Published: 25 Oct 2025 | Reading Time: 8 min read
Master Core Java 8 Features: Ace your 2025 Java interviews by mastering Java 8's core features like lambdas, streams, and Optionals—essential for 80% of backend roles.
Problem Solved: Confusing concepts and lack of practice lead to failed interviews; this guide provides clear explanations, code examples, and questions.
Key Benefit: 20 ready-to-use coding questions with answers, plus scenarios and MCQs, to boost confidence and skills in functional programming.
Java 8, released in March 2014, is an updated version of the Java programming language that introduced several features. These changes simplified coding practices and promoted a functional programming style, significantly improving developer productivity and code readability.
As the tech industry continues to evolve, demand for skilled Java developers proficient in these new features has increased. Coding interviews often focus on evaluating candidates' understanding and application of Java 8's core concepts. In this article, we will explore the top Java 8 coding interview questions and answers for 2025, suitable for beginners to seasoned professionals.
Java 8 is one of the most impactful updates to the Java programming language. It introduced several key features that fundamentally changed how developers write and structure their code.
Java 8 introduced a suite of powerful features that revolutionized how developers write, structure, and optimize Java applications. Below is a concise overview of the most impactful additions:
Lambda expressions allow you to write anonymous methods with clear, concise syntax, enabling functional-style operations. Method references further simplify code by referring directly to existing methods or constructors (e.g., System.out::println).
A shorthand notation of a lambda expression to call a method. It simplifies code by allowing existing processes to be referenced directly.
Example:
list.forEach(System.out::println);
Interfaces with a single abstract method. They can have multiple default or static methods.
Example: Runnable, Callable, or custom interfaces.
Facilitates functional-style operations on collections, enabling efficient data processing with methods like filter, map, and reduce.
Allow interfaces to have methods with a default implementation, enabling backward compatibility while adding new methods.
Example:
interface MyInterface {
default void defaultMethod() {
System.out.println("Default");
}
}
Provides a way to encode and decode data in Base64 format using the java.util.Base64 class.
Example:
String encoded = Base64.getEncoder().encodeToString("Hello".getBytes());
Interfaces can contain static methods that can be called independently of any instances.
Example:
interface MyInterface {
static void staticMethod() {
System.out.println("Static Method");
}
}
A container for optional values that helps avoid NullPointerExceptions by providing methods to handle the presence or absence of values.
Example:
Optional<String> optional = Optional.ofNullable(getValue());
Provides static methods to accumulate stream elements into collections or summarize statistics.
Example:
List<String> collected = stream.collect(Collectors.toList());
A method in the Stream API that allows you to act on each element of the stream.
Example:
stream.forEach(System.out::println);
A lightweight JavaScript engine that allows embedding JavaScript code in Java applications, replacing the older Rhino engine.
Example:
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("nashorn");
Introduces parallel sorting for arrays using the Arrays.parallelSort() method, which enhances performance on large datasets.
Example:
int[] array = {5, 3, 1, 2, 4};
Arrays.parallelSort(array);
Java 8 allows for the definition of repeating annotations, enabling the same annotation to be applied multiple times to the same declaration.
Example:
@Repeatable(Schedules.class)
@interface Schedule {
String day();
}
New java.nio.file package for improved file handling, and the introduction of new methods in Files class for easier file I/O operations.
Example:
Path path = Paths.get("file.txt");
Files.lines(path).forEach(System.out::println);
Introduces new classes and methods for better handling of concurrent programming, including the CompletableFuture for asynchronous programming.
Example:
CompletableFuture.supplyAsync(() -> {
// asynchronous task
return result;
});
Enhancements to JDBC include the introduction of the java.sql package, providing new methods for improved database operations.
Example: New Connection methods to manage SQL features like connection pooling.
The java.time package introduces a modern, immutable, and thread-safe API for date, time, duration, and period handling. Classes like LocalDate, LocalTime, and LocalDateTime replace the outdated Date and Calendar classes.
Java 8's CompletableFuture class, part of the java.util.concurrent package, makes it easier to write asynchronous, non-blocking code. It allows developers to compose, combine, and handle future results flexibly.
These features collectively make Java 8 more expressive, safer, and better suited for modern software development needs.
Lambda expressions are one of Java 8's most impactful features, enabling a functional programming style and significantly reducing boilerplate code. They allow you to write anonymous functions—functions without a name—which can be passed as arguments or stored in variables.
A lambda expression provides a clear and concise way to implement functional interfaces (interfaces with a single abstract method). The general syntax is:
(parameters) -> expression
// or
(parameters) -> { statements }
Before Java 8, developers often used anonymous inner classes to provide short implementations of interfaces. Lambda expressions make this much simpler and more readable.
Example (using Comparator):
// Using anonymous inner class
Comparator<String> comp = new Comparator<String>() {
@Override
public int compare(String a, String b) {
return a.compareTo(b);
}
};
// Using lambda expression
Comparator<String> compLambda = (a, b) -> a.compareTo(b);
Lambda expressions minimize the amount of code required to perform actions, improving code readability and maintainability.
Lambda expressions are heavily used with the Stream API for operations like filtering, mapping, and reducing collections.
Example (finding the maximum value):
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int max = numbers.stream()
.max((a, b) -> a - b) // Lambda as a Comparator
.get();
When a lambda expression simply calls an existing method, you can use a method reference for even greater clarity:
numbers.forEach(System.out::println);
Lambda expressions can access final variables and those that are effectively final (variables not changed after assignment) from the enclosing scope. This makes them similar to anonymous inner classes in terms of variable access.
Questions about Java 8 during interviews are frequently tailored to the applicant's experience, ensuring the assessment of the two groups — Java 8 interview questions for freshers and those for experienced professionals — is at the right level.
New graduates first and foremost need to demonstrate that they understand the fundamental concepts of Java and object-oriented programming (OOP), and that they are familiar with core Java libraries. The interviewer will be most eager to witness the applicant's problem-solving and quick learning.
1. Basic Java Concepts:
== and .equals() in Java?2. Lambda Expressions (Java 8 Introduction):
3. Optional Class (Java 8):
Optional.isPresent() and Optional.ifPresent() methods?4. Java Collections:
5. Basic Exception Handling:
throw and throws in Java exception handling?In the case of skilled candidates, the questions of the interview will explore in-depth complicated Java topics; thus, they should demonstrate how they efficiently use Java 8 features like the Stream API, functional programming paradigms, and performance optimization. Interviewers look for advanced problem-solving skills, code optimization, and profound knowledge of Java internals.
1. Lambda Expressions and Functional Programming (Java 8):
2. Stream API (Java 8):
3. Optional Class (Java 8):
map(), filter(), and flatMap().4. Concurrency and Multithreading:
5. Performance Optimization:
6. Design Patterns:
7. Scenario-Based and Problem-Solving Questions:
Here are the top Java 8 coding interview questions for freshers and experienced professionals in 2024:
import java.util.function.BiFunction;
public class LambdaAddition {
public static void main(String[] args) {
BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;
System.out.println(add.apply(5, 3)); // Output: 8
}
}
BiFunction is a functional interface that takes two arguments and returns a result. The lambda (a, b) -> a + b is an implementation of its apply method. apply(5,3) runs the addition and outputs the sum. This is a simple example of a lambda being used for arithmetic operations.
8
import java.util.Arrays;
import java.util.List;
public class FilterEvenNumbers {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
numbers.stream()
.filter(n -> n % 2 == 0)
.forEach(System.out::println); // Output: 2 4 6
}
}
The stream() creates a pipeline. filter() uses lambda to check evenness (mod 2 == 0), intermediate operation. forEach() terminal operation prints via method reference. Demonstrates filtering collections functionally.
2
4
6
import java.util.Arrays;
import java.util.List;
public class MapToSquares {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
numbers.stream()
.map(n -> n * n)
.forEach(System.out::println); // Output: 1 4 9 16
}
}
The function map() transforms each element using lambda n -> n*n. Chains to forEach for output. Shows data transformation in streams.
1
4
9
16
import java.util.Arrays;
import java.util.List;
public class FindMaxValue {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int max = numbers.stream()
.max(Integer::compare)
.get();
System.out.println(max); // Output: 5
}
}
max() reduces the stream using a Comparator via a method reference. .get() extracts Optional value (assumes non-empty). Efficient for aggregates.
5
import java.util.Arrays;
import java.util.List;
public class CountElements {
public static void main(String[] args) {
List<String> words = Arrays.asList("apple", "banana", "pear");
long count = words.stream().count();
System.out.println(count); // Output: 3
}
}
count() is a terminal operation returning stream size. Simple aggregation without lambda needed.
3
import java.util.Arrays;
import java.util.List;
public class ReduceToSum {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3);
int sum = numbers.stream().reduce(0, Integer::sum);
System.out.println(sum); // Output: 6
}
}
reduce() combines elements: identity 0, accumulator adds via method reference. Folds list into single value.
6
import java.util.Arrays;
import java.util.List;
public class PrintLengths {
public static void main(String[] args) {
List<String> words = Arrays.asList("Java", "Python", "JavaScript");
words.forEach(word -> System.out.println(word.length())); // Output: 4 6 10
}
}
forEach on list (not stream) uses lambda to compute and print length() for each. Basic iteration with lambda.
4
6
10
import java.util.Arrays;
import java.util.List;
public class DistinctElements {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 2, 3, 4, 4);
numbers.stream()
.distinct()
.forEach(System.out::println); // Output: 1 2 3 4
}
}
The distinct() intermediate removes duplicates (uses hash set internally). Then print.
1
2
3
4
import java.util.Arrays;
import java.util.List;
public class SortedOrder {
public static void main(String[] args) {
List<String> names = Arrays.asList("Charlie", "Alice", "Bob");
names.stream()
.sorted()
.forEach(System.out::println); // Output: Alice Bob Charlie
}
}
This program creates a list of names, sorts them alphabetically using the sorted() method, and prints them using forEach().
Alice
Bob
Charlie
import java.util.Optional;
public class OptionalExample {
public static void main(String[] args) {
Optional<String> name = Optional.of("Java");
name.ifPresent(System.out::println); // Output: Java
}
}
This program uses the Optional class to handle a possibly null value. If the value is present, it gets printed using ifPresent().
Java
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class GroupByLength {
public static void main(String[] args) {
List<String> words = Arrays.asList("a", "bb", "ccc", "dd");
Map<Integer, List<String>> grouped = words.stream()
.collect(Collectors.groupingBy(String::length));
System.out.println(grouped); // Output: {1=[a], 2=[bb, dd], 3=[ccc]}
}
}
This program groups strings by their lengths using Collectors.groupingBy() and prints the resulting map.
{1=[a], 2=[bb, dd], 3=[ccc]}
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class CollectToList {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3);
List<Integer> squares = numbers.stream()
.map(n -> n * n)
.collect(Collectors.toList());
System.out.println(squares); // Output: [1, 4, 9]
}
}
This program squares each number in the list using map() and collects the results into a new list with Collectors.toList().
[1, 4, 9]
import java.util.Arrays;
import java.util.List;
public class LimitAndSkip {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
numbers.stream()
.limit(3)
.forEach(System.out::println); // Output: 1 2 3
}
}
This program uses limit() to restrict the number of elements processed in the stream to the first 3 and prints them.
1
2
3
import java.util.Arrays;
import java.util.List;
public class FindFirst {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(5, 3, 1, 4, 2);
int first = numbers.stream()
.sorted()
.findFirst()
.get();
System.out.println(first); // Output: 1
}
}
This program sorts the list and then uses findFirst() to get and print the first element.
1
@FunctionalInterface
interface MyFunction {
int apply(int a, int b);
}
public class CustomFunctionalInterface {
public static void main(String[] args) {
MyFunction add = (a, b) -> a + b;
System.out.println(add.apply(10, 20)); // Output: 30
}
}
This program defines a custom functional interface MyFunction and uses a lambda expression to implement the apply() method for adding two numbers.
30
import java.util.Arrays;
import java.util.List;
public class FlatMapExample {
public static void main(String[] args) {
List<List<String>> list = Arrays.asList(
Arrays.asList("A", "B"),
Arrays.asList("C", "D"));
list.stream()
.flatMap(List::stream)
.forEach(System.out::println); // Output: A B C D
}
}
This program uses flatMap() to flatten a list of lists into a single stream and prints each element.
A
B
C
D
import java.util.Arrays;
import java.util.List;
public class PeekExample {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
numbers.stream()
.peek(n -> System.out.println("Processing: " + n))
.map(n -> n * n)
.forEach(System.out::println);
}
}
The peek() method lets you observe elements as the stream processes them. Here, it prints each element before it's squared. This is typically used for debugging.
Processing: 1
1
Processing: 2
4
Processing: 3
9
Processing: 4
16
import java.util.concurrent.CompletableFuture;
public class CompletableFutureExample {
public static void main(String[] args) {
CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 1 + 2);
future.thenAccept(System.out::println); // Output: 3
}
}
This program uses CompletableFuture.supplyAsync() to compute 1 + 2 asynchronously, then prints the result with thenAccept().
3
import java.util.Arrays;
import java.util.List;
public class CustomReduce {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
int prod = numbers.stream().reduce(1, (a, b) -> a * b);
System.out.println(prod); // Output: 24
}
}
The reduce() method is used to perform a reduction operation. In this case, it multiplies all the elements in the list to calculate the product. The starting value is 1.
24
import java.util.Arrays;
import java.util.List;
public class AnyMatchExample {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3);
boolean haseven = numbers.stream().anyMatch(n -> n % 2 == 0);
System.out.println(haseven); // Output: true
}
}
The anyMatch() method checks if any element in the list satisfies the given condition (in this case, being even). It returns true if at least one element matches the condition.
true
Modern Java 8 interviews often include scenario-based questions that test your ability to apply core concepts to real-world problems. Below are some sample scenarios and coding challenges using terms such as Arrays.asList, Collectors, List<String>, map, toList, toUpperCase, charAt, and isPalindrome. Practicing these will help you demonstrate both your technical skills and problem-solving abilities in the IT sector.
Suppose you are given a list of strings and need to filter out those that start with the letter 'A', then convert the remaining strings to uppercase.
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class ScenarioFilterAndUppercase {
public static void main(String[] args) {
// List of words to process
List<String> words = Arrays.asList("Apple", "Banana", "Apricot", "Orange");
// Stream processing: Filter words not starting with 'A' and convert to uppercase
List<String> result = words.stream()
.filter(s -> !s.startsWith("A"))
.map(String::toUpperCase) // Convert remaining words to uppercase
.collect(Collectors.toList()); // Collect the result into a list
// Output the result
System.out.println(result);
}
}
This program filters out words starting with the letter 'A' and converts the remaining words to uppercase. It uses filter() to exclude words beginning with 'A', then map() to transform the remaining words to uppercase. Finally, collect() gathers the results into a list.
[BANANA, ORANGE]
A common coding interview question is to check if a given string is a palindrome using Java 8 features, such as IntStream and charAt.
import java.util.stream.IntStream;
public class PalindromeCheck {
public static void main(String[] args) {
String input = "madam";
// Check if the string is a palindrome
boolean isPalindrome = IntStream.range(0, input.length() / 2)
.allMatch(i -> input.charAt(i) == input.charAt(input.length() - i - 1));
// Output the result
System.out.println(isPalindrome);
}
}
The program aims to verify if a given string is a palindrome or not, meaning the string reads the same way from left to right and from right to left. To perform the iteration over the first half of the string, the program uses IntStream.range(), and with the help of allMatch(), it checks whether the characters from both ends of the string are identical.
true
Given a string, count the number of occurrences of each character using Java 8 streams and collectors.
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
public class CharCount {
public static void main(String[] args) {
String input = "interview";
// Using stream to count character occurrences
Map<Character, Long> charCount = input.chars()
.mapToObj(c -> (char) c) // Convert int values to characters
.collect(Collectors.groupingBy(Function.identity(), Collectors.counting())); // Group by character and count
// Print the result
System.out.println(charCount);
}
}
This program counts the occurrences of each character in a given string. It converts the string into an IntStream of character codes with chars(), then maps these codes to Character objects. Using collect() with groupingBy() and counting(), it creates a map of character frequencies.
{i=2, n=1, t=1, e=2, r=1, v=1, w=1}
Such situational questions are typical of the difficulties that you might encounter in a Java 8 interview, particularly in the technology field. Working through these issues will enable you to take on actual coding tasks with confidence and demonstrate your skill with Java 8 features.
MCQs serve as an efficient way to assess your understanding of key Java 8 concepts. By answering these questions, developers can evaluate their proficiency with new features commonly used in real-world coding scenarios. Below are some of the critical Java 8 features that are often tested in coding questions for experienced developers:
These expressions are among the most popular Java 8 features, providing a clear and concise way to implement functional interfaces. Understanding how to implement and apply lambda expressions is vital for developers working with Java 8.
Example MCQ:
What is the primary advantage of using lambda expressions in Java 8?
The Optional class was introduced to avoid NullPointerExceptions and to handle null values more expressively and safely. It is a container object that may or may not contain a non-null value. Mastery of Optional is essential for any Java developer aiming for cleaner and more reliable code.
Example MCQ:
Which of the following methods is used to check if a value is present in an Optional object in Java 8?
The Stream API is another revolutionary feature introduced in Java 8 that enables functional-style operations on sequences of elements, such as collections. It allows developers to perform operations like filtering, mapping, and reducing in a more declarative way.
Example MCQ:
Which of the following is an example of a terminal operation in the Stream API?
Scenario-based MCQs test how well you can apply Java 8 concepts in real-life situations. These questions assess your problem-solving abilities and your understanding of how to use Java 8 features in practical applications.
Example Scenario-Based MCQ:
You have a list of employee objects, and you want to find the names of all employees who earn more than $50,000. How would you implement this using Java 8's Stream API?
In addition to fundamental features, MCQs also assess your ability to understand and work with advanced Java 8 features. These features include method references, default methods in interfaces, and more.
Simplify the usage of lambda expressions when invoking methods.
Example MCQ:
Which of the following is the correct way to use a method reference in Java 8?
Introduced to allow adding new methods to interfaces without breaking existing implementations.
Example MCQ:
What is the purpose of the default keyword in Java 8 interfaces?
In conclusion, while preparing for Java 8 coding interviews, it's crucial to focus on a variety of topics. Practice questions often involve writing code to manipulate collections using the Stream API. Solving Java 8 programming questions will sharpen your problem-solving skills and reinforce your understanding of modern Java features. Overall, consistent practice with Java 8 coding questions will help you excel in technical interviews.
Java 8 skills are foundational for frameworks such as Spring Boot and big-data tools such as Apache Spark. In 2025, they signal proficiency in functional programming, reducing bugs and improving maintainability in legacy systems still on Java 8.
peek()Common questions include those related to Stream API, lambda expressions, and functional programming concepts. They may also involve practical coding challenges to test problem-solving skills.
You can practice by solving problems on coding platforms like LeetCode, and HackerRank, or by working on personal projects that include Java 8 features.
The Stream API significantly simplifies data manipulation and enhances performance through parallel processing and functional programming techniques.
Yes, many online platforms, books, and coding challenge websites offer extensive resources on Java 8 interview questions and coding exercises.
Source: NxtWave - CCBP Blog
Original URL: https://www.ccbp.in/blog/articles/java-8-coding-interview-questions-and-answers