Java 8 Stream API Quiz | 25 Coding MCQs for Interviews & Exams

STUDYECRAT Java : Stream API

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Java Stream API

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Q1/4

What is the output?

System.out.println("Hello World");

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★ Java 8 Stream API: Key Interview Points

1. reduce() vs. collect()

List<Integer> nums = Arrays.asList(1, 2, 3);
int sum = nums.stream().reduce(0, (a, b) -> a + b);
List<Integer> doubled = nums.stream().collect(Collectors.toList());
// reduce() aggregates; collect() transforms to a container

• Tip: Use reduce() for immutable operations (e.g., sum). Use collect() for mutable results (e.g., lists).
• Real Use: reduce() is common in financial calculations (e.g., portfolio totals).

2. Stream Laziness & Terminal Operations

Stream<Integer> stream = Stream.of(1, 2, 3).filter(n -> {
  System.out.println("Filtering: " + n);
  return n > 1;
});
// No output until terminal op (e.g., count()) is called

• Tip: Intermediate ops (e.g., filter()) execute only after terminal ops (e.g., forEach()).
• Real Use: Debugging lazy streams requires adding peek() to log values.

3. flatMap() for Nested Collections

List<List<String>> nested = Arrays.asList(
  Arrays.asList("a", "b"),
  Arrays.asList("c", "d"));
List<String> flat = nested.stream()
  .flatMap(List::stream)
  .collect(Collectors.toList());
// Converts [[a,b], [c,d]] → [a,b,c,d]

• Tip: Use flatMap() to merge Kafka topic partitions or nested JSON arrays.
• Real Use: Processing database results with JOINs (e.g., orders and order items).

4. Optional with Streams

List<Integer> nums = Arrays.asList(1, 2, 3);
Optional<Integer> firstEven = nums.stream()
  .filter(n -> n % 2 == 0)
  .findFirst();
System.out.println(firstEven.orElse(-1));
// Safely handles empty results

• Tip: Always use orElse()/orElseThrow() with findFirst() to avoid NoSuchElementException.
• Real Use: API responses where a field might be missing (e.g., user addresses).

5. Parallel Stream Pitfalls

List<Integer> nums = Arrays.asList(1, 2, 3);
int sum = nums.parallelStream()
  .reduce(0, (a, b) -> a + b);
// Thread-safe for associative ops (e.g., sum)

• Tip: Avoid parallelStream() with stateful ops (e.g., sorted()) or non-thread-safe collectors.
• Real Use: Batch processing large datasets (e.g., log analysis).

💡 Pro Interview Tip

When asked about Streams, always mention lazy evaluation and immutability. Interviewers often test if you understand these core concepts.