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Java interview questions

9 November 2023 , by Josh Nel

When scaling your team, finding the right Java developer can be challenging, especially if you’re looking for experienced team members. A poor hiring decision can lead to setbacks, low-quality code, and wasted time. Here, we look at Java developer interview questions you can use to find the perfect technical and cultural fit for your team.

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Questions to ask when interviewing a Java developer

Before diving into the technical details, it can be helpful to start with a few questions to assess a candidate’s approach to technical problems, their knowledge of the latest ecosystem developments, and how they might collaborate with your team. Here are a few questions you can use in your next interview:

  • How do you handle and communicate technical issues or roadblocks with team members?
  • Describe your experience working in an Agile or Scrum development environment.
  • How do you stay up-to-date with the latest developments in the Java ecosystem and programming in general?
  • Can you share any personal projects or open-source contributions related to Java?
  • Walk us through your approach to solving complex programming challenges.
  • Can you provide an example of a time you had to make architectural decisions when designing a Java application?
  • Can you provide examples of projects or applications you've developed using Java?
  • Have you worked with any Java frameworks such as Spring or Hibernate? If so, can you explain your experience with them?
  • What is the significance of the "final" keyword in Java, and when would you use it in code?

Java technical interview questions

Now that you’ve covered a few behavioural questions, you can move on to assessing a candidate's technical ability. Here, we’ll look at some general interview questions, plus a few specific questions for Java developers at different levels of seniority.

  • What are the main principles of Object-Oriented Programming (OOP), and how do they apply to Java?
  • Explain the differences between the "final," "finally," and "finalise" keywords in Java.
  • How is memory managed in Java, and what is the purpose of the garbage collector?
  • Describe the differences between the String, StringBuffer, and StringBuilder classes in Java.
  • What is the purpose of the transient and volatile keywords in Java?
  • Compare and contrast primitive data types and reference data types in Java. Provide examples.
  • What is autoboxing and unboxing in Java, and when does it occur?
  • What is the purpose of the break and continue statements in Java?
  • How do you create and throw a custom exception in Java? When would you use this feature?
  • What is the difference between File, FileInputStream, and FileOutputStream classes in Java for file manipulation?
  • How do you read and write data to and from a file in Java using character streams?
  • Explain the Observer design pattern and provide an example of its use in Java.

If you’re interviewing for junior or intermediate Java developers, consider including a few of these questions:

  • Explain the concept of Object-Oriented Programming (OOP) and how Java supports it.
  • What is a class and an object in Java? Provide an example.
  • Differentiate between a class and an interface in Java.
  • Describe the purpose and use of the main method in Java.
  • Name and explain some of the primitive data types in Java.
  • What is the difference between int and Integer in Java?
  • What is version control, and why is it essential for collaborative software development?
  • Have you used a version control system like Git? If so, explain basic Git commands like "commit," "push," and "pull."

For more senior Java developers, more advanced questions might be appropriate:

  • Describe the differences between the Java Collections Framework and the Collections API. Provide examples of commonly used collections and their use cases.
  • How does the Java memory model impact the performance of collections, and what are the best practices for optimising collection usage?
  • Explain the purpose of the java.util.concurrent package and its key components, such as ‘ConcurrentHashMap’ and ‘ThreadPoolExecutor’.
  • Describe the Java EE architecture and its key components, such as EJBs, JPA, and servlets.
  • Explain the role of servlets and JSPs (JavaServer Pages) in web development, and provide examples of their use.
  • Discuss the use of RESTful web services and how they can be implemented in Java using JAX-RS.
  • Discuss techniques for optimising the performance of a Java application, including profiling, code analysis, and JVM tuning.
  • What are the best practices for optimising Java code for memory usage and CPU efficiency?
  • How do you address security concerns in Java applications? Explain common security vulnerabilities and how to mitigate them.
  • Discuss code quality and best practices for clean code in Java development.

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