Java 8 Interview Questions and Answers (2026)

The short answer: Java 8 questions are still a staple of Java interviews because its features, lambdas, functional interfaces, default methods, the Stream API, Optional and the java.time API, are how modern Java is written, and Java 8 itself is still supported: Oracle lists extended support for Java SE 8 until December 2030. Expect to explain each feature, write short stream pipelines (filter, map, group, collect), and discuss trade-offs such as when parallel streams help. Below are the common questions with short answers and code.

Why Java 8 still comes up

Java 8 was released in March 2014. Oracle’s support roadmap lists Java SE 8 as a long-term support release with extended support until December 2030, alongside later LTS releases (11, 17, 21 and 25). Many production systems still run it, and its features are the foundation of everything since, so interviewers use it to check modern Java fluency.

Lambdas and functional interfaces

“What is a lambda expression?”

Oracle describes lambdas as letting you “treat functionality as a method argument, or code as data.” A lambda is a concise implementation of a functional interface:

Comparator<String> byLength = (a, b) -> Integer.compare(a.length(), b.length());

“What is a functional interface?”

An interface with exactly one abstract method, such as Runnable, Comparator, or those in java.util.function: Predicate<T> (returns boolean), Function<T,R> (transforms), Consumer<T> (accepts, returns nothing), and Supplier<T> (produces a value). @FunctionalInterface makes the compiler enforce it.

“What are method references?”

Shorthand for lambdas that just call an existing method: String::toUpperCase, System.out::println, ArrayList::new.

“What can a lambda capture?”

Local variables that are final or effectively final, plus fields. That’s why you can’t increment a local counter inside a lambda.

Default and static methods in interfaces

“Why were default methods added?”

Oracle explains that “default methods enable new functionality to be added to the interfaces of libraries and ensure binary compatibility with code written for older versions.” That’s how forEach and stream() were added to existing collection interfaces.

“What happens if a class inherits two default methods with the same signature?”

It won’t compile until the class overrides the method; it can call a specific one with InterfaceName.super.method().

The Stream API

“What is a stream?”

A sequence of elements supporting functional-style operations, from the java.util.stream package, integrated with collections “which enables bulk operations on collections, such as sequential or parallel map-reduce transformations,” as Oracle puts it. A stream doesn’t store data and can be consumed only once.

“What’s the difference between intermediate and terminal operations?”

Intermediate operations (filter, map, sorted, distinct) return a new stream and are lazy; nothing runs until a terminal operation (collect, forEach, reduce, count, findFirst) is called.

“Group employees by department and count them.”

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

“Find the names of the three highest-paid employees.”

List<String> top3 = employees.stream()
    .sorted(Comparator.comparingDouble(Employee::getSalary).reversed())
    .limit(3)
    .map(Employee::getName)
    .collect(Collectors.toList());

“What’s the difference between map and flatMap?”

map transforms each element into one element; flatMap transforms each element into a stream and flattens the results, for example turning a list of orders into a stream of all their items.

“When should you use parallel streams?”

For large, CPU-bound workloads with independent, stateless operations and data structures that split well (arrays, ArrayList). Avoid them for small collections, I/O-bound work, or operations with shared mutable state; they use the common fork-join pool, so measure before and after.

Optional

“What is Optional for, and how should you use it?”

A container that may or may not hold a value, mainly as a return type to make “no result” explicit. Prefer map, orElse, orElseGet and orElseThrow over calling get() without checking, and don’t use Optional for fields or method parameters.

String city = findUser(id)
    .map(User::getAddress)
    .map(Address::getCity)
    .orElse("Unknown");

Date and time

“What problems did java.time solve?”

The old Date and Calendar classes were mutable and not thread-safe, with confusing APIs. The java.time package, which Oracle describes as “a comprehensive date-time model,” has immutable types: LocalDate, LocalDateTime, ZonedDateTime, Instant, Duration and Period, plus a thread-safe DateTimeFormatter.

Concurrency

“What is CompletableFuture?”

A future you can compose without blocking: chain steps with thenApply and thenCompose, combine with thenCombine or allOf, and handle errors with exceptionally or handle.

Other Java 8 changes worth knowing

• HashMap collisions: buckets with many entries switch from linked lists to balanced trees, improving worst-case lookups.
• PermGen was replaced by Metaspace for class metadata.
• Nashorn replaced the Rhino JavaScript engine (it was later removed in Java 15).

How to answer well

Explain the “why,” not only the syntax: readability, immutability, laziness, and when not to use a feature. Write code out loud, and mention what changed in later versions if you know it (for example var, records and Stream.toList()).

See also our Python interview questions for junior developers and system design interview guide.

Frequently asked questions

Sources

• Oracle, What’s New in JDK 8. Read on 1 October 2026.
• Oracle, Oracle Java SE Support Roadmap. Read on 1 October 2026.