Top 10 Java Interview Questions
Updated 20 December 2020
What Is a Java Interview?
Java programmers and developers work for companies that create web applications using the Java programming language.
As part of the application and recruitment process, employers administer a Java interview to determine the level of coding experience and expertise of each candidate.
Java developers can earn lucrative salaries and therefore competition is high. A bachelor’s degree in computer science or equivalent, and sometimes further IT qualifications, are a minimum requirement for most coding positions, so the caliber of applicants is high from the outset.
Therefore, a Java interview sets out a series of questions and tests to help employers identify the top candidates.
A software development team is made up of several roles that require Java competence.
A Java Fullstack programmer and Java Backend programmer are examples of jobs that would require a Java interview.
The responsibilities of an employee within such a role include:
- Writing code
- Performing compliance tests
- Working as part of a team
- Improving existing infrastructures
- Tracking procedures
The developer must also be able to take customer requirements and translate their needs into a Java algorithm that works.
There is no one standard set of questions in a Java interview, as the questions are usually determined by the level of experience required for the job role.
Coders with just one or two years’ experience will be asked about the fundamentals of Java coding, whereas more experienced coders are likely to be faced with more complex and technical challenges.
Recognizing this is an essential factor when preparing for your interview, as along with revising your generic knowledge of Java, you can tailor your preparations to reflect the level of expertise your interviewer may expect you to demonstrate.
Java interviews often involve a range of challenges that assess both hard skills and soft skills so the employer can get a measure of your skills, aptitude and general suitability for employment.
Candidates might face:
- Ice breaker questions
- Coding tests
- Behavioral questions
- Business problems
- Logic puzzles
- Brain teasers
Although it is advisable to research potential interview questions and Java technical challenges online, interviewers know that asking generic questions doesn’t thoroughly test the practical skills of the individual candidate.
Instead, you will likely face in-depth, unique tests that require genuine experience to solve.
How to Prepare for a Java Interview
There is no shortcut to a job working with Java coding. You simply must have the necessary skills and experience for the job to which you are applying.
Beyond this, there are some things you can do to improve your chances of success, including researching the types of questions you might be asked and common question formats.
Coding roles tend to be split into levels of experience; junior, middle and senior.
Junior positions will require knowledge of:
- Application Programming Interfaces (APIs)
- Data structure
At the other end of the scale, senior programmers are expected to be knowledgeable in areas such as:
- JVM internals
- Java Performance
- Java concurrency API
- GC tuning
Revisit the original job description to pick out the advertised duties, responsibilities, knowledge and skills and use them to focus your answers.
Knowing about the company and job role can influence the way you prepare for your interview.
Java interview questions are likely to be based around the 16 technical key areas that present the common challenges in coding.
If you need to brush up on Java, a good place to start is by revising each of these areas:
- Quality of Service
- Performance Considerations
- Design Concepts
- Specification Fundamentals
- Memory Considerations
- Best Practices
- Design Patterns
- Exception Handling
- Software Development Process
- Concurrency Management
- Platform Fundamentals
- Language Fundamentals
- Transaction Management
Referring to these key technical areas in your interview demonstrates that you have an awareness and overview of all aspects of the role, which helps you to stand out from your competitors.
Referring to real-life situations that the company may face shows that you have made an effort to understand their needs.
Java has been around for a long time, so there are many sources of information available to help you brush up on your skills, including respected books, video tutorials and websites.
The breadth and depth of Java programming mean that you will not be expected to know absolutely everything.
If you’re presented with a question or challenge you don’t know how to answer, an honest response with a willingness to learn can also reflect well on your attitude to approaching problems.
The Coding Test
When companies recruit for coding positions, they often administer challenges as part of the interview process.
A Java interview is always used as part of a broader interview process and, although it can come at any stage, it’s often used as a first-round interview to quickly identify a shortlist of candidates with the appropriate knowledge to progress.
Sometimes a coding test is followed by another technical interview.
There is no one standardized Java coding test as companies create their own to meet their recruitment requirements.
Some coding tests have a time limit, usually between thirty minutes and two hours in length, and others have no strict time limit. In either case, you’ll be asked to complete a series of coding problems and challenges.
You may be permitted to use your usual resources like Google or GitHub, as employers recognize that this is a common real-world way that developers work.
However, if they want you to complete the challenge without outside help, you will be unable to leave the testing platform while your challenge is in progress.
Employers can administer the coding test in one of two ways.
The first is via an online test accessed through a URL sent to the candidate that they then open and complete at home. The tests can be hosted on third-party platforms or the company’s own internal system.
It is always a good idea to write test cases to go alongside the core functionalities of the challenge to show that you are considering the maintainability of your code. It reflects well on you if you make it easy for other developers to understand and use your code.
If the job requires team approaches to coding, you may be asked to attend your coding test in-person to work alongside other candidates under the watchful eye of an assessor.
This shows the employer how well you work with others and gives you a chance to demonstrate communication skills as well as coding skills.
You may find that this method of interview feels more pressured, as you will try to solve a problem while being watched. However, the interviewer is observing the way you approach the challenge and may not necessarily be invested in whether you actually solve the problem.
It can be challenging for recruiters to strike the right balance between presenting a test at the appropriate depth and difficulty level while also giving the candidate a fair chance to demonstrate their skills.
As a result, applicants sometimes report finding Java interviews frustrating.
To avoid this situation, employers try to present real-life problems that occur in business and have an objective and standardized scoring system that awards an actual grade rather than a simple pass or fail.
As a guide, some of the areas covered in a Java coding test might relate to:
Application Design and Architecture
Testing fundamental skills in Java coding that all developers must have as a basis for their work.
Middle and senior developer roles may be required to demonstrate how they can design and build solutions that are maintainable and extendable.
Areas of knowledge include object-oriented programming and design patterns.
The Java Programming Language Itself
Including core APIs, methods, objects and syntax to test the developer’s knowledge of common functionalities and their use of data structures and APIs.
Problem Solving Using Algorithms and Data Structures
All developers are expected to know how to solve problems by writing Java algorithms. The more experienced the developer, the more complex the issues they are expected to solve.
Problem-solving tests will assess the candidate’s logical reasoning, analytical skills and knowledge of concepts such as number theory, graph theory and dynamic programming.
As well as being asked questions around problem-solving using Java, also expect to write code either into a code editor or onto a whiteboard to present to your interviewer.
As with all interviews and assessments, a key component of your preparation should be practice.
It is possible to access online practice coding tests from providers such as:
Accessing online practice tests helps you to become familiar with the format and questions within the test and the level of skill required to answer. You will also get access to answers and explanations to help you improve.
Make sure you include some timed practice tests so you can get a feel for how fast you must work to complete the challenges within the allowed time frame.
If you are successful in the coding test, you might get called in for a coding challenge review meeting to discuss your challenge with the recruitment team. This is a positive sign and usually means the employer sees potential in you that they want to explore.
Top 10 Core Java Interview Questions
There are several core Java interview questions that you can spend time researching and revising to improve your chances of interview success.
Other programming interviews may focus mainly on data structure and algorithms, but as Java is an application programming language, interview questions often have a heavy focus on APIs and core concepts.
1. ‘What Do You Like Best About Being a Java Developer?’
An honest answer is all this question requires. Every candidate will have their own reasons for getting into coding and developing, and the interviewer wants to get to know a bit about you and what motivates you.
Although this seems like a straight forward question, it’s wise to think through your answer beforehand so you can articulate your answer well.
2. 'Tell Me About a Time You Improved Some Code. What Did You Look For?'
The interviewer wants to get an idea of your basic understanding of coding and asks this somewhat open-ended question to hear how confidently you can talk about and explain the way you work.
Your answer could include how you identify low-quality code and distinguish it from high-quality code, moving on to explaining the factors you check for when assessing coding quality.
Common factors include:
End your answer by talking through what to do if your tests reveal low-quality code.
Solutions could be applying the applicable coding standard, following best practices and refactoring legacy code.
3. 'How Do You Test Your Code?'
The interviewer is assessing your level of basic coding knowledge.
The correct answer is to use test-driven development processes to repeatedly expose code to various test situations during the development phase, rather than completing coding first then testing later.
4. 'How Do You Write a Bubble Sort? What Is Its Computational Complexity?'
A bubble sort is a simple algorithm (often taught in basic computer science) to create a basic understanding of sorting.
This question is assessing whether the candidate has the foundational knowledge necessary for coding.
A bubble sort algorithm moves through a list, comparing adjacent elements and swapping them if they are in the wrong order. Its worst-case and average complexity is O(n2).
5. 'What Is a Binary Tree? How Would You Code One?'
A binary tree is a data structure in which each element, or node, has two children at most.
The child on the left is of an equal or lower value than the parent. The child on the right is of equal or greater value than the parent.
Coding a binary tree requires a recursive method of insertion, and your answer needs to cover inserting elements, finding elements and deleting elements.
6. 'What Are Some of the Most Common Disadvantages of Coding With Java?'
Java is a well-established technology that has consistently been a popular coding technique for twenty years.
However, like all programming languages, it does have its pitfalls.
Use this question as an opportunity to demonstrate your overall understanding and knowledge of Java.
Some of the common disadvantages you could list are:
Poor performance due to the complexity of the high-level language, which leads to slow speeds.
Complex code that may be easy to read but makes code lengthy and less readable and scannable.
Lack of ideal graphical user interface means it is difficult to create a native look and feel to the site.
Commercial license fees introduced in 2019 mean costs for businesses have increased.
7. 'How Can You Troubleshoot Code Errors in a Programming Team?'
Some common methods of troubleshooting code errors are to:
- Simplify the problem
- Ask other team members for advice
- Take some time away from the problem to clear your head
- Go through the problem step-by-step yourself
Often, talking through the problem out loud can help you see it more clearly and can help you find a solution.
8. 'What Are the Key Principles of Object-Oriented Programming?'
Object-oriented programming (or OOP) is a process consisting of four parts:
Interviewers commonly ask this question as it reveals whether the candidate is beyond the beginner stage of programming and that they have prepared adequately for the interview.
9. 'What Is Thread Safety?'
This question is testing your knowledge of concurrency; a complex subject that details multiple operations running in overlapping time periods.
Your answer might explain correctness before outlining that when a class behaves correctly, even when accessed from multiple threads, it is considered thread-safe.
10. 'Can You Explain the Differences Between Runnable and Callable?'
Although both interfaces carry a task that can be executed by several threads, there are differences between the two.
First, runnable has been in use since Java 1.0, with callable coming onto the scene in Java 1.5.
Other differences are that a callable object returns a result whereas a runnable object does not, and callable can throw a checked exception and runnable cannot.
When applying for a programming or developer role that requires a knowledge of Java programming language, you will be asked to attend a Java interview.
Java interview questions are designed to reveal the experience, understanding and skill of the candidate.
Although real-world experience is vital, there are also things candidates can do to prepare for interview success.
To prepare for a Java interview, first revisit the job description for clues about the skills and attributes you need to demonstrate.
As well as identifying specific tasks in which the company requires you to be proficient, you can also look for key phrases and words that you can reflect in your answers to show you can apply your knowledge to their company needs.
Practicing Java technical tests online is another key part of Java interview preparation as it will help you become familiar with the types of challenges and how fast to work to finish within the allotted time.
Researching common interview questions and answers and refreshing your basic knowledge of Java is also advisable.