Top 50+ Java 8 Interview Questions & Answers 2026 [Updated]

Java 8 introduced major functional programming features that are now standard interview topics. Here are the most frequently asked questions.

1. What are the major features introduced in Java 8?

Lambda expressions, the Streams API, functional interfaces, default and static methods in interfaces, and the new Date/Time API.

2. What is a lambda expression?

A concise way to represent an anonymous function, used mainly to implement functional interfaces with less boilerplate code.

Runnable r = () -> System.out.println("Running");

3. What is a functional interface?

An interface with exactly one abstract method, which can be implemented using a lambda expression, such as Runnable, Comparator, or Function.

4. What is the Streams API?

A way to process collections of data declaratively using operations like filter, map, and reduce, often chained together in a pipeline.

List<Integer> evens = numbers.stream()
    .filter(n -> n % 2 == 0)
    .collect(Collectors.toList());

5. What is the difference between map() and flatMap()?

map() transforms each element into another element, while flatMap() flattens nested structures (like a stream of lists) into a single stream.

6. What are default and static methods in interfaces?

Default methods provide a method body in an interface that implementing classes can use as-is or override; static methods belong to the interface itself and are called directly on it.

7. What is the Optional class?

Optional is a container object used to represent the possible absence of a value, helping avoid NullPointerException in a more explicit way.

Optional<String> name = Optional.ofNullable(getName());
name.ifPresent(System.out::println);

8. What is method reference in Java 8?

A shorthand notation for a lambda expression that calls an existing method, using the :: syntax, e.g. System.out::println.

9. What is the difference between intermediate and terminal operations in Streams?

Intermediate operations (like filter, map) return a new stream and are lazy; terminal operations (like collect, forEach) trigger the actual processing.

10. What is the new Date and Time API in Java 8?

The java.time package (LocalDate, LocalDateTime, etc.) provides an immutable, thread-safe replacement for the older, error-prone Date and Calendar classes.