55 Java 8 Interview Questions and Answers (2025)

Basic Java 8 Interview Questions

Have a look at the basic Java 8 interview questions to have a clear understanding of the concepts.

1. What is Java 8?

Java 8 is a revolutionary release that makes Java more functional and concise. Java 8’s features include Lambda Expressions, Streams API, Functional Interfaces, Optional, new Date/Time API, and Default Methods in interfaces.

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

The latest version has:

• Lambda Expressions
• Stream API
• Default and Static Methods in Interfaces
• New Date and Time API
• Optional
• Concurrency
• Parallelism Enhancements
• Method References

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

• To provide dramatic changes in hardware
• To modify current multi-core CPUs more efficiently
• To enable users to use new Functional Programming (FP) features

4. What advantages does Java 8 bring?

The advantages of Java 8 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
• Code is far more productive
• Users can write parallel code
• Users can write database-like operations
• Applications perform better

5. What is a Lambda Expression, and why use it?

A short way to write anonymous functions in Java is known as a Lambda Expression. It helps write cleaner and more readable code, especially for functional interfaces. An instance:

List<String> names = Arrays.asList("Tom", "Jerry");
names.forEach(name -> System.out.println(name));

6. What is a functional interface?

A functional interface in Java 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.

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

7. What is the difference between Function and BiFunction?

Code Example:

Function<Integer, Integer> square = x -> x * x;
BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;

8. When are the BinaryOperator and UnaryOperator interfaces used?

Both BinaryOperator and UnaryOperator are used when the arguments and the results need to be the same.

• BinaryOperator performs the same operation as BiFunction, but the arguments and the result are of the same type
• UnaryOperator performs a similar operation but only takes one argument

Also Read: Operators in Java: A Detailed Guide

9. How are functional interfaces and Lambda Expressions related?

A functional interface has only one abstract method. Lambda expressions are used to implement that method. This makes the code shorter and cleaner.

@FunctionalInterface  
interface MyInterface {
void sayHello();  
}
MyInterface obj = () -> System.out.println("Hello!");
obj.sayHello();

10. How can a user create a personal functional interface?

You can create a custom functional interface using the @FunctionalInterface annotation and write one abstract method inside it.

@FunctionalInterface  
interface Greet {  
void message(String name);  
}
Greet greet = name -> System.out.println("Hi " + name + "!");
greet.message("Ana");

11. What does “method reference” mean in Java 8?

A method reference is a short way to refer to a method without calling it. It uses a double colon (::). It makes the code cleaner and is often used with lambda expressions.

Syntax:

ClassName::methodName

Code Example:

Function<String, Integer> func = Integer::parseInt;
// Equivalent lambda: str -> Integer.parseInt(str)
System.out.println(func.apply("100")); // Output: 100

12. 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. It stores 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 if not properly managed, it can lead to an OutOfMemoryError.

In contrast, MetaSpace dynamically expands and shrinks based on the application's needs, making it more flexible and resilient to memory-related issues.

13. Can a functional interface contain static methods?

Yes, a functional interface can have static methods. These methods are not abstract and do not affect the interface's functional nature.

Code Example:

@FunctionalInterface
interface MyInterface {
void show();  // Single abstract method
static void display() {
System.out.println("Static method in interface");
}
}

14. What is the difference between a Lambda expression and an anonymous class?

Lambda Example:

Runnable r = () -> System.out.println("Run");

Anonymous Class Example:

Runnable r = new Runnable() {
public void run() {
System.out.println("Run");
}
};

15. How do you use the forEach method introduced in Java 8?

The forEach method in Java 8 iterates over elements in a collection like a list. It takes a lambda expression or method reference as an argument.

Example using Lambda:

List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
names.forEach(name -> System.out.println(name));

Example using Method Reference:

names.forEach(System.out::println);

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Intermediate Java 8 Interview Questions

The top intermediate Java 8 interview questions for 5 years of experience or even beginners are as follows:

16. What is the relation among Stream.collect() method, Collectors class, and Collector interface?

The Stream.collect() method transforms a stream into a different form, such as a collection. It relies on the Collector interface, which defines how to accumulate stream elements.

The Collectors class provides commonly used implementations of the Collector interface, such as toList(), toSet(), and joining(), making it easier to perform standard collection operations.

Stream.collect() uses Collector instances (often from Collectors) to perform various collection-related tasks.

17. What are the differences between collections and streams?

The collections in Java are mainly used to store and group the data. Meanwhile, streams are primarily used to process data. In collections, you can also add or remove elements from collections. But you cannot add or remove elements from streams.

Collections are iterated externally, while streams are internally iterated. Collections can be traversed multiple times, while streams can only be traversed once.

18. 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 exist or do not exist. Optional’s chief benefit is avoiding null checks, and there are no longer any “NullPointerException” results at run-time.

19. What is Type Inference?

With Type Inference, the compiler can guess the data type based on the context. So you do not always have to specify it.

Code Example:

static <T> T pick(T a1, T a2) { return a2; }
Serializable s = pick("hello", new ArrayList<String>());

Here, Java infers T as Serializable because both arguments implement it. This makes the code cleaner and less verbose.

20. List some Java 8 Date and Time APIs.

21. Why are default methods needed in interfaces?

Default interface methods let you add new methods without replacing existing ones.

They provide a default implementation so that classes implementing the interface don’t need to override them unless they want to. They are needed to support interface evolution (e.g., in Java 8) and maintain backward compatibility.

interface MyInterface {
default void show() {
System.out.println("Default implementation");
}
}

22. What is the Java 8 StringJoiner class used for?

Java 8 StringJoiner is easily used to create a string with custom delimiters, prefixes, and suffixes.

It helps join strings like:

StringJoiner sj = new StringJoiner(", ", "[", "]");
sj.add("A").add("B").add("C");
System.out.println(sj); // Output: [A, B, C]

You can also set a custom value if no elements are added using setEmptyValue(). It is useful for formatting lists or sets cleanly.

23. What are the more commonly found functional interfaces in the standard library?

• 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: Takes two arguments and returns a result, and they are all the same type
• UnaryOperator: Takes a single argument and returns a result of the same type

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

A stream in Java 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 is implemented inside the stream. Also, unlike collections, streams are inherently lazily loaded and processed.

25. What is a default method, and when is it used?

A default method in Java 8 is inside an interface with a body using the default keyword. It helps in adding new functionality to interfaces without breaking existing code.

Use Case: When you want to add new interface methods without forcing all implementing classes to override them.

Code Example:

interface MyInterface {
default void show() {
System.out.println("Default method");
}
}

26. How does Java 8 handle backward compatibility with default methods?

Here’s how backward compatibility is handled:

• If an interface in Java 8 introduces default methods, classes implementing that interface before Java 8 do not require any changes. These classes will continue functioning as before, even if the interface now includes default methods.
• Classes that implement interfaces with default methods are not forced to provide implementations unless they override the default method. The default implementation is used if the class does not override the default method.
• If a class implements two interfaces with conflicting default methods (i.e., methods with the same signature), the class must explicitly override the method to resolve the conflict. Java will raise a compile-time error if this is not done.

27. What are some of the Optional methods in Java 8?

• of: It makes an optional with a non-null value
• empty: It makes an empty optional
• ofNullable: It makes an optional with a nullable value
• get: It receives the value if present; if not, then it delivers an exception, i.e., NoSuchElementException
• orElseGet: It returns the value if present; if not, it returns the result of invoking the supplier function.
• orElse: It returns the value if present; if not, then it returns the given default value
• orElseThrow: It returns the value if present; if not, then it delivers an exception produced by the given supplier

28. What are the five methods of the Collectors class and their use?

The five methods of Collectors class and their use are,

• joining(): It concatenates input elements that are separated by the specific delimiter
• groupingBy(): It groups the input elements according to the supplied classifier and returns the outcome in a map
• counting(): It counts the number of input elements
• toList(): It collects all input elements into a new List
• partitioningBy(): It partitions the input elements according to the supplied predicate and returns a Map<Boolean, List<T>>

29. What are sorting operations given in Java 8 streams?

Java 8 streams have only one operation available, and it is sorted (). This has two versions:

• one that sorts the elements in natural order without any argument, and
• another that takes the comparator as an argument and sorts the elements according to the supplied comparator

30. What are the selection operations available in the Java 8 Stream API?

• filter(): It selects the element that satisfies the given predicate
• limit():  It selects the first n elements
• distinct(): It selects only unique elements
• skip (): It selects the elements after going over the first n elements

31. What are reducing operations? Name some of the reducing operations available in the Java 8 streams.

Reducing operations collect all the elements of a stream and continuously produce a single value. Examples include calculating the average of elements, counting the number of elements, finding the maximum or minimum of elements, etc.

Here are some of the reducing operations available in Java 8 streams:

• min(): It returns the minimum element
• max(): It returns the maximum element
• count(): It returns the number of elements
• collect(): It returns a mutable result container

32. What is the difference between flatMap() and map()?

The Java 8 flatMap() and map() are two significant methods of java.util.stream.Stream interface that are used for mapping or transformation operations.

Both of these are intermediate operations, and the only difference is that the flatMap() takes Stream<T> as input and returns Stream<R>. It removes the excess layer of nesting around input values, and the map() takes Stream<T> as input and returns Stream<R>.

33. What is the difference between findAny() and findFirst()?

findAny() is a terminal operation that returns any single element of the input stream, and the result is unpredictable. It can select any element in a stream, making it particularly useful in parallel streams.

On the other hand, the findFirst() is another terminal operation in Java 8 streams that returns the input stream’s first element.

34. How do you sort a list of objects using streams in Java 8?

You can sort a list of objects using Streams and the sorted() method in Java 8. You often use a comparator to define the sorting logic.

Code Example:

List<Person> people = Arrays.asList(
new Person("Ana", 30),
new Person("Bob", 25)
);
List<Person> sortedList = people.stream()
.sorted(Comparator.comparing(Person::getAge)) // sort by age
.collect(Collectors.toList());

This sorts the people list by age in ascending order.

35. What is the difference between map() and peek() in the Stream API?

Always use map() to modify data and peek() to observe elements without changing them.

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Advanced Java 8 Interview Questions

The following are the top Java 8 interview questions for individuals with 10 years of experience.

36. 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.

37. What is Stream Pipelining?

Stream pipelining is the process of chaining different operations together. It 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.

A user can set up an arbitrary number of intermediate operations to process data, forming a processing pipeline. At the end of the process, a terminal operation must return a final value and terminate the pipeline.

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

Use the following code to print ten random numbers using forEach.

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

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

Use the following code to get the highest number on a list.

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());

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

//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);

41. What is a Spliterator?

A Spliterator in Java 8 is a special iterator used for splitting and traversing elements, especially in parallel streams.

The key features are as follows:

• Traversal: Like an iterator, it visits elements one by one
• Partitioning: Can split the data source for parallel processing using trySplit()
• Parallelism: Optimized for use with parallel streams
• Characteristics: Informs about data traits (e.g., ORDERED, SIZED)
• Methods:
• tryAdvance(): Processes the next element
• trySplit(): Splits the data for parallel execution

List<String> names = List.of("Ana", "Bob", "Charlie");
Spliterator<String> splitr = names.spliterator();
splitr.tryAdvance(System.out::println);  // prints one element

42. What is the difference between a Predicate and a Function?

43. How do parallel streams in Java 8 improve performance, and what are their pitfalls?

Parallel streams in Java 8 improve performance by leveraging multiple CPU cores to process elements in parallel rather than sequentially. This can significantly speed up operations on large datasets.

Advantages:

• Automatic parallelism with .parallelStream()
• Ideal for CPU-intensive, stateless, and independent operations

Pitfalls:

• Overhead for small datasets might degrade performance
• Thread-safety issues with mutable/shared resources
• Unpredictable order, unless explicitly preserved
• Requires careful handling of side effects

44. What are intermediate and terminal operations?

45. Explain how you would use Collectors.groupingBy() for data aggregation.

Collectors.groupingBy() is used to group stream elements by a classifier and perform aggregation like counting or summing.

Sum of salaries by department:

Map<String, Integer> totalSalaryByDept = employees.stream()
.collect(Collectors.groupingBy(
e -> e.department,
Collectors.summingInt(e -> e.salary)
));

Employees are grouped by department, and their salaries are summed for each group.

46. What are the key components of the new java.time API, and how do they support time zones and formatting?

The java.time API introduced in Java 8 provides a modern and comprehensive date/time handling system. Its key components are:

47. How would you create a custom collector in Java 8?

To create a custom collector in Java 8, you use the Collector interface by providing its five core components: supplier, accumulator, combiner, finisher, and characteristics.

Custom Collector to concatenate uppercase strings:

Collector<String, StringBuilder, String> customCollector = Collector.of(
StringBuilder::new,
// Supplier
(sb, str) -> sb.append(str.toUpperCase()),
// Accumulator
StringBuilder::append,
// Combiner
StringBuilder::toString
// Finisher
);
List<String> names = Arrays.asList("alice", "bob", "charlie");
String result = names.stream().collect(customCollector);
System.out.println(result); // Output: ALICEBOBCHARLIE

This custom collector joins all strings in uppercase.

48. Can you describe a scenario where method references improve code readability over Lambda expressions?

Scenario: When you are just calling an existing method in a lambda.

Code Example:

List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
// Using Lambda
names.forEach(name -> System.out.println(name));
// Using Method Reference (More readable)
names.forEach(System.out::println);

It removes the boilerplate name -> part, which makes the code more concise.

49. 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 are improvements to the ConcurrentHashMap class and the introduction of the CompletableFuture class.

ConcurrentHashMap Enhancements

Java 8 introduced various 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 map segments concurrently.

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 various 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.

50. What is the difference between Java 8 Spliterator and the iterators that came before Java 8?

The iterators perform only iteration, but the spliterator performs both splitting and iteration. The iterator also iterates the elements one by one, but the spliterator processes the elements in chunks (or batches). The former is suitable for serial processing, but the latter is great for parallel processing. 

The iterator is designed primarily for iterating over collections, whereas a spliterator can work with collections, streams, and arrays. The collection size is unknown when using an iterator, whereas a spliterator can estimate or provide the exact size. 

You cannot extract the properties of the iterating elements through the iterator. However, you can extract some properties of the iterating element from the spliterator.

51. What does it mean when the stream operations do the iteration implicitly?

When the collection is to be iterated explicitly, then it means that one has to write the code to iterate over collections. All the stream operations internally iterate behind the scenes. So, you do not need to be concerned about iteration when writing the code using Java 8 streams API.

52. How are the signatures of lambda expressions determined?

The signature of the lambda expressions is received from the signature of the functional interface’s abstract method. For instance, the run() method of the Runnable interface returns nothing and accepts nothing. Then, the lambda expression’s signature implementing the runnable interface will be () -> void. 

The compare() method of the Comparator interface takes arguments of returns int and type object. The lambda expression’s signature for implementing this interface will become (object, object) -> int.

53. How does the compiler determine the lambda expression’s return type?

The compiler utilizes the target type to evaluate the lambda expression’s return type. 

For instance,

Runnable r = () -> System.out.println ( “Runnable Implementation Using Lambda Expressions” );

Then, the lambda expression implementing the Runnable interface will have the signature () -> void. The compiler utilizes the interface's run() method to examine the lambda expression’s return type.

54. 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 are shorthand for lambda expressions that only call a method. They use the :: operator.

Types:

• Static: ClassName::staticMethod
• Instance: object::instanceMethod
• Constructor: ClassName::new

List<String> list = Arrays.asList("a", "b", "c");
// Lambda
list.stream().map(s -> s.toUpperCase()).forEach(System.out::println);
// Method Reference
list.stream().map(String::toUpperCase).forEach(System.out::println);

Use-case: Preferably when just calling a method without adding extra logic.

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

Lambda Expressions:

• Introduced in Java 8
• Enable functional programming
• Provide cleaner and more concise code
• Allow passing behavior (functions) as parameters

Anonymous Inner Classes:

• Verbose and harder-to-read
• Create a separate class at runtime
• Don't support functional interfaces directly

Anonymous Class:

Runnable r = new Runnable() {
public void run() {
System.out.println("Running!");
}
};

Lambda Equivalent:

Runnable r = () -> System.out.println("Running!");

Lambdas revolutionized Java by introducing functional programming. It enables cleaner and more expressive code compared to older anonymous classes. Method references enhance this by reducing visual clutter when only method calls are involved.

7 Tips to Prepare for a Java 8 Interview

Use these seven direct tips, specific to Java 8 interview preparation and will help you stay confident and informed during the interview.

1. Master Lambda Expressions: Understand how to write and use lambda expressions to simplify code. Practice converting anonymous inner classes to lambda syntax.
2. Practice Stream API: Fluently use a filter, map, reduce, collect, groupingBy, and flatMap to process collections in a functional style.
3. Understand the Optional Class: Use Optional to avoid NullPointerException and write safer, cleaner code.
4. Know the New Date and Time API: Be familiar with LocalDate, LocalTime, and LocalDateTime and format with DateTimeFormatter.
5. Practice Method References and Default Methods: Understand how method references (ClassName::methodName) and default interface methods work.
6. Solve Java 8-based Coding Problems: Use coding or online teaching platforms to learn and know how to solve Java 8 coding interview questions. Practice converting traditional loops to stream-based solutions.
7. Know Where Java 8 Fits in Real Projects: Learn how Java 8 features are used in frameworks like Spring or for writing cleaner, more efficient backend code.

Conclusion

Java 8 offers various features, enhancements, and diverse possibilities for Java developers. It has 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 is assisting 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. Enrolling in the Full Stack Java Developer Masters Program can further solidify your expertise and readiness to excel in Java.