The complete guide to Java interview questions and interview prep

While an older language, Java remains one of the most popular high-level programming languages today. It entered the landscape as a high-level language designed for simplicity and platform independence. Java is running on most of our mobile devices and laptops, and Java developers remain in high demand.

Whether you’re preparing for a role in game development or Android development, mastering Java might take you closer to finding a developer job you love.

Today, we present you with this complete guide to preparing for Java interviews.

Common Java interview questions#

Core Java interview questions and answers for beginners#

Is Java an object-oriented programming language?#

Java can be considered a partially object-oriented programming language. Java supports object-oriented programming along with other programming paradigms. Java employs object-oriented programming (OOP) concepts including encapsulation, polymorphism, inheritance, and abstraction.

However, Java is not a pure object-oriented programming language. It fails to satisfy the pure OOP language requirement that all user-defined types are objects. Java supports primitive data types as well

Does Java support multiple inheritances for classes?#

No, Java doesn’t support multiple inheritances for classes. Java was designed for simplicity and multiple inheritances for classes was avoided to avoid the “Deadly Diamond Problem.” This problem results when a child class has two parent classes with two different methods for implementing a feature.

What is JDK?#

JDK stands for Java Development Kit. [JDK consists of the Java Virtual Machine (JVM) and Java Runtime Environment JRE, which are essential to Java development.

Method overloading: Which method will be invoked?#

Given two overloaded methods, which method will be invoked for the method call myOverloadedMethod(5)?

Execute the following code to verify the result.

What is a default constructor?#

If no constructors are defined for a class, the compiler generates a nullary constructor known as the default constructor.

Resources for common Java interview questions#

To prepare for basic Java interviews, check out these resources:

• Java interview prep: 15 Java interview questions: Blog post covering questions from bytecode to platform independence
• Exploring object-oriented programming concepts with Java: Blog post detailing object-oriented programming in Java
• Java Interview Handbook: Educative course with 300+ interview questions.
• Java for Programmers: Comprehensive learning path covering basic topics such as the main method and method overriding, as well as advanced topics.

Advanced Java interview questions and answers for experienced developers#

Can Enum implement an interface in Java?#

Enum is a special data type or class that’s used to store constants. Since enum is a type, it can implement an interface.

What are garbage collection (GC) roots?#

The JVM references GC roots to guide the garbage collector. GC roots allow the garbage collector to collect only those objects that need collection.

Java has four types of GC roots:

• Local variables
• Static variables
• Active Java threads
• JNI references

What are the three phases of garbage collection?#

Garbage collection in Java occurs in three phases: Marking, deletion, and promotion.

REST API questions#

REST APIs are very popular in Java development.

Some questions you may be asked to assess your depth of knowledge on REST include:

• What’s the difference between PUT and POST methods?
• What is CRUD?

Be sure to spend plenty of time studying REST API interview questions and answers.

Resources for advanced Java interview questions#

To prepare for advanced Java interviews, check out these resources:

• Java interview prep: 15 Java interview questions: Blog post covering common Java interview questions involving performance optimization, singleton classes, and more
• A quick guide to bitwise operators in Java: Blog post covering bit manipulation in Java
• Unit Testing Java Applications using JUnit 5: An Educative course covering writing and implementing tests with JUnit 5, and integrating them with Maven and Gradle build tools
• Java Interview Handbook: An Educative course covering topics including Lambda expressions and mutexes.
• Java for Programmers: A learning path covering advanced topics such as multithreading and JUnit.

Java coding interviews#

Coding interviews will test your familiarity with Java, your understanding of algorithms and data structures, and your problem-solving skills. The higher the level of the role, the more likely this test will touch on topics such as system design, bit manipulation, and concurrency.

Here are some resources to get you started preparing for a Java coding interview:

• Decode the Coding Interview in Java: Real-World Examples: Course covering real-world problems ranging from networks to company-specific projects
• Crack the top 40 Java coding interview questions: Blog post covering questions from data structures to recursion
• Java coding interview questions: Comprehensive learning path covering algorithms, dynamic programming in Java, and more
• Every developer’s coding interview roadmap: Detailed resource from our friends at codinginterview.com
• Bit manipulation in Java: Blog post introducing a concept highly relevant to embedded systems
• Dynamic programming: You can apply dynamic programming solutions to popular coding interview problems such as the knapsack problem).
  

Java algorithm interview questions#

As with any coding interview, you’ll want to study several algorithms and be prepared to implement them in Java.

Some algorithms to study include:

• Big O Notation
• Sorting and searching with quicksort, binary search, and more
• Greedy algorithms such as maximization
• Divide and conquer problems
• Graph algorithms

You can learn Java with common Java algorithm questions and more by taking our course, Algorithms for Coding Interviews in Java

Java data structure interview questions#

You’ll want to know the fundamentals of data structures in Java. You should know how to declare and initialize data structures, and practice performing different operations on them. It’s a good idea to spend time studying common Java data structure interview questions questions and answers.

Some topics you should be sure to study are:

• String data structures in Java
• Array data structures in Java
• Linked list data structure in Java
• How to use linked lists in Java
• How to use stacks and queues in Java
• How to use trees in Java
• Techniques for strings and arrays in Java
• How to declare and initialize Java arrays
  

Java collections interview questions and answers#

What are collections?#

Collections are objects grouping multiple elements into a single unit. The Java collections framework identifies classes and interfaces with which we can implement collections.

What is the collection interface?#

The collection interface is used to pass around collections of objects.

The core collection interfaces are:

• Set : A collection that can’t contain duplicate elements
• List : An ordered collection that can contain duplicate elements
• Queue : A collection used to hold multiple elements prior to processing
• Map : An object mapping at most one value to keys
  

What are collection interface bulk operations?#

Bulk operations are operations that can be performed on an entire Collection. Bulk operations include containsAll, addAll,removeAll and more.

What are the collection types in the Java collections framework?#

There are three collection types in the Java collections framework: Ordered lists, maps, and sets.

Java multithreading interview questions and answers#

Multithreading is increasingly important for senior level roles. Multithreading is a resource-efficient way to multitask in Java.

What is multithreading?#

Multithreading is a technique that allows us to efficiently leverage CPU by executing two or more threads concurrently.

What are the benefits of multithreading?#

Benefits of multithreading include:

• Increased throughput
• Increased responsiveness of applications
• More resource efficient than multiprocessing

Java 8 interview questions and answers#

Java 8 introduced several valuable new features, including the Lambda expression, virtual extension methods, a new Date and Time API, as well as support for functional programming. Check out our course for a deep dive on Java 8 for Experienced Developers

What is function composition?#

Function composition is one of the 5 key principles of functional programming. It refers to combining simple functions to create a complex function.

What is a functional interface?#

Functional interfaces are interfaces that have a single abstract method. Functional interfaces are implemented with Lambda expressions. The Lambda expression contains the body of the abstract method.

STREAM API questions#

Stream API questions questions can include:

• What’s the difference between streams and loops?
• What are advantages and disadvantages to the Stream API?

Check out our blog post on mastering Stream API for answers to these questions and more.

Java EE interview questions and answers#

If you’ll be working with the Java Enterprise Edition, or Jakarta EE, you’ll likely be asked about web frameworks such as Spring and Hibernate.

Spring framework interview questions#

Spring framework questions can include the following:

• What is aspect-oriented programming?
• What’s the difference between the IOC containers BeanFactory and ApplicationContext?
• What is the Bean lifecycle?

You can find answers to these among the top 40 Spring framework interview questions.

List the Hibernate framework’s core interfaces.#

Hibernate’s core interfaces are:

• Configuration
• Session
• SessionFactory
• Criteria
• Transaction
• Query
  

Are singleton beans thread-safe?#

Singleton beans are not thread-safe. A singleton is a design pattern for creation. Thread safety is specific toward execution.

How to prepare for Java interviews#

How much you prepare depends on your experience. If you’re interviewing for a job with additional language requirements, you won’t be able to spend all of your time on Java. You’ll want to use your time wisely and cover as many bases as you can during interview prep.

Study the fundamentals#

If you’ve been working with Java for a while, you can skip this.

Before you start preparing for interviews, make sure you’re well-versed on the fundamentals of Java programming. Check out our learning path tailored for Java for programmers .

Prepare for specific interview rounds#

You’ll often go through several rounds of interviews, from behavioral interviews to coding interview, up to System Design Interview.

Java coding interviews#

You’ll want to practice answering various interview questions. Since you have limited time to prepare, it’s important to have a strategic coding interview prep plan. In addition, you need to plan for what to do if you’re given a coding problem you can’t solve (Hint: You can still get the job).

Here are some resources to help you prepare:

• Decode the Coding Interview in Java: Real-World Examples: A comprehensive course with solutions to real examples
• Ace the Java Coding Interview: A learning path designed to cover the foundations of coding interview prep

Java system design interviews#

The system design interview is common practice if you’re interviewing for a role as a senior developer.

Here is a resource to help you prepare for the Java system design interview:

• Top 5 distributed system design patterns: Blog post covering design patterns including sharded services
  

Prepare for specific roles and companies#

All interviews vary! In addition to studying industry-standard interview questions, you should tailor your interview preparation for your unique circumstances:

• The level of the role: Expectations differ based on the level of the role you’re applying to. It’s difficult to provide a template, since every company is different, but a ballpark of the topics you could expect based on your level of experience include:
  • Java interview questions for junior developers will assess your general programming knowledge, OOP knowledge, and your understanding of Java basics.
  • Java interview questions for experienced developers are more likely to expect your knowledge of multithreading, concurrency, JVM internals, and Java collections.
• The individual job description: This is the most important aspect to consider. Your job description gives you a rubric for all the qualifications your interviewers hope to see in you. You should prepare to highlight and justify the qualifications you meet. If there are qualifications you can’t meet, don’t sweat it. The deal-breaker qualifications are already addressed during the phone screening.
• The company you apply for: Each company conducts interviews differently. Spend some time on forums and Glassdoor to explore what others have said about interviewing with the company. If you’re interviewing at a top tech company such as FAANG, you’ll be able to find many company-specific interview guides, such as Netflix coding interview guides and Facebook coding interview guides.

Here are some additional resources that are specific to interviews:

• Top 25 coding questions to crack the Microsoft interview
• Cracking the top Amazon coding interview questions
• Cracking the Uber Data Science Interview
• System Design Primer
• System Design Interview Questions

Wrapping up and next steps#

There are several ways to approach preparing for a Java interview. You’ll want to review fundamental concepts based on your experience level. You’ll also want to get hands-on practice with real-world problems. You’ll want to tailor your preparation based on the level and responsibilities of the job you’re applying for, as well as the company. While you can’t know everything about an interview in advance, there’s a lot you can do to be sure you walk in prepared.

At Educative, we love to support developers as they advance in their careers. That’s why we created Interview Prep with Educative. Here, you’ll find all our resources for Java interview prep in one place, from courses and tutorials to walkthroughs on real-world interview problems.