Top Java 8 Interview Questions and Answers with Code Examples

1️⃣ What are the key features introduced in Java 8?

Lambda Expressions
Functional Interfaces
Stream API
Optional Class
Default and Static methods in interfaces
New Date and Time API
Method References

2️⃣ What is a Lambda Expression in Java?

A Lambda Expression is an anonymous function that helps implement methods of functional interfaces in a cleaner way.


List<String> names = Arrays.asList("Tom", "Jerry", "Spike");
names.forEach(name -> System.out.println(name));

3️⃣ What is a Functional Interface?

A Functional Interface has exactly one abstract method and can have any number of default/static methods. Example: Runnable, Comparator, Function, Predicate.


@FunctionalInterface
interface MyFunctionalInterface {
    void execute();
}

4️⃣ What is the Stream API and how is it used?

Stream API is used to process collections of objects in a functional style (map, filter, reduce).


List<String> names = Arrays.asList("apple", "banana", "cherry");
List<String> result = names.stream()
                            .filter(s -> s.startsWith("b"))
                            .collect(Collectors.toList());

5️⃣ What is the difference between Stream and Collection?

Collection is an in-memory data structure.
Stream is for processing data using a pipeline of operations.
Streams don’t store data—they operate on source data.

6️⃣ What is the Optional class?

Optional is a container object which may or may not contain a non-null value, used to avoid NullPointerException.


Optional<String> optional = Optional.ofNullable("Hello");
optional.ifPresent(System.out::println);

7️⃣ What is the difference between map() and flatMap()?

map(): transforms elements.
flatMap(): flattens nested structures.


List<List<String>> data = Arrays.asList(Arrays.asList("a", "b"), Arrays.asList("c"));
List<String> flat = data.stream().flatMap(List::stream).collect(Collectors.toList());

8️⃣ How do you use the new Date/Time API?

Java 8 introduced java.time package with immutable and thread-safe classes.


LocalDate date = LocalDate.now();
LocalTime time = LocalTime.now();
LocalDateTime dateTime = LocalDateTime.of(date, time);

9️⃣ What are Method References?

Method references are a shorthand notation for lambda expressions that call a method.


List<String> names = Arrays.asList("A", "B", "C");
names.forEach(System.out::println);

🔟 What are Default and Static methods in interfaces?

Default methods: Provide method implementations in interfaces.
Static methods: Utility methods in interfaces.


interface MyInterface {
    default void show() {
        System.out.println("Default method");
    }

    static void utility() {
        System.out.println("Static method");
    }
}

📌 Final Thoughts

Java 8 brought functional programming to Java. Mastering its features like Lambda, Streams, Optional, and new Date APIs can significantly improve both code readability and performance, and help crack technical interviews with ease.

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