Java Coding Interview Questions: Answers and Explanations

Are you a Java Developer seeking a job in one of the top US MNCs? Or, are you a recruiter from a top US MNC looking for an excellent Java Developer? In either case, you have landed on the right page.

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Java is one of the most used programming languages. According to Statista, Java is used by 33.3% of programmers worldwide, making it the sixth-most popular coding language.

However, that doesn't mean hiring Java developers is easy. Not every Java developer has the skills, personality, and experience to work for your company. They may also lack the interpersonal and communication skills to gel with your existing staff.

One effective way to recruit suitable Java programmers is by asking them incisive Java interview questions. Read this guide to learn about the best Java developer interview questions and answers.

Core Concepts of Java

Before hiring a Java developer for your team, you should understand exactly what they do so you can tailor a compelling Java developer job description to attract top talent.

Once you have a number of candidates, you'll want to ask thoughtful and engaging questions to understand their experiences, education, skills, and motivation for being a Java developer. You should also test their understanding of practical Java concepts.

Basic Java Coding Interview Questions

Before you do anything else, you should ask basic interview questions to learn more about job seekers' motivations, experiences, and education. Examples include:

  • Tell me about yourself.   This question prompts the job seeker to tell you more about their areas of expertise, education, and skills. Once the applicant has talked a bit about themself overall, ask about their Java work experience and why they enjoy programming in Java.
  • How did you learn Java? Why did you become a Java developer? This question reveals how the candidate learned Java and their motivations for becoming a Java developer. While most Java developers have a bachelor's degree, the perfect hire doesn't have to have one. Many programmers with no degrees boast world-class experiences and certifications, including:
  • What do you like the most about being a Java developer? This question reveals why a candidate enjoys using Java. Make sure your hire is on fire for Java, Java trends, and Java development. They should also be passionate about developing the digital products you want them to create, whether those are websites, applications, or software.
  • Besides Java, what other languages do you know? If you want a hire to collaborate with other developers on other projects, they may need to know other languages, such as Python, C++, and C#.
  • How many Java applications have you designed, tested, and deployed? This question reveals the depth of an applicant's development experience.
  • What kind of Java projects have you worked on? This gives you a better idea of the breadth of the candidate's experience. Ideally, they should have worked on projects similar to the ones you're hiring for. For instance, if you're hiring a Java developer to create Android apps, your hire should have ample experience building, testing, and deploying Java Android apps.
  • What project management methodologies are you familiar with? Is the developer familiar with your company's preferred project management method? If not, consider hiring someone who is. For instance, if your company uses Scrum and Scrumban, you should hire Java developers who are familiar with developing in Scrum and Scrumban teams. Otherwise, your new hire may find it difficult to collaborate and communicate with the rest of the team.
  • How do you create secure Java applications and software? This question reveals how a candidate approaches security. An ideal Java developer should prioritize security and bake it into every step of the software development lifecycle (SDLC). They should also understand trending security practices like DevSecOps and DevOps.
  • What Java libraries are you familiar with? Libraries are collections of pre-written code that programmers can use to accelerate and optimize projects. An experienced Java coder should know how to create applications with a range of Java libraries, including:
  • Apache Commons
  • Jackson
  • Google Guava
  • Log4j 2
  • Mockito
  • JAXB
  • AssertJ
  • Apache HTTPComponents
  • Hibernate

Common and Practical Java Developer Interview Questions

After asking the questions above, you can start testing applicants' practical knowledge. Here are common Java developer practical questions and answers to get you started.

What Are the Differences Between JDK, JVM, and JRE?

The differences between these three are as follows:

Parameter JDK JVM JRE
What It Stands For Short for Java Development Kit Short for Java Virtual Machine Short for Java Runtime Environment
Function The JDK is a kit that compiles, documents, and packages Java programs. It consists of development tools and the JRE. You can use the JRE to execute Java programs. The JVM is a subclass of JRE that is responsible for minor tasks such as decoding Java bytecode into machine language. It is also called an interpreter. The JRE is an installation package that creates an environment to run the Java application or program on your machine. It cannot be used to develop Java programs.

Is Java an Object-Oriented Language? Why or Why Not?

Java is not entirely an object-oriented programming (OOP) language because it uses eight primitive data types that are not objects. These include:

  • Int
  • Boolean
  • Char
  • Byte
  • Double
  • Short
  • Long

What Are Abstract Methods and Classes?

An abstract class is a class that you declare to be abstract with the "abstract" keyword. You can use abstract classes to define or state general characteristics. Note that abstract classes can't be instantiated, which means you can't form an object from a Java abstract class. Trying to instantiate an abstract class will produce a compiler error.

An abstract method is a method that is declared by using abstract keywords in an abstract class. You can use abstract methods for abstraction — hiding unnecessary details from users and focusing on vital details. Abstraction boosts efficiency by reducing complexity.

What Are Constructors in Java?

Constructors are blocks of code for initializing objects. They must have the same name as the class and have no return types. Constructors are automatically called when you create an object.

There are two kinds of constructors:

  1. Parameterized Constructor: These constructors can initialize provided values with instance variables.
  2. Default Constructor: These constructors do not require any input. In other words, they are the no-argument constructors that will be created by default when the user does not define other constructors. The main purpose of default constructors is to initialize the default values with the instance variables. You can also use them for object creation.

What Are the Main Differences Between Java and C++?

The main differences between the two are as follows:

Parameter Java C++
Best-known characteristic Java is best known for its automatic garbage collection. C++ is best known for having features that are similar to object-oriented and procedural languages.
Semantics The object and primitive classes in Java are not consistent. Object and primitive types in C++ have the same type of semantics.
Interaction with the library Java's native libraries do not offer direct call support. Consider using Java Native Interface to access the libraries. C++ can access its native systems libraries directly. Accordingly, C++ is a better choice for system-level programming.
Interpreted vs. compiled Java is compiled and interpreted at the same time. C++ is compiled.

What Are the Main Features of Java?

Java's main features include:

  • User-friendliness: Java is considered one of the easiest languages to learn. That's because many Java processes run automatically. As such, you don't need to spend a lot of time studying how everything works.
  • OOP features: Java is not completely OOP, but it has many OOP principles. OOP is a computer programming model that organizes software development around objects rather than logic and functions. Because you can group objects into reusable classes, you can create reusable applications with less code and development time.
  • Top-notch security features: Java boasts a large set of tools; application programming interfaces (APIs); and implementations of popular security mechanisms, algorithms, and protocols. You can use these tools to create highly secure and tamper-proof applications.
  • Platform-independent: Java can work on any device that can run the JVM.

What Are the Memory Allocations in Java?

Java has five major memory allocation types:

  1. Native method stack memory
  2. Class memory
  3. Stack memory
  4. Heap memory
  5. Program counter-memory

What Is Object Cloning?

Object cloning is when you create an exact copy of an object. Here's what can happen when you clone an object in Java:

  • If the class only has primitive data types, the JVM will create a completely new copy of the object and return a reference to the new object copy
  • If the class has members of any class type, the JVM will only copy the object references to those class types. As a result, the member references in the cloned and original objects will refer to the same object.

Junior Java Developer Interview Questions

After the basic questions above, the next stage is to ask experience-specific interview questions. If you're hiring entry- or junior-level Java developers, you should ask questions specifically created for that experience level.

As their name suggests, junior- or entry-level Java developers have zero to five years of professional development experience. Because they have less experience, they have fewer responsibilities. Most companies only hire junior- and entry-level Java developers to do the following:

  • Create applications and software by coding, implementing, and maintaining Java-based interfaces and components
  • Contribute to the entire SDLC
  • Write well-designed, efficient, and testable code

Accordingly, you should be more lenient when assessing their skills, experiences, and portfolios. Instead of focusing on their hard skills, focus on their personalities, motivations, and soft skills. You can pair them with senior developers and give them learning and development opportunities to refine and expand their skills.

To help you get started, here are some sample junior- and entry-level Java developer interview questions. All of these questions reveal a junior- or entry-level developer's depth of technical knowledge.

What Are Static Variables and Static Methods in Java?

Static methods and variables belong to the class of the Java program rather than the object of the class. You can also directly call them using class names.

In One Sentence, What Is the Java Try...Catch Block?

The try...catch block in Java is for handling exceptions and preventing abnormal program termination.

What Is the Java Database Connectivity (JDBC)?

The Java Database Connectivity (JDBC) is one of the earliest libraries for the Java language. It is an API that issues commands and queries, connects to a database, and handles result sets obtained from the database. Developers can use the JDBC to interact with a database from inside a Java program.

The JDBC consists of two layers:

  1. The JDBC API, which supports communication between the JDBC manager and the Java application
  2. The JBDC driver, which supports communication between the database driver and the JDBC manager

What Are the Differences Between Sleep() and Wait() in Java?

Sleep() is a blocking operation that keeps a hold on the lock or monitor of the shared object for a specific number of milliseconds. Wait() pauses a thread without keeping a hold on the lock or monitor of the shared object until either of the following happens:

  1. The specific number of milliseconds have passed
  2. The program receives a desired alert from another thread

What Is the Java String Pool?

The Java Spring Pool is a collection of string objects in Java's Heap space, the JVM's garbage pile. Whenever you try to create a new string object, the JVM will first check for the string in the pool. If the string already exists in the pool, you will receive a reference to the pooled instance. If the string doesn't exist in the pool, the JVM will initialize a new string object and place it in the pool.

What Is the Purpose of the Finally Block?

In Java, the finally block is used for executing important code such as clean-up code. Many developers use the finally block for closing files, closing connections, and freeing up threads because it executes regardless of an exception.

What Is the Purpose of Garbage Collection?

The main goal of garbage collection is to free up the memory occupied by unreachable and unnecessary objects during Java program execution by deleting these objects. Deletion ensures that Java uses the memory resource efficiently. However, garbage collection doesn't necessarily guarantee sufficient memory for program execution.

What Is a Singleton Class in Java? Why Do They Exist?

A singleton class in Java is a class that can only have one object at once. The purpose of the singleton class is to control object creation and limit access to resources, such as sockets and database connections.

What Is the Final Keyword in Java?

The final keyword is one of the 48 reserved keywords in Java. You can use it to make classes, attributes, and methods non-changeable — impossible to override or inherit.

What Is JIT Compilation? What Methods of Machine Code Translation Does It Combine?

JIT or just-in-time compilation compiles code during program execution, while traditional compilers compile ahead of time. JIT compilation is an essential part of the JRE.

JIT compilation combines two traditional methods of machine code translation: ahead-of-time (AOT) compilation and interpretation. Specifically, JIT compilation combines the speed of AOT compilation with the flexibility of interpretation.

Advanced and Senior Questions Examples

Advanced or senior Java developers have over five years of professional knowledge. As such, they have more responsibilities than their entry-level and junior counterparts. Most companies hire them to do the following:

  • Identify and analyze consumer requirements
  • Develop, test, and deploy Java applications and software
  • Assign, execute, and prioritize tasks throughout the SDLC
  • Test and debug junior and entry-level developers' code
  • Design and implement database architecture
  • Train and mentor junior and entry-level developers' code
  • Create detailed reports on the projects completed and work performed
  • Determine software functions and establish objectives with team members

To see whether a senior Java developer has the skills you need, ask them the following questions:

What Is Spring? Why Do Developers Use It?

This question reveals whether a candidate understands Spring, a popular Java framework.

Spring is short for the Spring Framework, an open-source framework that offers infrastructure support for creating Java applications. Many Java developers use Spring to reduce development time and increase productivity.

Spring's main features include:

  • A robust set of core technologies, including data binding, dependency injection, events, validation, and type conversion
  • Testing, including Spring MVC Test, mock objects, and TestContext framework
  • Spring WebFlux and Spring MVC web frameworks
  • Data access, including DAO support, transactions, JDBC, and Marshalling XML
  • Support for other languages, including Groovy, Kotlin, and dynamic languages
  • Integration with email, scheduling, remoting, Java Management Extensions (JMX), Java Message Service (JMS), Java Connector Architecture (JCA), and cache

What Are Continuous Integration (CI), Continuous Delivery, and Continuous Deployment?

This question reveals the senior developer's experience with different SDLC practices.

Continuous integration (CI) is when programmers merge their code changes with the main branch as regularly as possible. By doing so, developers can sense and resolve conflicts earlier in the SDLC without implementing large-scale code changes. This practice makes software development faster, easier, and less risky for developers.

Continuous delivery (CD) is a software engineering approach where teams produce software in short cycles. It involves deploying changes to customers by clicking a button. Many teams use CD to ensure that they can release software anytime.

Continuous deployment is also abbreviated as CD, but it is a different concept. It involves producing software in short cycles through continuous automation. In other words, developers don't have to press any buttons to deploy changes.

What Is a JavaBean?

This question reveals a developer's experience with the JavaBeans architecture.

In the JavaBeans architecture, a JavaBean is a class that encapsulates several objects into one object. Developers use it to access objects from multiple places. JavaBeans has several elements like Getter/Setter Methods, Constructors, and much more.

You must follow several conventions when using JavaBeans:

  1. Beans must have a default constructor.
  2. Beans should implement because it lets you save, store, and restore the state of a JavaBean you're working on.
  3. Beans should have setter and getter methods. A getter method is for reading the value of a readable property, and a setter method is for updating the value.

Which Design Patterns Have You Used in Java Projects?

An experienced Java developer should have experience using different design patterns, including:

  • Decorator pattern: This is a structural design pattern that lets you attach new behaviors to objects. You can use this pattern by putting these objects inside special wrapper objects containing the behaviors.
  • Dependency injection (DI) pattern: This is a design pattern that allows you to create dependent objects outside of a class, such as the string class. It is an excellent way to reduce tight coupling. You can also use DI to manage future changes and other software complexities.
  • Behavioral design pattern: These patterns are concerned with assigning responsibilities between objects and algorithms. Examples include command, chain of responsibility, iterator, and memento.

Recruitment Java Developer Coding Challenges

As you can see, interview questions can reveal a lot about an applicant's experiences, education, and skills. However, they don't say much about an applicant's problem-solving abilities. This means that you can easily hire a Java developer who can't apply their technical knowledge in real life. For instance, they may know how to use the Spring framework in theory, but when push comes to shove, they can't use it to create a Java app.

To avoid this problem. you should get candidates to complete recruitment Java developer coding challenges before hiring them.  Coding challenges are in-person or online technical interviews or tests that show candidates' work ethic, communication, and problem-solving skills. The main types of recruitment Java developer coding challenges include:

  • Whiteboarding: Whiteboard interviews require the candidate to solve a Java problem in real time on a digital or physical whiteboard in front of an interviewer. The interviewer evaluates the job seeker's technical and soft skills as they solve the problem.
  • Pair programming: In pair programming, the candidate sits down with one of your Java developers to work on a Java project or problem. Your developer evaluates the job seeker's technical and soft skills as they go through the problem together.
  • Open coding challenges: These are online programming contests that test the abilities of Java developers. Because they are open to developers around the world, you can use them to recruit the best Java programmers worldwide.

How We Help You Find Java Developers for Hire

Having to hire Java developers with experience can be trying, especially if you're a small startup with limited resources and connections. That's why you need to ask incisive interview questions to narrow down the applicant pool.

Before you can start interviewing candidates, however, you need to source Java developers. One of the best ways to do this is by joining a first-class tech talent marketplace like Revelo. As Latin America's premier tech talent marketplace, we offer access to thousands of remote programmers, including Java web coders, big data programmers, quality assurance testers, Java mobile app developers, and more.

If you're interested in kickstarting your Java developer hiring process, hire through Revelo. After you fill in this form, we will send you a list of top-notch Java talent within three days. You can then interview and pick the coders you want.

Why Choose Revelo?

Quick turnaround for candidate shortlists

A vast talent pool of 
pre-vetted developers

Professional sourcing, vetting, and onboarding support

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Oscar D.
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11 years
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