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Java 8 brought a major paradigm shift to the language with the introduction of functional programming features and powerful APIs. Whether you're preparing for a job interview or brushing up on the latest Java features, these Java 8 interview questions will help you get ready with confidence.

🔹 1. What are the main features introduced in Java 8?

Java 8 introduced several key enhancements:

• Lambda Expressions: Enables functional programming by allowing code to be passed as data.
• Stream API: Facilitates functional-style operations on collections.
• Functional Interfaces: Interfaces with a single abstract method (e.g., Runnable, Comparator).
• Default and Static Methods in Interfaces: Allows method implementation inside interfaces.
• Optional Class: A container to avoid null references.
• New Date and Time API: A modern, immutable, and thread-safe date/time API (java.time package).
• Method References and Constructor References
• Nashorn JavaScript Engine (removed in later versions)

🔹 2. What is a lambda expression in Java? How is it used?

Lambda expressions provide a concise way to implement functional interfaces.

Syntax:

java

(parameter1, parameter2) -> { body }

Example:

java

List<String> names = Arrays.asList("Ava", "Emma", "Olivia");
names.forEach(name -> System.out.println(name));

Lambdas are often used with functional interfaces, making code more readable and reducing boilerplate.

🔹 3. Explain the Stream API. How is it different from Collections?

The Stream API allows processing sequences of elements (like Collections) in a functional style.

Key differences from Collections:

Stream APICollectionsDoes not store dataStores dataIs functionalIs imperativeSupports lazy evaluationEager operationsOften used with lambdaTraditional loops

Example:

java

List<String> names = Arrays.asList("Tom", "Jerry", "Spike");
names.stream().filter(n -> n.startsWith("T")).forEach(System.out::println);

🔹 4. What is a default method in an interface?

Java 8 introduced default methods so that interfaces can have method implementations without breaking existing code.

Example:

java

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

This enables backward compatibility for adding new methods to interfaces.

🔹 5. What is the java.util.function package?

This package provides functional interfaces such as:

• Predicate<T> – returns a boolean
• Function<T, R> – takes input T and returns R
• Consumer<T> – takes input T, returns nothing
• Supplier<T> – supplies objects of type T

These interfaces enable you to pass functions as parameters and use lambdas efficiently.

🔹 6. How does the Optional class work? When should it be used?

Optional<T> is a container object that may or may not contain a non-null value. It helps avoid NullPointerExceptions.

Example:

java

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

Use Optional for return values, not for fields or method parameters.

🔹 7. Explain the new Date and Time API introduced in Java 8.

Java 8 introduced a modern date/time API in the java.time package.

Key classes:

• LocalDate, LocalTime, LocalDateTime
• ZonedDateTime, Instant
• DateTimeFormatter for formatting

Example:

java

LocalDate today = LocalDate.now();
LocalDate birthday = LocalDate.of(1990, Month.JANUARY, 1);

The new API is immutable, thread-safe, and much more intuitive than Date or Calendar.

🔹 8. What are method references in Java 8? How do they differ from lambda expressions?

Method references are a shorthand notation for lambdas calling existing methods.

Example:

java

list.forEach(System.out::println); // Method reference

Equivalent Lambda:

java

list.forEach(x -> System.out.println(x));

They improve code readability when you're just forwarding arguments to a method.

🔹 9. What’s the difference between forEach() and parallelStream()?

• forEach() is used to iterate over elements (can be used on both streams and collections).
• parallelStream() processes elements in parallel, using multiple threads.

Example:

java

list.parallelStream().forEach(System.out::println);

Use parallelStream() for performance when processing large data sets, but be mindful of thread-safety.

🔹 10. How does the try-with-resources syntax improve resource management?

Java 8 allows you to declare resources that implement AutoCloseable inside the try block. These resources are automatically closed after the block execution.

Example:

java

try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    System.out.println(br.readLine());
} catch (IOException e) {
    e.printStackTrace();
}

This helps prevent resource leaks and eliminates the need for explicit finally blocks.

Conclusion:

Java 8 revolutionized how we write Java by introducing functional programming constructs and improving APIs. These interview questions and answers should prepare you well for discussions about Java 8 in technical interviews.