Question 1

What are the main features introduced in Java 8?

Accepted Answer

Java 8 introduced lambda expressions, the Stream API for functional-style operations on collections, default and static methods in interfaces, the Optional class for null-safety, a new Date and Time API (java.time package), method references, and functional interfaces. These features brought functional programming paradigms to Java, making code more concise, readable, and maintainable.

Question 2

What is a lambda expression in Java 8?

Accepted Answer

A lambda expression is a concise way to represent an anonymous function that can be passed as an argument or stored in a variable. It follows the syntax (parameters) -> expression or (parameters) -> { statements; }, eliminating the verbosity of anonymous inner classes and enabling functional programming in Java.

Question 3

What is a functional interface?

Accepted Answer

A functional interface is an interface with exactly one abstract method, marked with the @FunctionalInterface annotation. Lambda expressions are implementations of functional interfaces. Java 8 provides built-in functional interfaces like Predicate, Function, Consumer, and Supplier in the java.util.function package.

Question 4

What is the Stream API in Java 8?

Accepted Answer

The Stream API is a functional programming abstraction that allows processing sequences of elements in a declarative way. It supports operations like filtering, mapping, and reducing data with both sequential and parallel execution models. Streams don't store data but convey elements from a source through a pipeline of computational operations.

Question 5

What is the difference between map() and flatMap() in streams?

Accepted Answer

map() transforms each element into exactly one new element (one-to-one mapping), while flatMap() transforms each element into a stream of elements and flattens all those streams into a single stream (one-to-many mapping). Use map() for simple transformations and flatMap() when dealing with nested structures or when each element produces multiple values.

Question 6

What are intermediate and terminal operations in streams?

Accepted Answer

Intermediate operations (like filter, map, sorted) transform a stream into another stream and are lazily evaluated. Terminal operations (like collect, forEach, reduce) produce a result or side-effect and trigger the execution of the entire stream pipeline, consuming the stream. Intermediate operations don't execute until a terminal operation is called.

Question 7

What is the Optional class and why was it introduced?

Accepted Answer

Optional is a container object that may or may not contain a non-null value, introduced to reduce NullPointerExceptions. It forces developers to explicitly handle the absence of a value, making null-handling more explicit and code more readable. Optional is primarily designed for return types where absence is a valid, expected outcome.

Question 8

What are method references in Java 8?

Accepted Answer

Method references are a shorthand notation for lambda expressions that call a single method, using the :: operator. For example, System.out::println is equivalent to x -> System.out.println(x). They make code more concise and readable when a lambda expression does nothing but call an existing method.

Question 9

What are the four types of method references?

Accepted Answer

The four types are: static method references (ClassName::staticMethod), instance method references on a particular object (object::instanceMethod), instance method references on an arbitrary object of a type (ClassName::instanceMethod), and constructor references (ClassName::new). Each serves different use cases in functional programming.

Question 10

Why was the new Date and Time API introduced in Java 8?

Accepted Answer

The new java.time API replaced the legacy Date and Calendar classes which were poorly designed, not thread-safe, and difficult to use. The new API is immutable, thread-safe, follows clear naming conventions, and separates concerns between dates (LocalDate), times (LocalTime), date-times (LocalDateTime), and time zones (ZonedDateTime).

Question 11

What is the difference between LocalDate, LocalTime, and LocalDateTime?

Accepted Answer

LocalDate represents a date without time (e.g., 2024-01-15), LocalTime represents a time without date (e.g., 14:30:45), and LocalDateTime combines both date and time (e.g., 2024-01-15T14:30:45). None of them store time zone information - they represent local values without geographic location context.

Question 12

What are default methods in interfaces?

Accepted Answer

Default methods are interface methods with concrete implementations, introduced in Java 8 using the default keyword. They allow adding new methods to interfaces without breaking existing implementations, enabling interface evolution while maintaining backward compatibility. This was crucial for adding lambda-related methods to the Collections framework.

Question 13

What is the Predicate functional interface?

Accepted Answer

Predicate<T> represents a boolean-valued function that takes one argument and returns true or false through its test(T t) method. It's commonly used for filtering in streams, validation, and conditional logic. Predicates can be composed using and(), or(), and negate() methods for building complex conditions.

Question 14

What is the difference between sequential and parallel streams?

Accepted Answer

Sequential streams process elements one at a time in a single thread, while parallel streams divide elements across multiple threads for concurrent processing using the Fork/Join framework. Create parallel streams using parallelStream() or stream().parallel(). Parallel streams can improve performance on large datasets with computationally expensive operations, but have overhead that may make them slower for small datasets.

Question 15

How do you create an Optional object?

Accepted Answer

Optional objects are created using three static methods: Optional.of(value) for non-null values (throws NPE if null), Optional.ofNullable(value) for potentially null values (returns empty Optional if null), and Optional.empty() for explicitly empty optionals. Never use the constructor as it's private.

Java 8 Fundamentals(3 questions)

What are the main features introduced in Java 8?

Why was Java 8 considered a major release in Java's history?

What is the minimum JDK version required to use Java 8 features?

Other Java 8 Features(7 questions)

What is the StringJoiner class?

What are the new methods added to the Collection interface in Java 8?

What is the forEach() method and how does it differ from traditional for loops?

What are the new methods added to the Map interface (compute, merge, getOrDefault)?

What is the Base64 class in Java 8?

What are the improvements to the Comparator interface in Java 8?

How do you use Comparator.comparing() and thenComparing()?

Performance and Best Practices(5 questions)

What are the performance considerations when using streams?

When should you prefer traditional loops over streams?

What are the best practices for writing lambda expressions?

How do you debug streams and lambda expressions?

What is lazy evaluation in streams and why is it important?

Lambda Expressions(7 questions)

What is a lambda expression in Java 8?

What is the syntax of a lambda expression?

What is a functional interface and how does it relate to lambda expressions?

What is the difference between a lambda expression and an anonymous inner class?

Can lambda expressions access local variables? What are the restrictions?

What is "effectively final" and why is it important for lambdas?

How do you handle checked exceptions in lambda expressions?

Functional Interfaces(8 questions)

What is the @FunctionalInterface annotation and is it mandatory?

Explain the Predicate functional interface and its use cases.

Explain the Function<T, R> functional interface and its use cases.

Explain the Consumer functional interface and its use cases.

Explain the Supplier functional interface and its use cases.

What is the difference between Function and BiFunction?

What is the UnaryOperator and BinaryOperator interface?

How do you compose multiple functions using andThen() and compose()?

Method References(5 questions)

What are method references in Java 8?

What are the four types of method references?

When would you use a method reference instead of a lambda expression?

How do you reference a constructor using method references?

What is the difference between ClassName::instanceMethod and object::instanceMethod?

Stream API(14 questions)

What is the Stream API in Java 8?

What is the difference between a Collection and a Stream?

What are intermediate and terminal operations in streams?

Explain the difference between map() and flatMap().

What is the difference between filter() and distinct()?

How do you use reduce() operation in streams?

What is the difference between findFirst() and findAny()?

What are short-circuiting operations in streams?

How do you convert a Stream to a Collection or Array?

What is the Collectors class and what are its common methods?

How do you perform grouping and partitioning with Collectors?

What is the difference between sequential and parallel streams?

When should you use parallel streams and what are the pitfalls?

How do you create a Stream from different sources?

Optional Class(8 questions)

What is the Optional class and why was it introduced?

How do you create an Optional object?

What is the difference between Optional.of() and Optional.ofNullable()?

How do you safely retrieve a value from Optional?

What is the difference between orElse() and orElseGet()?

When should you use orElseThrow()?

How do you use map() and flatMap() with Optional?

What are the anti-patterns to avoid when using Optional?

Date and Time API (java.time)(8 questions)

Why was the new Date and Time API introduced in Java 8?

What is the difference between LocalDate, LocalTime, and LocalDateTime?

What is ZonedDateTime and when would you use it?

What is the difference between Instant and LocalDateTime?

How do you format and parse dates using DateTimeFormatter?

What is the Period class and how does it differ from Duration?

How do you perform date arithmetic in Java 8?

How do you convert between legacy Date/Calendar and java.time classes?

Default and Static Methods in Interfaces(5 questions)

What are default methods in interfaces and why were they introduced?

What is the diamond problem and how does Java 8 resolve it?

What are static methods in interfaces?

What is the difference between default methods and abstract methods?

Can you override default methods in implementing classes?