Top 20 Stream API Interview Questions with Answers

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Recent surveys show that demand for Stream API expertise is on the rise. Why? Companies like Infosys, TCS, Wipro, HCL Technologies, and Tech Mahindra are increasingly relying on it for real-time data processing. And, if you’re getting ready for a technical Java interview, expect questions on Stream API. But don’t worry! This article is your guide to Java Stream API interview questions, helping you prepare thoroughly. By going through these 15+ commonly asked questions, you’ll be better equipped to showcase your skills during technical interviews. 

So, let us begin!

Basic Stream API Interview Questions for Freshers

Here are some important Java Stream API interview questions and answers for freshers. 

1. What is Stream API in Java?

Stream API in Java is a toolset for processing sequences of elements in a functional programming style. It provides a way to perform operations such as filtering, mapping, and reducing collections of objects.

2. Can you explain the difference between collection and stream?

Collections and Streams serve different purposes. Collections are used for storing and managing groups of data, while streams are utilized for data processing. 

Collections allow for adding or removing elements, which isn’t possible with streams. Moreover, collections iterate externally, while streams handle iterations internally.

3. How do you create a stream in Java?

To create a stream in Java, you can use the stream() method on collections like lists or arrays. For example, to create a stream from a list, you would call list.stream(). This returns a sequential stream containing the elements of the collection.

Also Read - Top 20+ Java Collections Interview Questions With Answers

4. What are intermediate operations in Stream API?

Intermediate operations in Stream API transform or filter elements of a stream, such as filter(), map(), and sorted(). These operations return a new stream, allowing for chaining.

5. What are terminal operations in Stream API?

Terminal operations in Stream API produce a result or side-effect, like forEach(), count(), or collect(). They mark the end of a stream pipeline, triggering execution.

6. How do you map elements in a stream?

To map elements in a stream, use the map() method, which applies a function to each element, transforming it into another form. This creates a new stream with the transformed elements.

7. What is the difference between intermediate and terminal operations in Stream API?

Intermediate operations return a new stream and can be chained together, while terminal operations produce a result and mark the end of a stream pipeline.

Stream API in Java 8 Interview Questions for Experienced

These are some commonly asked Java 8 Stream API interview questions and answers for experienced candidates. 

8. What is the purpose of the Stream API in Java 8?

The Stream API in Java 8 is used to process sequences of elements in a functional programming style, making data manipulation more efficient.

9. How do you filter elements in a stream? 

You can filter elements in a stream using the filter() method, providing a condition that determines which elements to include.

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

The map() method independently transforms each element of a stream. On the other hand, flatMap() transforms each element into a stream of zero or more elements, consolidating the outcome into a unified stream.

11. How do you handle parallel streams? 

Parallel treams in Java 8 are created using the parallelStream() method. It divides the data into multiple parts and processes them concurrently, improving performance for certain operations.

12. Can you convert an array to stream? How?

Yes, you can convert an array to a stream in Java. One way is using the Arrays.stream() method. Alternatively, you can use the Stream.of() method. Both methods allow for easy conversion, enabling further stream operations on array elements.

13. Can you explain how collectors work in Stream API? 

Collectors in Stream API accumulate elements of a stream into a mutable result container, performing various mutable reduction operations like grouping, partitioning, or summing elements. They facilitate the transformation of streams into collections or other data structures.

Also Read - Top 30+ Java 8 Interview Questions With Answers

Advanced Stream API Java 8 Interview Questions

Here are some advanced Stream API interview questions with answers. 

14. What is the difference between sequential and parallel streams?

Sequential streams process elements one after the other, maintaining the order of operations. On the other hand, parallel streams split data into segments, executing them simultaneously, which can improve performance for operations suitable for parallel processing.

15. Explain the concept of lazy evaluation in Stream API.

Lazy evaluation means that elements in a stream are processed only when needed. This helps in optimizing performance by avoiding unnecessary computations until a terminal operation is invoked.

16. What is the purpose of the reduce() method in Stream API? 

The reduce() method in Stream API combines elements of a stream into a single result using a binary operation. It’s useful for aggregating elements or performing calculations on streams.

17. How do you handle exceptions in Stream API operations? 

Exceptions in Stream API operations can be handled using try-catch blocks within lambda expressions or by wrapping the stream operation in a try-with-resources statement.

Also Read - Top 25 Exception Handling Questions In Java Interview

Stream API Coding Interview Questions

Here are some important Java 8 Stream API programming questions and answers. 

18. How do you find the sum of all elements in a list using Stream API?

You can use the stream() method to convert the list into a stream, then apply the mapToInt() method to extract integers, and finally use the sum() method to calculate the sum.

19. How do you filter even numbers from a list using Stream API?

You can use the stream() method to convert the list into a stream and then apply the filter() method with a predicate to select only the even numbers.

20. How do you find the maximum element in a list using Stream API? 

You can use the stream() method to convert the list into a stream and then apply the max() method, providing a comparator to compare elements and find the maximum.

Wrapping Up

So, these are the top 15+ Stream API interview questions with answers you can follow for your interview preparation. Learning these concepts will boost your confidence and improve your chances of success in landing your dream job.

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