Top 25 Java 8 Interview Questions and Answers in 2020

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.”

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Java 8 Basic Interview Questions

Here are some easy ones to get us warmed up.

1. What new features did Java 8 introduce?

The latest version has:

  • An improved, immutable JodaTime-inspired Date and time API
  • A new language called Lambda Expressions that treats actions as objects
  • Method References, which enable defining Lambda Expressions by referring to methods directly using their names
  • Default methods, which give users the ability to add full implementations in interfaces besides abstract methods
  • Nashorn, a high-performance Java-based engine integrated to JDK used to evaluate and execute JavaScript code
  • Stream API, a special iterator class that allows processing object collections in a functional manner

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 is an interface that contains just one abstract method.

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.

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Java 8 Intermediate Interview Questions

Now let’s increase the difficulty a bit.

9. 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.

10. What is Type Inference?

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

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

The core API classes are:

  • LocalDate
  • LocalTime
  • LocalDateTime

12. 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.

13. 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.

14. 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

15. 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.

16. 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.

17. What is jjs in Java 8?

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

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Java 8 Advanced Interview Questions

Finally, here come the tough questions.

18. 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.

19. 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.

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

Use the following code segment:

Random random = new Random();


21. 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 = −> x).summaryStatistics();

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

22. 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(;

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

23. 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.

24. 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.

25. 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.

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.

The article Top 6 Skills to Boost Java Developer Salaries gives you insights on which skills are most sought after by businesses and organizations, and consequently, the skills that help you get the best salary.

There is also another set of Java interview questions you can check out. There’s no such thing as too much preparation!

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