Must Java Developer Interview Questions And Answers

Real Questions. Right Answers. Ready for Any Java Interview.

What you'll learn

Gain in-depth knowledge of core Java topics such as exception handling, multithreading, and collections, essential for acing Java developer interviews.

Develop a strong understanding of advanced concepts like Streams, Lambdas, JMM, and garbage collection to confidently address tough interview questions.

Build the skills to answer scenario-based questions on optimisation, & debugging, aligning with industry expectations for Java developers.

Gain more confidence before attending interviews by getting yourselves ready with the most commonly asked Java developer interview questions.

Requirements

Generally, you should be familiar with: Core Java concepts, including object-oriented programming principles like inheritance, encapsulation, polymorphism, and abstraction. Basic understanding of data structures and algorithms. Experience with Java development tools and environments. Familiarity with common Java libraries and frameworks.

Description

Are you preparing for your next Java developer interview and feeling a bit unsure about the kinds of questions you might face? Don’t worry — you’re not alone! Whether you're a fresher stepping into your very first interview or an experienced developer looking to take the next big leap in your career, this course is your ultimate companion to help you crack Java interviews with clarity and confidence.In "Must Java Developer Interview Questions and Answers," we’ve handpicked and curated the most frequently asked and important interview questions based on real-world interviews from top tech companies like Infosys, TCS, Accenture, and more. Each question is carefully broken down with clear, detailed explanations, coding examples, and real-life scenarios so that you're not just memorising answers — you’re truly understanding the concepts and learning how to apply them in real interview settings.We’ll cover everything from Java basics to advanced topics like OOPs, Collections, Multithreading, Java 8 features, Exception Handling, and even tricky logic-based questions that often stump candidates. Plus, you’ll get bonus tips on give interviews with confidence if you opt for mock interviews that I conduct free of charge.By the end of this course, you'll feel prepared, confident, and interview-ready. Let's get started!

Overview

Section 1: Introduction

Lecture 1 Introduction

Section 2: OOPs Concepts

Lecture 2 What is the difference between abstraction and encapsulation?

Lecture 3 Can you explain(Liskov Substitution Principle) with an example.

Lecture 4 Explain the concept of 'Composition' and 'Aggregation' in Java.

Lecture 5 How can you achieve multiple inheritance in Java? Why is it limited?

Lecture 6 What are covariant return types in Java?

Lecture 7 What is method hiding in Java? How is it different from method overriding?

Lecture 8 How does the final keyword affect inheritance and polymorphism?

Lecture 9 What are the implications of using hashCode() and equals() methods incorrectly?

Lecture 10 Can you use finally without a try block? What would happen?

Section 3: Collections and Generics

Lecture 11 Explain the role of load factor in HashMap.

Lecture 12 Why is HashSet implemented using HashMap internally?

Lecture 13 Explain the contract between equals & hashcode. What happens when it breaks?

Lecture 14 How would you implement a custom HashMap?

Lecture 15 How to create immutable collections in Java?

Lecture 16 Why are generics introduced in Java collections?

Lecture 17 Explain type erasure in Java generics.

Lecture 18 What is the difference between ArrayList and LinkedList?

Lecture 19 Difference between Collections.synchronizedList and CopyOnWriteArrayList.

Section 4: Exception Handling

Lecture 20 What is the difference between checked and unchecked exceptions?

Lecture 21 Explain the importance of try-with-resources. How does it work internally?

Lecture 22 What happens if an exception is thrown in a finally block?

Lecture 23 Is it ever appropriate to catch Error or OutOfMemoryError? Why or why not?

Lecture 24 How do exceptions impact transaction management in Java?

Lecture 25 How do you handle uncaught exceptions in threads?

Lecture 26 How does Spring handle exceptions in controllers?

Lecture 27 What happens if multiple exceptions are thrown in a try-with-resources block?

Lecture 28 What does the throw keyword without any arguments inside a catch block do?

Section 5: Java Memory Management

Lecture 29 Explain the differences between Young Generation, Old Generation, and Metaspace.

Lecture 30 How does Garbage Collection (GC) work in Java? Explain G1GC.

Lecture 31 Explain the structure of JVM memory.

Lecture 32 What are memory leaks in Java?

Lecture 33 How can you monitor and troubleshoot memory leaks in Java applications?

Lecture 34 What is the role of soft references, weak references, and phantom references?

Lecture 35 What happens if a thread allocates more stack memory than available?

Section 6: Streams, Functional Interfaces, and Lambdas

Lecture 36 How do map() and flatMap() differ in Java Streams?

Lecture 37 Explain the difference between intermediate and terminal operations in Streams.

Lecture 38 What is the difference between filter() and findAny() or findFirst() in Streams?

Lecture 39 How do you handle checked exceptions within Java 8 streams or lambdas?

Lecture 40 How do streams handle large datasets efficiently?

Lecture 41 Explain the types of method references in Java.

Lecture 42 Can lambda expressions capture local variables?

Lecture 43 Explain how Predicate, Function, & Supplier differ in terms of purpose and usage

Lecture 44 What is the purpose of the Spliterator interface?

Lecture 45 How would you implement a custom functional interface?

Lecture 46 What is the difference between stateful and stateless intermediate operations?

Lecture 47 How do default methods work in interfaces? Why were they introduced in Java 8?

Lecture 48 What is the use of functional interfaces like Predicate, Supplier, and Consumer?

Lecture 49 What are the key components of a lambda expression?

Lecture 50 How this keyword behaves differently in lambda expression vs an anonymous class?

Lecture 51 How do you override a default method in a class that implements an interface?

Lecture 52 What happens if a class implements two interfaces that have same default method?

Lecture 53 Can a default method call another method in the same interface?

Lecture 54 What are the limitations of using default methods?

Section 7: Multithreading and Concurrency

Lecture 55 What are the differences between Thread, Runnable, and Callable?

Lecture 56 What is the difference between synchronized block and ReentrantLock?

Lecture 57 How does the CountDownLatch differ from CyclicBarrier?

Lecture 58 What are the differences between parallel streams and sequential streams?

Lecture 59 What is a ForkJoinPool? How does it differ from an ExecutorService?

Lecture 60 What is the difference between parallelism and concurrency?

Lecture 61 What is the difference between volatile and synchronized?

Lecture 62 What is the difference between a livelock and a deadlock?

Lecture 63 What is the purpose of the ConcurrentHashMap?

Lecture 64 How does the ThreadLocal class work?

Section 8: Java 8 Concepts

Lecture 65 How are Lambda expressions different from anonymous inner classes?

Lecture 66 What is a functional interface? Can you create your own?

Lecture 67 What are intermediate and terminal operations in streams?

Lecture 68 What is method reference and constructor reference? Syntax and examples?

Lecture 69 What are default methods in interfaces? Why were they introduced?

Lecture 70 What if two interfaces have the same default method and a class implements both?

Lecture 71 What are static methods inside interfaces? How are they used?

Lecture 72 Explain Predicate, Function, Consumer, Supplier & BiFunctional interfaces.

Lecture 73 Difference between findFirst(), findAny(), and anyMatch(), allMatch()?

Lecture 74 How can you handle NullPointerException in Java 8 using Optional?

Lecture 75 How would you refactor a traditional for-loop to use streams?

Lecture 76 What is the difference between map() and flatMap() in streams?

Lecture 77 What is the Spliterator in Java 8?

Lecture 78 Explain LocalDate, LocalTime, LocalDateTime classes in Java 8 Date-Time API.

Lecture 79 How are lambdas internally implemented?

Lecture 80 What are the performance impacts of using parallel streams?

Section 9: Java 9 and beyond Concepts

Lecture 81 How do sealed classes compare to abstract classes and interfaces?

Lecture 82 Can a sealed class have both final and non-sealed subclasses?

Lecture 83 How does pattern matching simplify the instanceof operator?

Lecture 84 What are the key components of a record in Java 16?

Lecture 85 What is the difference between permits and extends in sealed classes?

Lecture 86 What are text blocks, and how do they improve code readability?

Lecture 87 How are switch expressions different from traditional switch statements?

Lecture 88 What is the HTTP Client API in Java 11?

Lecture 89 How does the lines method work, and what are its use cases?

Lecture 90 What is var in Java, and when should it be used?

Lecture 91 What is the difference between List.copyOf and Collections.unmodifiableList?

Lecture 92 Explain difference between requires, exports, and opens in the module system?

Lecture 93 Explain List.of, Set.of, and Map.of methods.

Section 10: Spring Boot

Lecture 94 What is Dependency Injection (DI), and how does Spring implement it?

Lecture 95 What is the difference between @Autowired and @Inject?

Lecture 96 How does Spring Boot achieve auto-configuration?

Lecture 97 How does Spring Boot optimize DI with its auto-configuration mechanism?

Lecture 98 What is the role of @SpringBootApplication?

Lecture 99 What is the purpose of the @Primary, and how does it interact with @Qualifier?

Lecture 100 How do you configure a multi-module Spring Boot application with proper DI?

Lecture 101 @Configuration vs @Component in the context of creating beans.

Lecture 102 Explain the different bean scopes in Spring.

Lecture 103 Difference between application.properties and application.yml in Spring Boot.

Lecture 104 How SpringBoot manages externalized configuration using @ConfigurationProperties

Section 11: REST APIs

Lecture 105 How do you handle partial updates in REST APIs? Explain PATCH vs PUT requests.

Lecture 106 What is HATEOAS in REST APIs, and how is it implemented in Spring Boot?

Lecture 107 How do you design APIs to be backward compatible during versioning?

Lecture 108 How do you handle large file uploads/downloads efficiently in a REST API?

Lecture 109 What is the difference between @Valid and @Validated) in Spring?

Lecture 110 How do you secure REST APIs using Spring Security? Explain the use of JWT.

Lecture 111 How would you prevent CSRF and CORS vulnerabilities in REST APIs?

Lecture 112 How do you implement an API Gateway using Spring Cloud Gateway?

Section 12: Microservices

Lecture 113 What are the key principles of microservices architecture?

Lecture 114 What are the pros and cons of synchronous vs. asynchronous communication?

Lecture 115 What are the challenges of managing data in a microservices architecture?

Lecture 116 How do you debug high latency in a microservices environment?

Lecture 117 How do you approach end-to-end testing in a microservices architecture?

Lecture 118 What are the roles of service registries like Eureka, Consul, or Zookeeper?

Lecture 119 How do you ensure fault tolerance in service discovery mechanisms?

Lecture 120 How do you test the performance of individual microservices?

Lecture 121 How do you secure inter-service communication in microservices?

Lecture 122 How do you implement rate limiting and IP whitelisting in an API Gateway?

Lecture 123 What are circuit breakers, and how do they help in microservices?

Lecture 124 How would you use patterns like retry, fallback, and timeout?

Lecture 125 What are some best practices for logging and monitoring microservices?

Section 13: Conclusion

Lecture 126 Congratulations!!

Lecture 127 Mock Interviews

This course is for Java developers who are planning to give interviews and want to brush-up on the topics most frequently asked in the common Java developer interviews.