Java is a very popular programming language, found everywhere from Android apps to the Internet of Things (IoT). In fact, Java was #1 in job postings in 2019, according to Codeplatoon. Considering its ubiquity, it’s no surprise that there continues to be a high demand for professionals who are proficient in Java.

That’s why we are presenting this collection of the most common Java 8-related questions and answers found in job interviews. It’s not enough that you have extensive training and understanding in a given subject (in this case, for instance, Java). You need to organize your thoughts, review the information on the topic, and focus on the most likely asked questions.

We’ll begin with the basics of Java 8 and work our way up to the tougher questions. Once you get through this material, you will be in a better position to own that critical interview!

What is Java 8?

Before we tackle the questions, let’s do a little fundamental review here, and nail down what Java 8 is. Java 8 was released on March 14, 2014, and is described by Java as “…the latest release for Java that contains new features, enhancements and bug fixes to improve efficiency to develop and run Java programs.”

Check out the video below on corporate level Java Interview Questions and Answers in detail to help beginners in a theoretical way for a better learning experience.

Java 8 Interview Questions - Basic Level

Here are some Java8 interview questions to get us warmed up.

1. What are the newly added features in Java 8?

The latest version has:

Java 8 introduced several significant features and enhancements to the language and platform. Some of the key features include:

Lambda Expressions: Lambda expressions allow developers to write more concise and expressive code by enabling the use of functional programming constructs. They simplify the syntax for defining anonymous functions, making it easier to work with collections and perform operations like filtering, mapping, and reducing.

Stream API: The Stream API provides a new way to work with collections in Java. It allows developers to perform bulk operations on collections, such as filtering, mapping, sorting, and aggregating, using functional-style operations. Streams enable more declarative and concise code, improving readability and reducing boilerplate code.

Default and Static Methods in Interfaces: Java 8 introduced the ability to define default and static methods in interfaces. Default methods provide a way to add new methods to interfaces without breaking existing implementations, while static methods allow interfaces to have utility methods that can be called without an instance.

New Date and Time API: Java 8 introduced the java.time package, which provides a modern date and time API to address the shortcomings of the older java.util.Date and java.util.Calendar classes. The new API is more comprehensive, flexible, and easier to use, with support for operations like date arithmetic, time zones, and formatting.

Optional: Optional is a new class introduced in Java 8 to represent optional values, reducing the need for null checks and improving code clarity and safety. Optional provides methods to handle the presence or absence of a value in a more functional and idiomatic way.

Concurrency and Parallelism Enhancements: Java 8 introduced enhancements to the ConcurrentHashMap class, making it more efficient for concurrent updates. It also introduced the CompletableFuture class, which provides a flexible and composable way to work with asynchronous computations and handle concurrency-related tasks.

Method References: Method references allow developers to refer to methods or constructors using a concise syntax, improving code readability and reducing boilerplate code. They complement the use of lambda expressions and provide an alternative way to achieve the same functionality.

Overall, Java 8 introduced several powerful features and enhancements that improve developer productivity, code readability, and performance, making it a significant milestone in the evolution of the Java language.

2. Why was a new version of Java needed in the first place?

There are two main reasons:

  • Dramatic changes in hardware created the need for Java to use current multi-core CPUs more efficiently
  • Enable users to use new Functional Programming (FP) features

3. So, what actual advantages does Java 8 bring?

The advantages include:

  • Code is more concise and readable
  • Code is more reusable
  • Code is more testable and maintainable
  • Code is now both highly concurrent and scalable
  • Users can write parallel code
  • Users can write database-like operations
  • Applications now perform better
  • Code is far more productive

4. What is a Lambda Expression, and why use them?

It’s a function that can be referenced and shared as an object. Lambda Expressions require less coding, provide a means of implementing the Java 8 functional interface, and let users encapsulate one behavior unit to pass around to other code.

5. What is a functional interface?

A functional interface in Java is an interface that contains exactly one abstract method. It is also known as a Single Abstract Method (SAM) interface. Functional interfaces serve as the basis for working with lambda expressions and other functional programming constructs in Java.

In addition to the single abstract method, a functional interface may contain default methods, static methods, and other abstract methods inherited from the Object class. However, it must have only one abstract method to qualify as a functional interface.

Functional interfaces provide a way to represent behaviors as first-class citizens in Java. They enable the use of lambda expressions to define anonymous functions that implement the single abstract method of the interface. This allows developers to write more concise and expressive code, especially when working with APIs that expect functional interfaces as parameters.

6. How are functional interfaces and Lambda Expressions related?

Lambda expressions are applied only to the functional interface’s abstract method.

7. Can users create a personal functional interface?

Yes, they can.

8. What does the term “method reference” mean in the context of Java 8?

Method reference is a Java 8 construct used to reference a method without having to invoke it. It is a compact method of Lambda expression.

9. What is MetaSpace? How does it differ from PermGen?

MetaSpace is a new memory space introduced in Java 8 to replace the Permanent Generation (PermGen) space used in earlier versions of Java. MetaSpace is used to store metadata related to classes and class loaders, such as class definitions, method data, and constant pools.

One of the main differences between MetaSpace and PermGen is that MetaSpace is not fixed in size like PermGen. In PermGen, the size of the memory space is fixed and can lead to OutOfMemoryError if not properly managed. In contrast, MetaSpace dynamically expands and shrinks based on the application's needs, making it more flexible and resilient to memory-related issues.

Another difference is that MetaSpace is allocated from the native heap, whereas PermGen is allocated from the Java heap. This separation allows MetaSpace to be managed more efficiently by the JVM, reducing the risk of memory leaks and improving garbage collection performance.

Overall, MetaSpace offers several advantages over PermGen, including improved memory management, better performance, and reduced risk of memory-related errors.

Now that we know basic java 8 interview questions, lets check the intermediate level questions.

Java 8 Interview Questions - Intermediate Level

Now let’s increase the difficulty a bit.

10. What is optional, and what is it best used for?

Optional is a new container class defined in the java.util package, and used to represent optional values that either exist or do not exist. Optional’s chief benefit is avoiding null checks, and there are no longer any “NullPointerException” results at run-time.

11. What is Type Inference?

Type inference helps the compiler determine the argument types by looking at each method invocation and corresponding declaration.

12. List some Java 8 Date and Time API’s

The core API classes are:

  • LocalDate
  • LocalTime
  • LocalDateTime

13. Why are default methods needed in the interface?

Default methods let you add new functionality to your libraries’ interfaces and ensure binary compatibility with older code written for the interfaces.

14. What is Java 8 StringJoiner class used for?

Java 8 StringJoiner class constructs a sequence of characters separated by a delimiter so that users can create a string by passing delimiters such as hyphens and commas.

15. Describe the more commonly found functional interfaces in the standard library.

Although many functional interfaces exist, these are the one's users most likely encounter:

  • Function. Takes one argument and returns a result
  • Consumer. Takes one argument and returns no result
  • Supplier. Takes a not argument and returns a result
  • Predicate. Takes one argument and returns a boolean
  • BiFunction. Takes two arguments and returns a result
  • BinaryOperator. It’s like a BiFunction, except it takes two arguments and returns a result, and they are all the same type
  • UnaryOperator. It’s like a Function, but it takes a single argument and returns a result of the same type

16. What is a stream, and how does it differ from a collection?

A stream is an iterator whose function is to accept a set of actions and apply them to each of the elements it contains. A stream represents an object sequence from a collection or other source that supports aggregate operations. Unlike collections, iteration logic implements inside the stream.

Also, streams are inherently lazily loaded and processed, unlike collections.

17. What is a default method, and when does it get used?

The default method involves an implementation, and it is found in the interface. The method adds new functionalities to an interface while preserving backward compatibility with the classes that already implement the interface.

18. What is jjs in Java 8?

Jis is the new executable or command-line tool used at the console to execute JavaScript code.

Now that we learned the java 8 interview questions for intermediate level, let’s have a look at the experienced level questions.

Java 8 Interview Questions - 12 years/ 10 years/ 5 years Experienced Level 

Finally, here come the tough questions.

19. What is Nashorn, and what advantages does it provide?

Nashorn is the new JavaScript processing engine that shipped with Java 8. Previously, the Java platform used Mozilla Rhino. Nashorn offers better compliance with ECMA normalized JavaScript specifications and provides faster run-time performance than its predecessor.

20. What is stream pipelining?

Stream pipelining is the process of chaining different operations together. Pipelining accomplishes this function by dividing stream operations into two categories, intermediate operations, and terminal operations. When each intermediate operation runs, it returns an instance of the stream. Thus, a user can set up an arbitrary number of intermediate operations to process data, thereby forming a processing pipeline.

At the end of the process, there must be a terminal operation to return a final value and terminate the pipeline.

21. How do you print ten random numbers using forEach?

Use the following code segment:

Random random = new Random();

random.ints().limit(10).forEach(System.out::println);

22. How do you get the highest number that exists on a list?

Use the following code segment:

List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);

IntSummaryStatistics stats = integers.stream().mapToInt((x) −> x).summaryStatistics();

System.out.println("Lowest number in List : " + stats.getMin());

23. How do you get the second Friday of next month?

Use the following code segment:

//get the second friday of next month

LocalDate firstInYear = LocalDate.of(date1.getYear(),date1.getMonth(), 1);

LocalDate secondFriday = firstInYear.with(TemporalAdjusters.nextOrSame(DayOfWeek.FRIDAY)).with(TemporalAdjusters.next(DayOfWeek.FRIDAY));

System.out.println("Second Friday on : " + secondFriday);

24. What is a Spliterator?

The term is a blend of “splittable” and “iterator” and is a new feature in Java SE 8. It is used in Stream API to iterate streams in a parallel or sequential order by internal iteration.

25. Explain the difference between predicate and function.

Although they are both functional interfaces, Predicate<T> is a single argument function that returns either true or false. Function<T,R> is also a single argument function, although it returns an object instead. In this case, the “T” represents the type of function input, and the “R” denotes the type of result.

26. What’s the difference between findFirst() and findAny()?

findFirst() returns the first element meeting the criterion, while findAny()returns any element meeting the standard, a feature that is very useful when working with a parallel stream.

27. Explain how Java 8 has enhanced interface functionality with default and static methods. Why were these features introduced?

Java 8 introduced default and static methods in interfaces to enhance the functionality and flexibility of interfaces without breaking existing code.

Default methods allow interfaces to provide default implementations for methods, which can be overridden by implementing classes if needed. This allows interfaces to evolve over time without breaking backward compatibility. It also enables the addition of new methods to interfaces without requiring all implementing classes to implement them.

Static methods in interfaces provide a way to define utility methods that can be called without an instance of the interface. These methods are similar to static methods in classes but are scoped within the interface namespace. They allow interfaces to provide helper methods or utility functions that are relevant to the interface's purpose.

These features were introduced to address the limitations of interfaces in previous versions of Java, where interfaces could only define abstract methods. By allowing interfaces to have default and static methods, Java 8 enables interfaces to act more like abstract classes and provides a way to share code among multiple classes without inheritance.

28. Discuss the significance of the Stream API introduced in Java 8 for data processing. How does it improve application performance and developer productivity?

The Stream API introduced in Java 8 provides a powerful way to perform bulk operations on collections in a functional-style manner. Streams allow developers to express complex data processing tasks using a declarative and fluent API, making code more readable and concise.

One of the main benefits of the Stream API is its ability to enable parallelism and optimize performance for multi-core processors. Streams can automatically parallelize operations on collections, allowing them to take advantage of multiple CPU cores and improve application performance, especially for computationally intensive tasks.

Streams also improve developer productivity by providing a higher level of abstraction for working with collections. They allow developers to express operations like filtering, mapping, sorting, and aggregating in a more expressive and declarative way, reducing the need for boilerplate code and making code easier to understand and maintain.

29. Java 8 introduced Lambda Expressions. Can you explain their impact on the way Java handles functional programming and how they differ from anonymous inner classes?

Lambda expressions in Java 8 revolutionized the way Java handles functional programming by enabling the use of functional-style programming constructs in a more concise and expressive manner. Lambda expressions allow developers to define anonymous functions inline, making it easier to work with collections, perform functional operations, and write more expressive code.

One of the main differences between lambda expressions and anonymous inner classes is their syntax and verbosity. Lambda expressions provide a more concise and readable syntax for defining anonymous functions, making code more expressive and easier to understand. They eliminate much of the boilerplate code associated with anonymous inner classes, resulting in cleaner and more maintainable code.

Another difference is that lambda expressions can capture variables from their enclosing scope without requiring them to be declared as final or effectively final, as is the case with anonymous inner classes. This makes lambda expressions more flexible and easier to use in practice.

Overall, lambda expressions in Java 8 have a profound impact on the way Java handles functional programming, making it more accessible and expressive for developers, and enabling the adoption of functional programming paradigms in Java applications.

30. Describe the new Date and Time API in Java 8. How does it address the shortcomings of the older java.util.Date and java.util.Calendar?

Java 8 introduced the java.time package, which provides a modern date and time API to address the shortcomings of the older java.util.Date and java.util.Calendar classes. The new Date and Time API offers improved functionality, better performance, and more flexibility for working with dates and times in Java.

Some key features of the new Date and Time API include:

  • Immutability: The new API is immutable, meaning that instances of date and time classes are unchangeable once created. This reduces the risk of unintentional modification and ensures thread safety in concurrent environments.
  • Clarity and Readability: The new API provides clear and intuitive class names, such as LocalDate, LocalTime, and LocalDateTime, making code more readable and self-explanatory.
  • Thread Safety: Unlike the older java.util.Date and java.util.Calendar classes, which are not thread-safe, the new Date and Time API is designed to be thread-safe, making it safer to use in concurrent applications.
  • Better Handling of Time Zones: The new API provides better support for working with time zones, including classes like ZoneId and ZoneOffset, allowing developers to easily convert between time zones and perform calculations accurately.

31. What are method references in Java 8, and how do they complement the use of lambda expressions? Provide an example where a method reference is more suitable than a lambda expression.

Method references in Java 8 provide a shorthand syntax for referring to methods or constructors using a concise and readable syntax. Method references complement the use of lambda expressions by providing an alternative way to achieve the same functionality, especially when working with existing methods or constructors.

There are four types of method references in Java 8:

  • Reference to a static method
  • Reference to an instance method of a particular object
  • Reference to an instance method of an arbitrary object of a particular type
  • Reference to a constructor

Method references are often more suitable than lambda expressions when:

The lambda expression simply calls an existing method or constructor: In such cases, using a method reference can make the code more readable and concise.

The lambda expression contains a single method call: If the lambda expression contains only a single method call, it can be replaced with a method reference, improving code clarity and reducing boilerplate.

Here's an example where a method reference is more suitable than a lambda expression:

By studying these questions, you can refresh your Java 8 knowledge and take on that interview with new confidence. But if you would like to brush up on some more Java knowledge, you should consider the following.

// Lambda expression
List<String> names = Arrays.asList("John", "Alice", "Bob");
names.forEach(s -> System.out.println(s));

// Method reference
List<String> names = Arrays.asList("John", "Alice", "Bob");
names.forEach(System.out::println);
In this example, both the lambda expression and the method reference achieve the same result of printing each element of the list to the console. However, the method reference System.out::println provides a more concise and readable syntax, especially when calling an existing method like println().

32. Can you explain the concept of Optional in Java 8? Discuss its advantages over traditional null checks.

Optional is a class introduced in Java 8 to represent optional values, which may or may not be present. It is designed to address the problem of null references and NullPointerExceptions (NPEs) in Java by providing a more structured and safer way to handle potentially absent values.

Some key advantages of Optional over traditional null checks include:

  • Expressive API: Optional provides a set of methods to work with optional values in a more expressive and functional way, such as orElse(), orElseGet(), orElseThrow(), isPresent(), ifPresent(), and map().
  • Reduces NullPointerExceptions: By encouraging developers to explicitly handle the absence of values, Optional reduces the risk of NullPointerExceptions in code, leading to more robust and reliable applications.
  • Improves Readability: Optional makes code more readable by clearly indicating the possibility of a value being absent and providing methods to handle both present and absent cases in a concise and idiomatic way.
  • Encourages Defensive Programming: Optional encourages a defensive programming style by forcing developers to handle potential null values explicitly, rather than relying on implicit null checks or passing null values around.

33. What improvements does Java 8 offer for multithreading and concurrency over its predecessors? Specifically, discuss enhancements in ConcurrentHashMap and CompletableFuture.

Java 8 introduced several enhancements for multithreading and concurrency to improve performance, scalability, and developer productivity. Two key enhancements introduced in Java 8 are improvements to the ConcurrentHashMap class and the introduction of the CompletableFuture class.

ConcurrentHashMap Enhancements

Java 8 introduced several improvements to the ConcurrentHashMap class, making it more efficient for concurrent updates and reducing contention under high concurrency.

ConcurrentHashMap now uses a more fine-grained locking strategy called lock striping, which reduces contention by allowing multiple threads to update different segments of the map concurrently.

Additionally, ConcurrentHashMap provides methods for performing atomic updates and bulk operations, such as compute(), merge(), and forEach(), making it easier to work with concurrent collections.

CompletableFuture

CompletableFuture is a new class introduced in Java 8 to provide a flexible and composable way to work with asynchronous computations and handle concurrency-related tasks.

CompletableFuture allows developers to define chains of asynchronous tasks and compose them using a fluent API, making it easier to express complex asynchronous workflows.

CompletableFuture supports a wide range of operations, including combining multiple asynchronous computations, handling errors and exceptions, and executing tasks asynchronously on different threads.

CompletableFuture also integrates with other asynchronous APIs in Java, such as CompletableFuture.supplyAsync() and CompletableFuture.thenApply(), making it a powerful tool for building responsive and scalable applications.

Job Opportunities in Java 8

Java 8, released in March 2014, marked a significant milestone in the evolution of the Java programming language. It introduced several groundbreaking features and enhancements that revolutionized the way Java applications are developed, making Java 8 developers highly sought after in today's job market.

Lambda Expressions and Functional Programming

One of the most prominent features introduced in Java 8 is lambda expressions. Lambda expressions enable developers to write more concise and expressive code by allowing them to treat functionality as a method argument or pass code more efficiently. This paradigm shift towards functional programming has opened up new opportunities for Java developers to work on projects that leverage the benefits of functional programming paradigms.

Stream API for Data Processing

Java 8 also introduced the Stream API, which provides a powerful and declarative way to perform bulk operations on collections. The Stream API allows developers to express complex data processing tasks, such as filtering, mapping, and reducing, in a more intuitive and efficient manner. With the increasing demand for data-driven applications and real-time analytics, proficiency in the Stream API has become a valuable skill for Java developers.

Modern Date and Time API

Prior to Java 8, working with dates and times in Java was often cumbersome and error-prone. Java 8 addressed this issue by introducing a modern Date and Time API that offers improved functionality and flexibility. The new Date and Time API simplifies common date and time operations, such as date arithmetic, time zone handling, and formatting, making it easier for developers to work with dates and times in their applications.

Concurrency and Parallelism Enhancements

Java 8 brought significant enhancements to concurrency and parallelism, making it easier for developers to write concurrent and scalable applications. Improvements to classes like ConcurrentHashMap and the introduction of CompletableFuture have empowered developers to tackle the challenges of concurrent programming more effectively. As the demand for high-performance and scalable applications continues to grow, expertise in concurrency and parallelism has become increasingly valuable in the job market.

Job Opportunities for Java 8 Developers

With its rich set of features and enhancements, Java 8 has opened up a wide range of job opportunities for developers across industries. Companies are actively seeking Java 8 developers who are proficient in lambda expressions, the Stream API, the new Date and Time API, and concurrency and parallelism concepts. Whether it's developing data-driven applications, building scalable web services, or optimizing performance-critical systems, Java 8 developers are in high demand and can command competitive salaries in today's job market.

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Conclusion

Java 8 has ushered in a new era of possibilities for Java developers, offering a rich array of features and enhancements that have transformed the way Java applications are designed and developed. From lambda expressions and the Stream API to the modern Date and Time API and concurrency improvements, Java 8 has empowered developers to write more efficient, expressive, and scalable code. By mastering the concepts and techniques covered in these Java 8 interview questions and answers, developers can position themselves as invaluable assets in today's competitive job market, ready to tackle the challenges and opportunities of modern software development. Furthermore, investing in a comprehensive Java training certification program can further solidify one's expertise and readiness to excel in the dynamic field of Java development.

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