Top 10 Java Streams Interview Questions with Answers

Here are the top 10 Java Stream interview questions with concise, single-line answers:

1. What is a Java Stream?
   A Java Stream is a sequence of elements supporting sequential and parallel aggregate operations.

2. How do you create a Stream in Java?
   Streams can be created using methods like Stream.of(), Collections.stream(), or Arrays.stream().

3. What is the difference between Stream and Collection?
   A Stream is not a data structure but a pipeline for processing data, whereas a Collection stores data.

4. What are intermediate and terminal operations in Streams?
   Intermediate operations (e.g., map, filter) transform a stream; terminal operations (e.g., collect, forEach) produce a result or side-effect.

5. What is the purpose of the filter() method in Streams?
   The filter() method selects elements from a stream based on a predicate.

6. How does the map() method work in Java Streams?
   The map() method transforms each element in a stream using a provided function.

7. What is lazy evaluation in Java Streams?
   Lazy evaluation means intermediate operations are not executed until a terminal operation is invoked.

8. What is the difference between Stream and ParallelStream?
   Stream processes data sequentially, while ParallelStream processes data in parallel using multiple threads.

9. How do you collect Stream results into a List?
   Use stream.collect(Collectors.toList()) to collect stream elements into a List.

10. What is Optional in the context of Java Streams?
    Optional is a container object used to handle potentially null values, often returned by stream operations like findFirst().