: TTL, size-based eviction, compute-if-absent. Google Guava Cache inspired Caffeine.

Question 1

What are Java's key features?

Accepted Answer

Object-oriented, platform-independent (JVM), automatic GC, strong typing, multithreaded, rich standard library. Compiles to bytecode, runs on JVM. OOP: encapsulation, inheritance, polymorphism, abstraction. Modern Java (17+): records, sealed classes, pattern matching, text blocks, virtual threads.

Question 2

What is JDK vs JRE vs JVM?

Accepted Answer

JVM executes bytecode; JRE = JVM + libraries for running Java; JDK = JRE + development tools (compiler, debugger). JVM: abstract machine executing bytecode, platform-specific. JRE: JVM + core libraries (java.lang, java.util). JDK: JRE + javac compiler, debugger, docs. Since Java 11, JRE not distributed separately.

Question 3

Abstract classes vs interfaces?

Accepted Answer

Abstract classes have state and constructors; interfaces define contracts with default methods, support multiple inheritance. Abstract: instance variables, constructors, mixed methods, single inheritance. Interfaces (Java 8+): default methods, static methods, constants, multiple inheritance. Use abstract for sharing state; interface for contracts across unrelated classes.

Question 4

What are Java Collections?

Accepted Answer

Framework with List (ArrayList, LinkedList), Set (HashSet, TreeSet), Map (HashMap, TreeMap), Queue (PriorityQueue). List: ordered, duplicates OK. ArrayList (O(1) get), LinkedList (O(1) add/remove). Set: no duplicates. HashSet (O(1)), TreeSet (sorted). Map: key-value. HashMap (O(1)), TreeMap (sorted), ConcurrentHashMap (thread-safe). Choose based on access patterns.

Question 5

What is Spring Framework?

Accepted Answer

Comprehensive Java framework: dependency injection, Spring MVC, Spring Boot (auto-config), Spring Data, Spring Security. Core: IoC container, DI. Spring Boot: opinionated defaults, embedded server. Spring Data: database abstraction. Spring Security: auth. Spring Cloud: microservices. @Autowired for DI, @RestController for APIs. De facto standard for Java web apps.

Question 6

Explain Java multithreading.

Accepted Answer

Concurrent execution via Thread class, Runnable, ExecutorService. Synchronization, locks, and CompletableFuture for async. Creating: extend Thread or implement Runnable. ExecutorService manages pools. synchronized keyword, Lock interface, volatile. Java 21+: virtual threads (Project Loom) for million-scale concurrency. Issues: deadlocks, race conditions.

Question 7

How does Java GC work?

Accepted Answer

Generational collection (young/old gen) with collectors: G1 (default), ZGC (ultra-low latency), Shenandoah. Young gen: Eden + Survivor (fast minor GC). Old gen: Tenured (slower major GC). G1: balanced. ZGC: <1ms pauses. Tuning: -Xms/-Xmx heap size. Monitor: JVisualVM, GC logs. Leaks: unclosed resources, static collections, listener registration.

Question 8

What are records and sealed classes?

Accepted Answer

Records are immutable data carriers with auto-generated methods. Sealed classes restrict which classes can extend them. record Point(int x, int y) auto-generates constructor, getters, equals, hashCode, toString. sealed class Shape permits Circle, Square restricts subclasses. Together they create algebraic data types for pattern matching.

Question 9

Java app has memory leak. How to find and fix?

Accepted Answer

Use heap dumps with JVisualVM or Eclipse MAT, identify retained objects, fix unclosed resources and growing collections. Steps: (1) Monitor heap with JVisualVM. (2) Take heap dump: jmap -dump. (3) Analyze with Eclipse MAT. (4) Check: static collections, unclosed streams (try-with-resources), event listeners, ThreadLocal. Use -XX:+HeapDumpOnOutOfMemoryError.

Question 10

Google: Design a thread-safe LRU cache.

Accepted Answer

Use ConcurrentHashMap with doubly-linked list, or LinkedHashMap with removeEldestEntry, or Caffeine library. Simple: synchronized LinkedHashMap with removeEldestEntry. Production: Caffeine library (near-optimal hit rate). Features: TTL, size-based eviction, compute-if-absent. Google Guava Cache inspired Caffeine.

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Variables, Data Types & Operators

Control Flow & Loops

Object-Oriented Programming

Classes, Inheritance & Polymorphism

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Exception Handling

File I/O & Streams

Multithreading & Concurrency

Java Project: Banking Application

Spring Framework & Modern Java

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