15 tricky Java interview questions and answers

Hiring the right Java developer is crucial for your business. A bad hire can lead to inefficient code, missed deadlines, and poor team dynamics. That’s why it’s essential to vet every candidate thoroughly before you bring them on board. Part of that process is asking the right questions when interviewing your Java developer candidates.

With our list of 15 tricky Java interview questions, you can assess your applicants’ theoretical knowledge, practical skills, and ability to think quickly and creatively. Coupling these questions with a Java skills test will ensure you select the best talent for your team.

What are tricky Java interview questions?

Java is a widely used programming language known for its object-oriented programming (OOP) capabilities. It’s instrumental in creating web applications, mobile apps, and software tools. 

When we talk about “tricky” Java interview questions, we’re referring to queries that delve deeper into the Java language – questions designed to evaluate a candidate’s understanding of Java’s concepts and principles. 

Compared to regular Java interview questions, tricky questions go beyond basic syntax and principles. They aim to assess candidates’ ability to solve complex problems, implement intricate designs, and use Java’s advanced features. 

These questions are meant to be answered in an interview setting, as they require candidates to explain concepts or outline code structure rather than write lines of code. Detailed coding tasks are better suited for Java skill tests, such as TestGorilla’s Java Data Structures and Java Debugging skill tests.

Why include tricky Java questions in your interviews?

Incorporating tricky Java questions in your interview process allows you to identify candidates who can handle complex Java-based tasks. 

The value of these questions isn’t just in assessing technical knowledge, though. They also help you understand the candidate’s problem-solving abilities, communication skills, and approach under pressure. 

Below, we cover the key reasons why including challenging Java questions is a win for your hiring process.

Identify in-depth understanding and application of Java

Complex Java questions explore deeper layers of a candidate’s knowledge. By these questions, you can determine how well your applicants grasp advanced Java concepts and how they apply them in real-world scenarios. 

Candidates’ responses can indicate their ability to solve problems, think critically, and make decisions – all vital aspects of a Java developer role.

Evaluate thought processes

By asking tricky Java questions in interviews, you give candidates a chance to showcase their unique perspectives and ability to think on their feet. Their answers shed light on their thought processes – including how they approach a problem, the logic they follow, and the solutions they propose. These responses offer deep insights into their cognitive abilities.

Assess communication skills

Most Java developers need to communicate technical topics effectively to do their job. And in an interview setting, candidates’ ability to articulate their thoughts is tested as much as their technical prowess. 

Tricky Java questions take this further by requiring candidates to express complex ideas clearly and succinctly. You get to observe how well they communicate intricate concepts, which is essential for team collaboration.

Create a more thorough evaluation process

Complex Java questions can bring an extra dimension to your interview process, complementing the information you get from resume screening and Java tests. While skill tests accurately assess a candidate’s coding capabilities, tricky interview questions enable candidates to demonstrate their knowledge verbally. This allows for a more well-rounded evaluation of each applicant.

Overall, including tricky Java questions in your interview process helps you identify top candidates who truly understand and can apply Java’s advanced principles. Along with skill tests, personality tests, and behavioral interview questions, these questions provide a robust and comprehensive evaluation system, helping you make the best hiring decisions for your business.

15 tricky Java interview questions and answers

Listed below are 15 tricky Java questions you can use in your interviews. We’ve provided examples of solid, thoughtful replies to each question – but there’s often more than one “correct” answer. Candidates might offer equally valid yet different responses. 

1. What is the Java Memory Model, and how does it support multi-threaded applications?

Answer:

The Java Memory Model (JMM) is an abstraction of the underlying hardware’s memory architecture. It provides a framework for understanding how threads interact through memory. In the context of multi-threaded applications, the JMM provides guarantees about when changes made by one thread are visible to others.

JMM offers a set of rules that programs must follow to ensure consistent behavior in multi-threaded environments. It addresses the “happens before” relationship, which is the foundation of visibility and ordering guarantees. If one action happens before another, the memory effects of the first will be visible to the second.

2. Explain the differences between the synchronized keyword, Lock, and Semaphore for managing thread synchronization.

Answer:

Synchronized is a keyword in Java that controls access to an object to prevent data inconsistencies due to concurrent access. It provides an implicit lock and release mechanism but no explicit methods to lock or unlock, giving the programmer less control.

The Lock interface in the java.util.concurrent.locks package provides more extensive operations than the implicit locking mechanism. It allows more flexible structuring and can be non-blocking, making it more resilient to deadlock situations.

A Semaphore controls access to a shared resource using permits. It’s used when the number of resources to access is fixed. It allows multiple threads to access a fixed set of resources in parallel.

3. What are the benefits and drawbacks of using Java’s Executor Framework over directly creating threads?

Answer:

The Executor Framework simplifies managing and controlling threads. It comes with several benefits. It provides thread pooling, which helps to reduce the overhead of thread creation and destruction. It also offers convenient scheduling capabilities and lets you more finely control how tasks are executed.

However, there are some drawbacks. The Executor Framework introduces an additional layer of abstraction, which can make it more complex. Also, a thread pool can cause resource exhaustion if not correctly configured.

4. Describe the concept of Java NIO (New I/O) and compare it with traditional I/O (java.io)

Answer:

Java NIO (New I/O) is an alternative I/O API introduced in Java 1.4. It provides non-blocking I/O capabilities, allowing a thread to handle multiple input and output channels simultaneously. This leads to more efficient use of threads and can improve the performance of I/O-intensive applications.

In contrast, the traditional I/O API (java.io) is blocking. This means that when a thread invokes a read or write operation, it gets blocked until the operation is completed.

Furthermore, NIO provides additional features unavailable in java.io, such as file system notifications, scalable network sockets, and character set encoders and decoders. But NIO is generally more complex to use than java.io.

5. How do you handle concurrent modifications to a collection using the ConcurrentHashMap and CopyOnWriteArrayList?

Answer:

Both ConcurrentHashMap and CopyOnWriteArrayList are designed for multithreaded environments and are part of the java.util.concurrent package.

ConcurrentHashMap allows high concurrency for updates (via segment locking) and retrieval operations. Iterators of this class reflect the state of the map at the time they were created, so they don’t throw ConcurrentModificationException. It’s particularly useful when you need a thread-safe map where reads are more common than writes.

CopyOnWriteArrayList uses a different approach. Any modification operation (add, set, remove, etc.) on this list results in a new copy of the underlying array, while read operations work on the existing copy. This approach eliminates the need for locking for reads, making CopyOnWriteArrayList excellent when iteration is a more common operation than modification.

6. What are CompletableFutures in Java, and how do they enable asynchronous programming?

Answer:

CompletableFuture is a Future that may be explicitly completed by setting its value and status. It’s part of the java.util.concurrent package and supports asynchronous programming in Java.

CompletableFuture offers a variety of methods to handle the results of asynchronous computations. You can chain together multiple asynchronous operations, handle exceptions, combine results, or run additional tasks when the computation completes.

By allowing you to write non-blocking code, CompletableFuture makes it possible to maximize the usage of computational resources. It makes it possible to perform other tasks instead of waiting for a long-running operation to complete.

7. Explain the different types of garbage collectors available in the JVM and their use cases (e.g., G1, CMS, Parallel).

Answer:

The JVM comes with several garbage collectors, each designed for specific use cases:

1. Serial Garbage Collector. This collector is single-threaded and best suited for simple, single-processor machines. It’s designed for small applications with low memory footprints.

2. Parallel Garbage Collector (Throughput Collector). This collector uses multiple threads for garbage collection, making it suitable for medium to large-sized data sets in multiprocessor environments. It aims to maximize throughput by minimizing the time spent in garbage collection.

3. Concurrent Mark Sweep (CMS) Collector. The CMS collector aims to minimize application pauses by doing most of its work concurrently with the standard application threads. It’s best suited for responsive applications that can’t tolerate long garbage collection pauses.

4. Garbage-First (G1) Collector. The G1 collector is designed for applications running on multiprocessor machines with large memories. It aims to provide high throughput and predictable pause times by dividing the heap into regions and prioritizing garbage collection in the regions that contain the most garbage.

Each garbage collector has its trade-offs, and the best choice depends on the specific needs of your application.

8. How can you achieve high-performance serialization and deserialization in Java, considering various formats (JSON, XML, Protocol Buffers)?

Answer:

To achieve high-performance serialization and deserialization in Java, you can use various libraries and techniques depending on the data format.

For JSON, popular libraries include Jackson and Gson. These provide comprehensive APIs to convert objects to JSON and vice versa. Jackson is generally faster and has a smaller memory footprint than Gson.

For XML, JAXB is a standard Java API for object/XML mapping and provides mechanisms for efficient serialization and deserialization.

Protocol Buffers (protobuf), a binary protocol developed by Google, allows faster serialization/deserialization than JSON and XML. It also results in smaller payloads. For Java, protobuf provides an API to generate Java classes from a .proto file, which you can use for serialization and deserialization.

In all cases, custom serializers and deserializers can improve performance, as you can optimize them according to the specific application’s needs.

9. What is the Java Flight Recorder (JFR), and how can it be used to monitor and diagnose Java applications?

Answer:

Java Flight Recorder (JFR) is a tool integrated into the Java Virtual Machine (JVM) for collecting diagnostic and profiling data about a running Java application. It has a minimal performance overhead, making it safe to use in production environments.

JFR can capture many event types covering areas like CPU usage, memory management, network usage, I/O, garbage collection, Java exceptions, and more. It’s a valuable tool for understanding the behavior of your Java application and the underlying JVM.

The collected data can be analyzed with the Java Mission Control (JMC) tool, providing a graphical interface with detailed metrics and diagnostics. Therefore, JFR helps you identify performance bottlenecks and memory leaks.

10. Describe the principles of the Dependency Injection design pattern and how it can be implemented using frameworks like Spring.

Answer:

Dependency Injection (DI) is a design pattern where an object’s dependencies are injected by an external entity instead of the object creating them itself. This promotes loose coupling, enhances testability, and enables better separation of concerns.

DI has three types: constructor injection, setter injection, and field injection.

• In constructor injection, dependencies are provided through the class constructor. 

• Setter injection involves providing the dependency using a setter method. 

• Field injection injects dependencies directly into the fields.

The Spring framework provides comprehensive support for implementing DI. In Spring, the ApplicationContext is the container that provides DI features.

You define your beans and their dependencies in a configuration file (XML, Java Config, or Annotation-based). Spring’s container then creates these beans, resolves their dependencies, and provides the right beans when needed. The @Autowired annotation is often used to wire beans automatically.

Spring’s DI helps in building flexible, reusable, and testable Java applications.

Question 11

How does the Stream API in Java 8 handle parallelism, and what considerations should you consider when using it?

Answer 11

The Stream API in Java 8 can facilitate parallel execution of operations. By invoking the parallelStream() method instead of stream() on a collection, you can create a stream that processes elements in parallel.

Underneath, the Stream API uses the Fork/Join Framework to divide tasks into smaller subtasks and assign them to different threads. It uses the common ForkJoinPool, which by default has as many threads as you have processors, as defined by Runtime.getRuntime().availableProcessors().

However, you must consider several factors when using it. Parallel streams work best when: 

1. Your data is large enough. Smaller data sets may not benefit much from parallelism and might even suffer due to overhead. 

2. Your tasks are CPU-intensive. If they’re I/O bound, parallelism might not improve performance.

3. Your operations are independent. Ensure your lambda functions don’t have a shared mutable state to avoid thread-safety issues.

12. Discuss the pros and cons of using the ThreadLocal class in Java for managing thread-local variables

Answer:

ThreadLocal in Java provides a way to create variables that can only be read and written by the same thread. This encapsulation makes having per-thread singleton instances and per-thread context information easy.

Pros include:

• Thread safety. Since each thread has its own instance, it doesn’t need synchronization.

• Performance. It can lead to more efficient code by reducing the overhead of synchronization.

Cons include:

• Memory leaks. If you don’t remove a ThreadLocal key from its ThreadLocalMap, it could potentially lead to memory leaks, especially in thread pool scenarios. 

• Misuse. You should use ThreadLocal sparingly, as it can make the code harder to reason about and maintain.

It’s important to use ThreadLocal responsibly and clean up any set variables when they’re no longer needed.

13. How can you implement a custom ClassLoader in Java, and what are the potential use cases?

Answer:

To implement a custom ClassLoader in Java, you typically extend the ClassLoader class and override the findClass() method. This method is called after the parent ClassLoader fails to find the class in its classpath.

The key steps typically include the following: 

1. Convert the binary name of the class to a file name. 

2. Read that file into a byte array. 

3. Call defineClass() to convert the byte array into a Class object.

Use cases for a custom ClassLoader could include:

• Loading classes dynamically from sources other than the classpath, such as a network location or a database.

• Implementing hot-swapping of classes in a running JVM.

• Isolating or sandboxing specific parts of an application by controlling class loading behavior.

Creating a custom ClassLoader should be done with caution due to the complexity involved.

14. Describe the java.lang.instrument package and how it enables bytecode manipulation in Java.

Answer:

The java.lang.instrument package provides services that enable Java programming language agents to instrument programs running on the JVM. It allows an agent (in the form of a JAR file) to transform the bytecode of classes loaded by the JVM at runtime.

The key class in the package is Instrumentation, which provides methods to retransform classes and query class status. An agent receives an instance of Instrumentation in its premain or agentmain methods.

The instrument package is primarily used for profiling, monitoring, and logging purposes. For instance, you can use it to measure the execution time of a method, monitor memory usage, or add additional logging for debugging purposes.

15. What is Double-Checked Locking, and why is it considered an anti-pattern for implementing Singleton in multi-threaded environments?

Answer:

Double-Checked Locking is an idiom that reduces synchronization overhead when implementing Singleton classes in a multithreaded environment. In this pattern, the Singleton instance is checked twice before it’s initialized – once without synchronization (first check) and once with synchronization (second check).

The problem with Double-Checked Locking in Java arises due to the Java Memory Model. Without the proper use of the volatile keyword, there’s a risk that the Singleton instance will not be seen as fully initialized by all threads. 

This is because the Java compiler is allowed to reorder instructions, meaning that the Singleton instance could be set before it’s fully initialized. Consequently, another thread could see it as not null and use it before it’s fully initialized, leading to unpredictable results.

How to add tricky Java interview questions to your hiring campaign

Complex Java interview questions can provide a better understanding of a candidate's technical competency. But they shouldn’t be the only tool you rely on when evaluating potential hires. 

It’s best to use them as part of a comprehensive screening process that also considers applicants’ problem-solving skills, critical thinking, teamwork, and cultural fit. 

This is where a pre-employment testing platform like TestGorilla can be highly beneficial.

With TestGorilla, you can access a range of scientifically validated tests tailored for assessing tech professionals, including Java coding tests. These tests allow you to objectively measure a candidate’s proficiency in Java and other related technical areas.

However, TestGorilla’s capabilities extend far beyond technical testing. Our platform also includes personality tests like the DISC Personality and Big 5 (OCEAN) Personality tests. These can help you assess if a candidate’s temperament and working style are compatible with your team’s culture and dynamics.

Additionally, you can add custom questions to your assessment to cover specific areas relevant to your business. For instance, you can include problem-solving questions to evaluate a candidate’s logical thinking and creativity and behavioral questions to uncover their teamwork and conflict resolution approach.

Another unique feature of TestGorilla is the one-way interview system. This allows candidates to respond to your preset interview prompts at their convenience, and you can review their responses whenever it suits you. This approach offers greater flexibility to both you and the candidate while also standardizing the interview process to ensure fairness.

Combining tech skill tests with behavioral and personality assessments gives you a complete picture of each candidate’s skills, personality, and potential fit within your team. You can make better hiring decisions and avoid bias along the way.

How TestGorilla can help you find the best Java developers

Tricky Java interview questions are vital in assessing your Java developer candidates’ technical competency. However, you should also use them as part of a holistic assessment approach covering soft skills, cognitive abilities, and cultural fit. Pairing these elements ensures you have a comprehensive view of each candidate’s strengths and areas for development.

TestGorilla provides an extensive library of tests, customizable assessments, and one-way interview tools to help you do just that. To get hands-on experience with our platform and its features, sign up for your free TestGorilla plan account today.