Top 10 Java 8 Interview Questions

Java 8 introduced several powerful features that revolutionized the way Java is used for programming. These features are frequently discussed during interviews. Here, we’ll explore the Top 10 Java 8 Interview Questions along with comprehensive answers to help you ace your interview.

1. What are the key features introduced in Java 8?#

Answer:

Java 8 introduced several groundbreaking features, including:

• Lambda Expressions: Enables functional programming by allowing concise syntax for implementing functional interfaces.
• Streams API: Facilitates functional-style operations on collections and sequences of elements.
• Functional Interfaces: Interfaces with a single abstract method, like Runnable and Callable. Annotated with @FunctionalInterface.
• Optional Class: Provides a way to handle null values gracefully and avoid NullPointerException.
• Default Methods: Allows interfaces to have methods with implementations.
• New Date and Time API: Introduced java.time package for better date-time handling.
• Method References: Simplifies referring to methods by their names instead of invoking them.
• Nashorn JavaScript Engine: Allows integration of JavaScript with Java.

2. What are Lambda Expressions, and why are they useful?#

Answer:

Lambda Expressions are a way to represent anonymous functions in Java. They enable functional programming and concise code. The syntax is:

(parameters) -> expression
(parameters) -> { statements; }

Example:

// Traditional way
Runnable r1 = new Runnable() {
    @Override
    public void run() {
        System.out.println("Hello World");
    }
};

// Using Lambda
Runnable r2 = () -> System.out.println("Hello World");

Benefits:

• Reduces boilerplate code.
• Improves readability.
• Enables functional-style programming.

3. What is the Streams API, and how is it used?#

Answer:

The Streams API is used for processing collections of data in a functional style. Streams support operations like filtering, mapping, and reducing.

Key Characteristics:

• Lazy Evaluation: Operations are not performed until a terminal operation is invoked.
• Stateless: Intermediate operations don’t store state.
• Parallelism: Simplifies parallel execution with parallelStream().

Example:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
List<Integer> evenNumbers = numbers.stream()
    .filter(n -> n % 2 == 0)
    .collect(Collectors.toList());

System.out.println(evenNumbers); // Output: [2, 4]

4. What is a Functional Interface? Give examples.#

Answer:

A Functional Interface is an interface with exactly one abstract method. It can have any number of default or static methods.

Examples:

• Built-in Functional Interfaces:
  • Predicate<T>: Accepts a single input and returns a boolean.
  • Consumer<T>: Accepts a single input and performs an operation.
  • Supplier<T>: Provides a result without taking any input.
  • Function<T, R>: Accepts an input and produces an output.

Example Code:

@FunctionalInterface
interface MyFunctionalInterface {
    void display();
}

MyFunctionalInterface mfi = () -> System.out.println("Hello from Lambda");
mfi.display();

5. What are Method References in Java 8?#

Answer:

Method References provide a shorthand for referring to methods by their names. Syntax:

ClassName::methodName

Types:

1. Static Method Reference: ClassName::staticMethod
2. Instance Method Reference: object::instanceMethod
3. Constructor Reference: ClassName::new

Example:

List<String> names = Arrays.asList("John", "Jane", "Jack");

// Using Lambda
names.forEach(name -> System.out.println(name));

// Using Method Reference
names.forEach(System.out::println);

6. What is the Optional class in Java 8? Why is it useful?#

Answer:

The Optional class is a container for holding nullable values and helps avoid NullPointerException.

Example:

Optional<String> optional = Optional.ofNullable(null);
System.out.println(optional.orElse("Default Value"));

optional.ifPresent(value -> System.out.println(value));

Benefits:

• Encourages writing cleaner and safer code.
• Eliminates explicit null checks.

7. What are Default Methods in Interfaces?#

Answer:

Default Methods are methods with implementations inside interfaces. They were introduced to provide backward compatibility to existing interfaces.

Example:

interface Vehicle {
    void run();

    default void stop() {
        System.out.println("Stopping the vehicle.");
    }
}

class Car implements Vehicle {
    public void run() {
        System.out.println("Car is running.");
    }
}

Car car = new Car();
car.run();
car.stop();

8. How does Java 8 handle Date and Time?#

Answer:

Java 8 introduced the java.time package to replace the outdated java.util.Date and Calendar classes.

Key Classes:

• LocalDate: Represents a date without a time zone.
• LocalTime: Represents a time without a date.
• LocalDateTime: Combines date and time.
• ZonedDateTime: Handles time zones.

Example:

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

System.out.println(date);      // Output: 2024-12-12
System.out.println(time);      // Output: 14:45:30
System.out.println(dateTime); // Output: 2024-12-12T14:45:30

9. What are Streams’ Terminal and Intermediate Operations?#

Answer:

• Intermediate Operations: Return another stream. Examples: filter(), map(), sorted().
• Terminal Operations: Trigger the processing of streams. Examples: collect(), forEach(), reduce().

Example:

List<String> names = Arrays.asList("Alice", "Bob", "Charlie");

// Intermediate: Filter and Map
List<String> upperCaseNames = names.stream()
    .filter(name -> name.startsWith("A"))
    .map(String::toUpperCase)
    .collect(Collectors.toList());

System.out.println(upperCaseNames); // Output: [ALICE]

10. What are the differences between a Stream and a Collection?#

Answer:

Conclusion#

These questions represent some of the most important Java 8 concepts frequently asked in interviews. Mastering these will not only help you in interviews but also enhance your coding skills for real-world applications. Good luck!