55 Java 8 interview questions (+ sample answers) to assess developers’ skills

Even though Java 8 was introduced over 10 years ago, it’s still super relevant today. Its new capabilities over earlier versions have made developers and businesses worldwide adopt it at scale, to the point that Java 8 became an industry standard.

So, when hiring developers, you might still want to review their Java 8 skills. Make sure they’re familiar with its key features (such as lambda expressions, streams, and the new Date and Time API) and that they’re able to use them to create elegant, clean code.

To help you gauge applicants’ skills and hire the right talent, we’ve selected 55 Java 8 interview questions and provided sample answers to 20 of them. So, even if you don’t know Java 8 inside out yourself, you can still evaluate applicants’ expertise.

What's the best way to evaluate candidates’ Java 8 skills?

To bring on board developers with top-notch Java skills, we advise you to:

• Use a skills assessment that includes Java tests to screen out unqualified candidates

• Follow up with an interview with shortlisted candidates to evaluate their practical knowledge and skills

For the first step, check out our test library to find the best tests for the role.

Here are some of our top tests to consider when hiring a Java developer:

• Java (Coding) Debugging: Check candidates' debugging skills before you commit time to interviewing them.

• Java (Coding) Data Structures: Assess applicants’ ability to manipulate core data structures in Java with this test.

• Object-oriented Programming (OOP): This test is perfect for roles where candidates need a deep understanding of OOP principles.

• Clean Code: Make sure applicants know how to write code that is clean, efficient, and performant.

Add one of our personality and culture tests to these – or evaluate candidates’ skills in another programming language.

To support you in the second step, we've prepared a list of the best 55 Java 8 interview questions (and 20 sample answers) below.

Top 20 Java 8 interview questions to hire the best developers

Look for answers that show that candidates know Java 8 in depth and know how to use their knowledge to write more concise, readable, and efficient Java code.

1. How does Java 8 compare to Java 7 in terms of features and performance improvements?

Here, candidates should discuss the most significant changes between Java 7 and Java 8. Examples of key features and improvements include:

• Lambda expressions, which brought a functional programming aspect to Java, allowing for cleaner and more concise code

• Stream API, which provides a new abstraction for processing collections of objects in a functional manner

• Default and static methods in interfaces, enabling more flexible evolution of interfaces

• Improvements to the Java Virtual Machine (JVM), a virtual machine that enables a computer to run Java programs

• A new Date-Time API that helps bring consistency and clarity in handling dates and times

2. How do lambda expressions work in Java 8, and how are they different from anonymous inner classes?

Lambda expressions are an easy way to create instances of single-method interfaces (functional interfaces), making the code more readable and concise. Expect candidates to explain that lambdas are treated as functions, so they can be passed around and executed on demand.

Some of the key differences between lambda expressions and anonymous inner classes are that:

• Lambda expressions don’t have their own scope but operate in the enclosing scope instead

• Lambdas can perform better than anonymous inner classes in some contexts, because the JVM (Java Virtual Machine) optimizes them differently

3. Can you explain the concept of functional interfaces in Java 8?

A functional interface is an interface with just one abstract method (aside from the methods of Object); developers can use them as the assignment target for lambda expressions.

Good answers will also mention the FunctionalInterface annotation, which is not mandatory but helps with clarity and ensures the interface meets the criteria of a functional interface.

Candidates may also explain how functional interfaces support the functional programming style introduced in Java 8.

4. What is a stream in Java 8 and how does it differ from a collection?

A stream is a sequence of elements supporting sequential and parallel aggregate operations.

Candidates should explain that streams enable high-level, functional-style operations on sequences of elements, such as map-reduce transformations, without modifying the underlying data source.

The key difference from collections is that streams are not data structures that store elements. Instead, they carry values from a source (which could be a collection) through a pipeline of computational steps.

5. How do you convert a list to a stream and then back to a list?

This question enables you to check whether candidates know their APIs.

To convert a list to a stream, they might use the .stream() method on the List instance. For converting a stream back to a list, they could use the .collect(Collectors.toList()) terminal operation.

The best candidates might include a quick example to show their practical skills.

6. Explain the difference between intermediate and terminal operations in streams.

Intermediate operations transform a stream into another stream, such as filter, map, and sorted. These operations are lazy; they don't perform any processing until a terminal operation is invoked.

Terminal operations, such as forEach, collect, or reduce, produce a result or a side-effect, after processing all the elements of the stream.

Intermediate operations allow for chaining (building a pipeline), while terminal operations close that pipeline, triggering the processing of the data.

7. How would you use streams to filter and collect elements of a list based on a condition?

Candidates should explain a process that looks something like this:

• Use .stream() on a collection to create a stream

• Use .filter() with a lambda expression to specify the condition for filtering elements

• Use the .collect(Collectors.toList())terminal operation to collect filtered elements back into a list

Top answers will include a brief code example, such as filtering a list of integers to find even numbers: List<Integer> evenNumbers = myList.stream().filter(n -> n % 2 == 0).collect(Collectors.toList());.

8. Describe how to use the new Date and Time API in Java 8.

This API offers a few improvements over the old java.util.Date and java.util.Calendar, such as immutability, clarity, and a rich set of operations for date and time manipulation.

Candidates should mention key classes like:

• LocalDate

• LocalTime

• LocalDateTime

• ZonedDateTime

• Period

Look for examples of how these classes can help create instances for specific dates, adding or subtracting time units, and formatting dates for display.

9. How do you create an instance of LocalDate and find the current date?

Candidates should explain they’d use LocalDate.now() to get the current date according to the system clock and the default time zone. They could also create a LocalDate for a specific date by using the .of(year, month, dayOfMonth) method.

Skilled candidates might mention that LocalDate instances are immutable and thread-safe.

10. Explain the differences between LocalDate, LocalTime, and LocalDateTime.

The three classes handle different aspects of time:

• LocalDate presents a date without time and time zone (year-month-day)

• LocalTime presents time without a date and time zone (hour-minute-second-nanosecond)

• LocalDateTime combines date and time but still lacks time-zone information, representing a date-time without a time zone

Use cases skilled candidates might mention include using LocalDate for birthdays, LocalTime for alarm clocks, and LocalDateTime for precise moments of time.

11. What improvements did Java 8 bring to the ConcurrentHashMap?

Java 8 introduced several performance improvements to ConcurrentHashMap, such as:

• Concurrent updates for an improved scalability by transitioning bins to tree nodes

• Computational methods, which introduced atomic operations like compute, computeIfAbsent, and merge for direct, thread-safe updates

• Stream support, which Enabled parallel stream operations

• Improved iteration performance to reflect the map's state more accurately at creation

• Key and value views, which are better methods for working with map views for keys, values, and entries

• Size estimation for more accurate and efficient size and emptiness checks

12. What is the Optional class and how do you use it?

Expect candidates to describe Optional as a container object which may or may not contain a non-null value.

Its purpose is to provide a better alternative to returning null from methods, helping to avoid NullPointerException. They should talk about some of the following methods:

• For creating instances: Optional.of(value), Optional.empty(), and Optional.ofNullable(value)

• For accessing contained value: isPresent(), ifPresent(consumer), and orElse(defaultValue)

Look for applicants who mention how Optional makes code more readable and robust by explicitly handling the presence or absence of a value.

13. How do you retrieve the value of an Optional object?

There are several ways to safely retrieve the value from an Optional object, such as:

• get(), which gets the value, but will throw a

  NoSuchElementException if the Optional is empty, making it risky to use without prior checks

• orElse(T other), which returns the value if present, or otherwise returns a default value provided

• orElseGet(Supplier<? extends T> other), which is similar to the previous one, but a supplier function retrieves the default value (only if necessary)

• orElseThrow(Supplier<? extends X> exceptionSupplier), which throws an exception created by the provided supplier if the Optional is empty

Skilled candidates would also mention isPresent() and ifPresent(Consumer<? super T> consumer) for checking and acting upon the presence of a value.

14. What advantages do lambda expressions bring to Java?

Expect candidates to mention several benefits such as:

• Conciseness:

  Lambda expressions allow for shorter and more readable code by eliminating boilerplate code required for anonymous classes

• Functional programming:

  They introduce functional programming features to Java, enabling operations on collections in a functional style

• Parallel processing:

  Lambdas work well with the Stream API, making parallel operations on collections more straightforward

• Higher-order functions:

  They enable functions to be passed as arguments, returned as values, and stored in variables, which makes them more modular

15. What is a method reference and do you use it in Java 8?

A method reference is a shorthand notation of a lambda expression to call a method.

Candidates should explain that they can use method references to refer directly to methods by their names, in case the method's parameters match the parameters of the functional interface method.

Types of method applicants might mention include:

• Static methods

• Instance methods of a particular object

• Instance methods of an arbitrary object of a particular type

• Constructors (using :: syntax)

Look for answers that include an example, like String::toLowerCase referring to the toLowerCase() method of an instance of String.

16. Explain the differences between forEach, map, and reduce operations.

Expect candidates to explain that:

• forEach

  is a terminal operation that applies an action to each element of a stream, helping iterate over elements and invoke a side-effect action

• map

  is an intermediate operation that transforms each element of a stream using a provided function; it serves to convert elements of a stream from one form to another

• reduce

  is a terminal operation that combines all elements of a stream into a single result using a provided binary operation and is useful for producing a single result from a collection of elements, like summing all numbers in a list

17. How do you create a custom functional interface and use it with a lambda expression?

A custom functional interface is defined like any other interface but with a single abstract method.

Applicants should mention the use of the functional interface annotation to explicitly indicate that the interface is intended for use with lambdas. The best answers will include an example of this.

18. What is the difference between Collectors.joining() and String.join()?

The main difference between the two is the following:

• Collectors.joining() is part of the Stream API. Developers use it in stream operations to link together elements according to a delimiter, providing a prefix and suffix. It's flexible and enables the joining of stream elements.

• String.join() is a static method in the String class that joins sequences of characters or strings with a delimiter. It's straightforward but less flexible compared to

  Collectors.joining() because it works with iterable collections of strings directly, not streams.

Top candidates will show a nuanced understanding of when to use each in streams versus collections.

19. Explain the concept of laziness in stream operations.

Laziness in stream operations refers to the way Streams delay their computations.

Essentially, stream operations do not immediately execute when they are called. Instead, they wait and only execute when a terminal operation (like .collect(), .forEach(), or .reduce()) is invoked. This means that intermediate operations (like .map() or .filter()) are ready but don't actually process any data on their own.

This approach is efficient because it allows for fewer iterations through the data, because multiple operations can be combined and executed in a single pass.

20. How do you decide when to use parallel streams in your application?

Candidates should know the benefits and downsides of parallel streams.

Expect them to mention that parallel streams can leverage multicore processors, improving performance when dealing with large datasets or other intensive tasks.

However, they should also explain that not all tasks are suitable for parallelization; improper use can lead to worse performance or unpredictable results.

Key factors to consider include:

• The size of the dataset

• The complexity of the operations

• The computational cost of processing elements

35 more Java 8 interview questions you can ask developers

If you need more Java 8 questions for your interviews, look no further. Below, we’ve selected 20 more interview questions you can use to assess applicants’ skills and knowledge of Java.

1. What is the significance of the main method in a Java program?

2. How does Java achieve platform independence?

3. Describe the difference between == and .equals() in Java.

4. How do you handle exceptions in Java?

5. Explain the difference between overloading and overriding in Java.

6. What is an abstract class and how is it different from an interface?

7. What is polymorphism in Java?

8. How do you implement encapsulation in Java?

9. What is the use of the final keyword?

10. What are the differences between List, Set, and Map in Java?

11. How do you ensure a collection is thread-safe?

12. Explain type erasure in the context of generics.

13. What are the benefits of using generics in Java?

14. What is the difference between Runnable and Callable in Java?

15. How do you use the ExecutorService to manage concurrent tasks?

16. Explain what happens in a deadlock situation.

17. What is reflection and how would you use it?

18. How do you implement a singleton pattern in Java?

19. What is the difference between the Strategy pattern and the State pattern?

20. How would you design a cache system in Java?

21. How does the static keyword work in the context of interfaces in Java 8?

22. Can you describe the difference between map and flatMap in the Stream API?

23. What are collectors in Java 8 and how do you use them?

24. What issues with the old date and time API does Java 8 address?

25. How does the CompletableFuture class work, and how does it improve upon Future?

26. Can you write a lambda expression that takes two integers and returns their sum?

27. Explain how to perform a join operation on two collections using streams.

28. How would you use streams to filter null values from a collection?

29. How do you create a range of numbers in Java 8 and process them with a stream?

30. Describe how to use the Optional class to prevent NullPointerException.

31. When would you choose to use Optional over null checks?

32. What are some practical use cases for using lambda expressions and streams in Java applications?

33. How do you handle exceptions in lambda expressions?

34. Can you explain how to use Java 8 features to read and write files more efficiently?

35. How would you refactor a for-loop that processes elements of a list into Java 8 style using streams?

Need more questions? Check out our 100+ Java interview questions.

Assess Java 8 skills quickly and efficiently with the right tests and interview questions

To hire a skilled Java developer, you need to look beyond the resume and arm yourself with the right questions to ask candidates.

Skills tests and our selection of the best Java 8 interview questions will help you uncover the best talent in your talent pool and make the right hiring decisions confidently and, most importantly, without delay.

Gone are the days when hiring a single employee took more than 40 days!

Book a free 30-minute live demo to chat with one of our team members and see how TestGorilla can help you find the right talent. Or, sign up for our free forever plan and start evaluating candidates’ skills today.