Top Java 8 Coding Interview Questions and Answers (With Code)

Q: What’s the difference between map() and flatMap() in streams?

map() is used to transform each element. flatMap() is used when you have a stream of collections (e.g., List<List<Integer>>) and you want to flatten it into a single stream. Think of map() as one-to-one and flatMap() as one-to-many.

Q: How do I find duplicates in a list using Java 8 Streams?

You can track duplicates using a Set. As you stream through the list, use filter() to check if the element is already seen. If it is, it's a duplicate. This is a common interview favorite.

Are you preparing for a Java interview focused on Java 8 features? This blog will help you master the most frequently asked Java 8 coding questions, especially around Streams, Collectors, and functional-style programming. From beginner-level problems to advanced real-world challenges, we’ve covered them all with concise explanations and working examples.

🧠 Why Java 8 Coding Interview Questions Matter in Interviews

Java 8 introduced powerful functional programming capabilities with Streams, Lambda expressions, and Collectors. These concepts have become essential in real-world Java development and are now a core part of interviews.

✅ Beginner to Intermediate Java 8 Coding Questions

1. Find even numbers from a list using streams

list.stream()
    .filter(n -> n % 2 == 0)
    .collect(Collectors.toList());

2. Convert a list of strings to uppercase using streams

list.stream()
    .map(String::toUpperCase)
    .collect(Collectors.toList());

3. Filter names starting with ‘S’ from a list

list.stream()
    .filter(name -> name.startsWith("S"))
    .collect(Collectors.toList());

4. Count the number of elements in a list using streams

long count = list.stream().count();

5. Sort a list of integers in descending order using streams

list.stream()
    .sorted(Comparator.reverseOrder())
    .collect(Collectors.toList());

6. Remove duplicate elements from a list using streams

list.stream()
    .distinct()
    .collect(Collectors.toList());

7. Find the first non-repeated character in a string using streams

Character result = str.chars()
    .mapToObj(c -> (char) c)
    .collect(Collectors.groupingBy(c -> c, LinkedHashMap::new, Collectors.counting()))
    .entrySet().stream()
    .filter(e -> e.getValue() == 1)
    .map(Map.Entry::getKey)
    .findFirst().orElse(null);

8. Find the maximum and minimum element in a list using streams

int max = list.stream().max(Integer::compareTo).get();
int min = list.stream().min(Integer::compareTo).get();

9. Find the sum and average of a list of integers

int sum = list.stream().mapToInt(Integer::intValue).sum();
double avg = list.stream().mapToInt(Integer::intValue).average().orElse(0.0);

10. Check if all elements are even using allMatch()

boolean allEven = list.stream().allMatch(n -> n % 2 == 0);

🔥 Advanced Java 8 Stream Interview Questions

11. Group a list of employees by department

Map<String, List<Employee>> deptMap =
    employees.stream().collect(Collectors.groupingBy(Employee::getDepartment));

12. Count the number of employees in each department

Map<String, Long> countMap =
    employees.stream().collect(Collectors.groupingBy(Employee::getDepartment, Collectors.counting()));

13. Find the second highest salary

Optional<Integer> secondHighest = salaries.stream()
    .sorted(Comparator.reverseOrder())
    .distinct()
    .skip(1)
    .findFirst();

14. Partition a list into even and odd using partitioningBy

Map<Boolean, List<Integer>> partitioned =
    list.stream().collect(Collectors.partitioningBy(n -> n % 2 == 0));

15. Flatten a list of lists using flatMap

List<Integer> flatList =
    listOfLists.stream().flatMap(List::stream).collect(Collectors.toList());

16. Find duplicate elements in a list using streams

Set<Integer> seen = new HashSet<>();
Set<Integer> duplicates =
    list.stream().filter(n -> !seen.add(n)).collect(Collectors.toSet());

17. Find the most frequent element in a list

Integer mostFrequent = list.stream()
    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
    .entrySet().stream()
    .max(Map.Entry.comparingByValue())
    .map(Map.Entry::getKey).orElse(null);

18. Sort a map by its values using streams

map.entrySet().stream()
    .sorted(Map.Entry.comparingByValue())
    .collect(Collectors.toMap(
        Map.Entry::getKey, Map.Entry::getValue,
        (e1, e2) -> e1, LinkedHashMap::new));

19. Convert a list to a map using Collectors.toMap

Map<Integer, String> map =
    list.stream().collect(Collectors.toMap(Employee::getId, Employee::getName));

20. Create a comma-separated string from a list

String result = list.stream()
    .map(String::valueOf)
    .collect(Collectors.joining(", "));

📌 Final Tips for Java 8 Interviews

Use Streams when dealing with collections.
Always prefer Collectors.groupingBy, map, filter, and flatMap in chained operations.
Master Optional, Predicate, Function, and Consumer interfaces.

🧾 Helpful Resources

What are Java 8 Streams and why are they important in interviews?

Java 8 Streams are used to process collections of data in a functional style. They’re popular in interviews because they help write cleaner, more readable, and more efficient code — especially for filtering, mapping, and aggregating data.

How can I get better at solving Java 8 coding interview questions?

Start by solving common tasks like filtering, grouping, sorting, and mapping collections. Practice on real-world datasets, and explore interview-style questions like “second highest salary” or “grouping by department.” Focus on writing code using map, filter, collect, flatMap, and groupingBy.

Are Java 8 Streams faster than traditional loops?

Not always. Streams are more readable and declarative, but they’re not always faster than traditional for-loops. For large datasets or performance-critical operations, use them carefully and benchmark if needed. However, their clarity and expressiveness make them a strong choice in interviews.

What’s the difference between `map()` and `flatMap()` in streams?

map() is used to transform each element.
flatMap() is used when you have a stream of collections (e.g., List<List<Integer>>) and you want to flatten it into a single stream.
Think of map() as one-to-one and flatMap() as one-to-many.

How do I find duplicates in a list using Java 8 Streams?

You can track duplicates using a Set. As you stream through the list, use filter() to check if the element is already seen. If it is, it’s a duplicate. This is a common interview favorite.

Can I use Java Streams to modify the original list?

No, streams are designed to be non-mutating. They don’t change the original collection. Instead, they return a new stream or result. If you need to modify the original list, use traditional loops or the List.replaceAll() method.

How do I answer Java 8 stream questions in interviews under pressure?

Stay calm. Break the problem into smaller parts. Think about whether you need to filter, transform, group, or aggregate. Start with a simple stream, and build the logic step-by-step. Communicate your thought process clearly — that matters as much as the final code.

🙋 Got a Question?

Drop your queries in the comments or contact us here. We’re here to help you ace your Java interview! 🚀