Forty-six Java interview questions and answers for hiring

Identifying, attracting and retaining top talent is paramount to an organization’s success, particularly in software development. Java is one of the most used programming languages, the backbone for countless applications and enterprise solutions. Therefore, effectively identifying candidates who possess not just proficiency in Java but also the critical thinking and problem-solving skills necessary for innovation is essential.

This blog lists the top Java interview questions designed to empower hiring managers in their quest to uncover exceptional talent. These questions include a broad spectrum of topics, ranging from fundamental principles of object-oriented programming to advanced concepts and Java libraries. By leveraging such insights, organizations can improve their hiring processes, ensuring they build agile, capable Java development teams that are well-equipped to drive their business forward.

Whether one is a seasoned recruiter or new to the technical hiring process, this guide offers insights to make informed recruitment decisions and build a knowledgeable and capable Java development team.

Hiring decisions should be driven by data-backed insights and well-planned core Java interview questions that can help identify and hire top talent. Structured Java programming interview questions enable recruiters to gather unbiased insights into the abilities of candidates so they can reduce the time to hire and make informed hiring decisions.

After candidate screening, Java programming interview questions should be integrated into the recruitment process. Pre-employment assessments can be administered first to screen candidates suitable for the job and help ensure that only the top candidates are cleared for the interview round.

Hiring managers can consider using pre-employment assessments relevant to the organization’s open position to evaluate candidates for their technical skills and knowledge, coding expertise, soft skills, cognitive abilities, and more. Additionally, pre-employment assessments can provide recruiters with deep insights into the candidates, ensuring a suitable cultural fit and helping curate a targeted set of core Java interview questions for different candidates.

Here are some of the Java interview questions for freshers with answers below-

1. Give two differences between Java and C++.
   C++ is only a compiled language, whereas Java is compiled and interpreted. Java programs are machine-independent, whereas a C++ program can run only in the machine in which it is compiled. Java also supports garbage collection, which is not a native feature in C++.
2. How is an array list different from a vector?
   An ArrayList is not synchronized and is generally faster for non-threaded applications, while a Vector is synchronized, making it thread-safe but slower due to the synchronization overhead. Additionally, ArrayList can grow dynamically, whereas Vector doubles its size when it needs to expand.
3. How to override a static or private method in Java?
   It is impossible to override a static or private method.
4. Explain what break and continue statements are.
   Break statements terminate loops early based on certain conditions and pass control from the loop to the statement immediately after the loop. On the other hand, continue statements are used to skip the current loop iteration. After the current iteration is skipped, the next iteration begins immediately.
5. What is a map in Java?
   In Java, a Map is an object that maps keys to values. A map cannot include duplicate keys. Each key can be mapped to at least one value. It models the mathematical function abstraction.
6. What is object cloning?
   Object cloning in Java refers to making an exact copy of an object with the clone() method, which is part of the Cloneable interface.
7. What are errors and exceptions in Java?
   Error is an illegal operation performed by users that causes abnormalities in the program. Exceptions are the unexpected events or conditions that come while running the program; exception disrupts the normal flow of the program’s instructions.
8. Are default parameter values supported in Java?
   Java does not support default parameter values; instead, method overloading, optional parameters, or the Builder pattern is used to achieve similar functionality.
9. Define reflection and its uses.
   The term reflection describes the inspection capability of a code on another code, either of itself or its system, and modifies it during runtime.
10. Define marker interface.
    Marker interfaces, or tagging interfaces, have no methods and constants defined in them. They are there to help the compiler and JVM to get run time-related information regarding the objects.
11. Why is Java not a pure object-oriented language?
    Java supports primitive data types, including boolean, byte, short, char, int, float, long, and double, and these operate outside of objects. Therefore, it is not a pure object-oriented language.
12. What is an instance variable?
    Instance variables are those that are accessible by all the methods in the class. They are declared outside the methods and inside the class.
13. What do you mean by data encapsulation?
    Data Encapsulation is an object-oriented programming concept that hides data attributes and their behaviours in a single unit.
14. What is constructor overloading?
    Constructor overloading creates multiple constructors in the class with the same name but different constructor parameters.

1. What distinguishes fail-fast iterators from fail-safe iterators?
   Fail-fast iterators will throw a ConcurrentModificationException if the collection is altered during iteration. In contrast, fail-safe iterators do not throw exceptions when the collection is modified; instead, they operate on a clone of the collection.
2. What is the role of the volatile keyword in Java, and how does it connect to the happens-before relationship?
   The volatile keyword in Java guarantees that changes to a variable can be immediately visible to other threads. It creates a happens-before relationship, which means that any write operation to a volatile variable will be visible to any subsequent read of that variable by any thread.
3. What are the key differences between ArrayList and LinkedList? When would you choose one over the other?
   The choice of ArrayList and LinkedList comes down to the trade-off between array access and array modification. ArrayList, which utilizes a dynamic array, offers quick random access but has slower insertion and deletion times. Conversely, LinkedList employs a doubly-linked list structure, allowing for faster insertions and deletions but at the expense of slower random access.
4. What is the concept of functional interfaces in Java 8, and how do they relate to lambda expressions?
   A functional interface can be understood through a simple analogy. It resembles a straightforward job description that specifies a single task. For instance, consider a ‘Chef’ position where the only requirement is to ‘cook a meal.’ Lambda expressions act like quick, on-the-spot hires for these straightforward jobs. Instead of undergoing a lengthy hiring process and drafting an entire employee contract, one can say, ‘Here is how to cook the meal,’ and the task gets done.
5. What is the purpose of the Fork/Join framework in Java?
   The Fork/Join framework, introduced in Java 7, facilitates the parallel processing of tasks. It divides tasks into smaller subtasks (fork), executes them concurrently, and then combines the results (join). This framework is beneficial for processing large datasets or implementing recursive algorithms like quicksort.
6. Differentiate between the Runnable and Callable interfaces in Java concurrency.
   Both Runnable and Callable interfaces define tasks that threads can execute. Runnable doesn’t return a result and cannot throw checked exceptions, making it simpler for basic tasks. In contrast, Callable can return a result and can throw checked exceptions. Callable is typically used with ExecutorService for asynchronous task execution. One can think of Runnable as a worker who completes the job without providing any information beyond success or failure, while Callable represents a more professional worker who not only performs the task but also compiles a detailed report on the results and any issues encountered during execution.
7. What is the Singleton pattern? Provide an example of a thread-safe implementation in Java.
   The Singleton pattern guarantees that a class has only one instance, which helps manage global states or resources and scenarios requiring centralized control, such as logging or database connections.
8. What is the Observer pattern, and how can it be implemented using Java’s built-in classes?
   The Observer pattern allows objects to be automatically notified when another object changes, which is great for loose coupling in systems like GUIs or event handling. In Java, developers now use PropertyChangeListener instead of the deprecated Observable. A quick example is a WeatherStation class that utilizes PropertyChangeSupport.
9. What are the distinctions between abstract classes and interfaces in Java 8 and beyond?
   In Java 8 and later, abstract classes are partially implemented classes that cannot be instantiated. They are intended to provide a common foundation for related classes within a hierarchy. Interfaces act as contracts defining abstract methods applicable to unrelated classes, enabling them to share similar abilities.
10. How is the hashCode() method connected to the equals() method? What are the consequences of overriding one without the other?
    When the .equals() method is invoked on objects, Java compares their hash codes to perform the check. If one overrides the equals() method in classes, it is essential to override the hashCode() method, as objects considered equal must have equal hash codes. Not adhering to this can lead to problems with hash-based collections.
11. What is the purpose of the synchronized keyword in Java, and what are its limitations?
    The synchronized keyword serves an essential function in Java when used correctly. It controls access to a block of code or method in multi-threaded environments, ensuring that only one thread executes the synchronized code at any given time. However, overusing it can lead to performance issues due to thread contention, where multiple threads compete for resources. Additionally, if not used carefully, it can result in deadlocks.
12. What are the key distinctions between the Comparable and Comparator interfaces, and when should each be used?
    The Comparable and Comparator interfaces both facilitate the sorting of objects but serve different purposes. One can implement a Comparable in the class to define its natural ordering and use it when there is a clear default way to compare objects of that class. In contrast, Comparator is a separate interface that allows for the definition of custom orderings. It is particularly beneficial when multiple sorting methods are needed or when the original class cannot be modified.
13. What are the differences between HashMap, LinkedHashMap, and TreeMap?
    Here’s a summary of the differences:

    HashMap: It is unordered, allows null keys and values, and has an average time complexity of O(1) for basic operations.
    LinkedHashMap: It maintains the insertion order (or access order), allows null keys and values, and is slightly slower than HashMap.
    TreeMap: It sorts entries by keys, does not allow null keys, and has a time complexity of O(log n) for basic operations.

14. What is the concept of try-with-resources in Java, and how does it enhance resource management?
    Try-with-resources is a language feature introduced in Java 7 that automatically closes resources implementing AutoCloseable. This construct simplifies resource management and helps prevent resource leaks.
15. Explain the differences between unchecked and checked exceptions in Java. When should each be used?Checked and unchecked exceptions indicate how they should be handled. Checked exceptions are verified at compile-time, requiring the programmer to catch or declare them in the method signature. They are typically used for recoverable conditions that may occur. Unchecked or runtime exceptions are not checked at compile-time, meaning the programmer does not need to take any action, such as checks or declarations. These exceptions usually indicate programming errors and bugs, such as null pointer references or illegal arguments, which should be addressed and fixed.
16. What is the purpose of the transient keyword in Java?
    The transient keyword prevents passwords from being included in the serialized data when a User object is serialized. This keyword indicates which fields should not be serialized during the conversion of an object to a byte stream. It is also useful for fields that hold temporary data or derived values that can be recalculated. Additionally, it can be essential for fields containing data that do not support serialization.
17. What is the purpose of default methods in interfaces introduced in Java 8?
    Previously, adding a method to an interface required implementing it in every class that utilized it, which could be cumbersome, especially in large codebases. With default methods in Java 8, developers can provide a default implementation of a method directly within the interface, allowing existing code to remain unaffected.
18. Explain the concepts of method overloading and method overriding. How do they relate to polymorphism?
    Method overloading and method overriding are two kinds of polymorphism in Java. Method overloading occurs when multiple methods within the same class share the same name but have different parameters. In contrast, method overriding occurs when a subclass offers a different implementation for a method defined in its superclass, which is also known as runtime polymorphism.
19. What is the purpose of the Java.util.concurrent package? Provide examples of classes from this package and their use cases.
    The Java.util.concurrent package is designed for concurrent programming in Java, offering thread-safe, high-performance alternatives to traditional synchronization methods. Key classes in this package include:

    ExecutorService: Manages and executes tasks asynchronously.
    ConcurrentHashMap: A thread-safe variant of HashMap.
    CountDownLatch: Synchronizes threads that are waiting for operations to complete.
    AtomicInteger: Facilitates lock-free atomic operations on integers.

20. Explain the difference between synchronized collections and concurrent collections in Java.
    Synchronized collections are found in Java.util.Collections (e.g., Collections.synchronizedList) are thread-safe, but they achieve this by synchronizing all methods, which can lead to contention in multi-threaded environments. In contrast, concurrent collections are located in Java.util.concurrent (e.g., ConcurrentHashMap), allow multiple threads to access them concurrently by using more fine-grained locks or lock-free algorithms, thereby enhancing performance in concurrent settings.

Here are some Java interview questions for ten years of experienced professionals-

1. What is the concept of lock-free programming in Java, and what are its advantages and challenges?
   Lock-free programming aims to achieve thread safety in multi-threaded environments without relying on locks. It utilizes atomic operations and compare-and-swap (CAS) mechanisms to accomplish this. This programming approach provides advantages such as improved scalability, elimination of deadlocks, and often enhanced performance. However, it also presents challenges, including more complex implementation, the potential for ABA (Atomic-Borrow-Assign) problems, and increased memory usage.
2. How would one design a highly scalable, distributed caching system using Java?
   One could utilize technologies like Hazelcast or Apache Ignite to create a scalable, distributed caching system. Key considerations for the design would include:

   A partitioning strategy for effective data distribution
   Replication methods for ensuring fault tolerance
   A consistency model, such as eventual consistency versus strong consistency
   Eviction policies
   Network topology and communication protocols
   Integration with existing systems.

3. How would one design and implement a custom annotation processor in Java?
   To create a custom annotation processor, one should implement the javax.annotation.processing.Processor interface. The key steps involved include defining the annotation, implementing the processor logic, and registering the processor using the ServiceLoader mechanism. Common use cases for custom annotation processors include code generation, compile-time checks, and metadata processing.
4. What is the difference between weak references and soft references in Java?
   A weak reference allows objects to be garbage collected when no strong references exist, and it is commonly used in WeakHashMap. In contrast, a soft reference retains objects until the system is low on memory and is often used for caching.
5. Describe the Garbage Collection process in Java. How can one tune GC for a high-throughput, low-latency application?
   Java’s Garbage Collection (GC) automatically identifies and removes unused objects to free up memory. For applications that handle large amounts of data quickly without delays, one can make several adjustments to the GC mechanism, such as using newer GC types like G1GC or ZGC for faster cleanup with shorter pauses, adjusting the amount of memory Java can use, and controlling the number of ‘cleaner’ threads used by the GC.
6. Describe the internals of the ConcurrentHashMap class. How does it achieve its high level of concurrency?
   ConcurrentHashMap employs a segmented structure, dividing the map into segments that can be locked independently. It utilizes CAS operations for updates when feasible, which reduces contention. It does not use locks at all for read operations, enabling high concurrency. The internal structure has evolved across different Java versions to enhance performance and minimize memory usage.
7. How would one implement a thread-safe singleton in Java? Discuss the trade-offs of different approaches.
   Several approaches exist for implementing a thread-safe singleton, including eager initialization, lazy initialization with double-checked locking, the initialization-on-demand holder idiom, and the enum singleton. Each approach has trade-offs regarding thread safety, lazy loading, and serialization behaviour. The enum singleton is often regarded as the best option due to its conciseness, free serialization support, and inherent thread safety.
8. Explain the concept of reactive programming in Java. How does it vary from traditional imperative programming?
   Reactive programming is a declarative programming paradigm focusing on data streams and change propagation. In Java, it is commonly implemented using libraries like RxJava or Project Reactor. This approach differs from traditional imperative programming in that it emphasizes the flow of data and responses to events rather than the sequential execution of commands. It can result in more scalable and responsive systems, particularly for I/O-bound applications.
9. How does the Java Memory Model relate to the happens-before relationship? Provide examples of how this impacts concurrent programming.
   The Java Memory Model (JMM) establishes the rules governing how Java programs interact with computer memory, especially in multi-threaded environments. The happens-before relationship is a crucial aspect of this model, ensuring that memory operations in one thread are correctly ordered concerning operations in another thread. This relationship is vital for writing correct concurrent code.
10. How does the CompletableFuture class improve asynchronous programming in Java?
    CompletableFuture offers a more flexible and functional approach to asynchronous programming than the Future class. It enables the chaining of tasks using methods such as .thenApply(), .thenAccept(), and .thenCompose().
11. What is the concept of Java agents, and how can they be used for application monitoring and profiling?
    Java agents are specialized tools that monitor and modify the execution of a Java program without requiring changes to the original code. They can be thought of as invisible assistants that can:

    Observe the internal workings of the program
    Make minor adjustments to its functionality
    Gather information about the program’s activities
    Measure the duration of various parts of the program
    Monitor memory usage
    Detect if certain sections of the program become unresponsive
    To utilize a Java agent, one typically writes specific code instructing the agent on what to monitor, packages this code into a JAR file, and specifies the agent to be used when running the Java program.

12. What is the class loading process in Java, and how can one implement a custom class loader? What are some use cases for doing so?
    Class loading in Java consists of three primary steps:

    Loading: The class loader reads the .class file and creates a Class object.
    Linking: This step involves verifying, preparing, and resolving symbolic references.
    Initialization: Static variables are initialized, and static initializers are executed.
    To create a custom class loader, one would extend the ClassLoader class and override the findClass method.
    Use cases for custom class loaders include:
    Loading classes from non-standard locations, such as databases or networks
    Implementing plugin systems
    Modifying bytecode dynamically
    Enforcing security policies
    Hot-swapping classes in a running application

Mercer | Mettl offers an expansive test library of scientifically validated, data-driven tests that hiring managers can use to optimize and streamline recruitment processes effortlessly. It helps improve the hiring process with a wide selection of tested, data-driven assessments. These tools make it easier for hiring managers to choose the most suitable candidates.

Hiring managers can use customizable assessments for Java developers to check candidates’ technical skills and coding abilities. It ensures that the evaluations meet the organization’s specific needs and accurately represent the candidate’s capabilities.

Companies can reduce the chances of making uninformed hiring decisions by using effective hiring practices, such as pre-employment assessments and structured interview questions for Java programming. This approach guarantees that candidates have the required skills and fit the company’s culture.

By focusing on these practices, organizations can improve recruitment strategies, build strong teams, and achieve long-term success.