Java 8 Features

1. What is a lambda expression?

A lambda expression is a concise way to represent an anonymous function that can be passed around as a parameter.

(a, b) -> a + b; // Example of a lambda expression

2. How do you use lambda expressions with functional interfaces?

Lambda expressions can be used to implement functional interfaces, which are interfaces with a single abstract method.

@FunctionalInterface
interface MyFunctionalInterface {
    void myMethod();
}
MyFunctionalInterface myFunc = () -> System.out.println("Hello from lambda!");
myFunc.myMethod();

3. What is the Stream API?

The Stream API allows you to process sequences of elements (like collections) in a functional style.

4. How do you create a stream from a collection?

You can create a stream from a collection using the stream() method.

List<String> list = Arrays.asList("a", "b", "c");
Stream<String> stream = list.stream();

5. What is the filter() method in streams?

The filter() method is used to filter elements based on a predicate.

List<String> filtered = list.stream()
    .filter(s -> s.startsWith("a"))
    .collect(Collectors.toList());

6. What is the map() method in streams?

The map() method is used to transform each element in the stream.

List<Integer> lengths = list.stream()
    .map(String::length)
    .collect(Collectors.toList());

7. What is the reduce() method in streams?

The reduce() method is used to combine elements in the stream into a single result.

Optional<String> concatenated = list.stream()
    .reduce((s1, s2) -> s1 + s2);

8. What is the collect() method in streams?

The collect() method is used to accumulate the elements of a stream into a collection.

List<String> collected = stream.collect(Collectors.toList());

9. What is a functional interface?

A functional interface is an interface that contains exactly one abstract method.

10. What is the Optional class?

The Optional class is a container object which may or may not contain a value, used to avoid null references.

11. How do you create an Optional?

You can create an Optional using the Optional.of(), Optional.ofNullable(), or Optional.empty() methods.

Optional<String> optional = Optional.of("Hello");

12. How do you check if an Optional has a value?

You can check if an Optional has a value using the isPresent() method.

if (optional.isPresent()) {
    System.out.println(optional.get());
}

13. What is the flatMap() method in Optional?

The flatMap() method is used to transform the value inside an Optional and return another Optional.

Optional<String> transformed = optional.flatMap(value -> Optional.of(value.toUpperCase()));

14. What is the ifPresent() method in Optional?

The ifPresent() method executes a block of code if the Optional contains a value.

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

15. What is the orElse() method in Optional?

The orElse() method returns the value if present, otherwise returns a default value.

String value = optional.orElse("Default");

16. What is the orElseGet() method in Optional?

The orElseGet() method returns the value if present, otherwise invokes a supplier to provide a default value.

String value = optional.orElseGet(() -> "Default");

17. What is the orElseThrow() method in Optional?

The orElseThrow() method returns the value if present, otherwise throws an exception.

String value = optional.orElseThrow(() -> new IllegalArgumentException("Value not present"));

18. What is the method reference in Java 8?

A method reference is a shorthand notation of a lambda expression to call a method.

List<String> list = Arrays.asList("a", "b", "c");
list.forEach(System.out::println);

19. What is the default method in interfaces?

A default method is a method defined in an interface with a body, allowing interfaces to have methods without breaking existing implementations.

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

20. What is the static method in interfaces?

A static method in an interface can be called without an instance of the interface.

interface MyInterface {
    static void myStaticMethod() {
        System.out.println("Static method");
    }
}
MyInterface.myStaticMethod();

21. What is the Collectors class?

The Collectors class provides various implementations of the Collector interface for use with the Stream API.

22. How do you use Collectors.toList()?

You can use Collectors.toList() to collect the elements of a stream into a List.

List<String> collected = list.stream().collect(Collectors.toList());

23. How do you use Collectors.joining()?

You can use Collectors.joining() to concatenate the elements of a stream into a single string.

String result = list.stream().collect(Collectors.joining(", ")); // "a, b, c"

24. How do you use Collectors.groupingBy()?

You can use Collectors.groupingBy() to group elements of a stream by a classifier function.

Map<Integer, List<String>> grouped = list.stream()
    .collect(Collectors.groupingBy(String::length));

25. How do you use Collectors.partitioningBy()?

You can use Collectors.partitioningBy() to partition elements of a stream into two groups based on a predicate.

Map<Boolean, List<String>> partitioned = list.stream()
    .collect(Collectors.partitioningBy(s -> s.length() > 1));

26. What is the java.time package?

The java.time package provides a comprehensive date and time API introduced in Java 8.

27. How do you create a LocalDate?

You can create a LocalDate instance using the LocalDate.now() method or by specifying a date.

LocalDate date = LocalDate.now(); // Current date
LocalDate specificDate = LocalDate.of(2023, 1, 1); // January 1, 2023

28. How do you create a LocalTime?

You can create a LocalTime instance using the LocalTime.now() method or by specifying a time.

LocalTime time = LocalTime.now(); // Current time
LocalTime specificTime = LocalTime.of(12, 30); // 12:30 PM

29. How do you create a LocalDateTime?

You can create a LocalDateTime instance using the LocalDateTime.now() method or by combining a date and time.

LocalDateTime dateTime = LocalDateTime.now(); // Current date and time
LocalDateTime specificDateTime = LocalDateTime.of(2023, 1, 1, 12, 30); // January 1, 2023, 12:30 PM

30. How do you format a LocalDate?

You can format a LocalDate using the DateTimeFormatter class.

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy");
String formattedDate = date.format(formatter);

31. How do you parse a date string into LocalDate?

You can parse a date string into a LocalDate using the LocalDate.parse() method.

LocalDate parsedDate = LocalDate.parse("01-01-2023", formatter);

32. What is the Duration class?

The Duration class represents a time-based amount of time, such as "34.5 seconds".

33. How do you create a Duration?

You can create a Duration instance using the Duration.ofSeconds() method or other similar methods.

Duration duration = Duration.ofSeconds(60); // 60 seconds

34. What is the Period class?

The Period class represents a date-based amount of time, such as "3 years, 2 months, and 1 day".

35. How do you create a Period?

You can create a Period instance using the Period.of() method.

Period period = Period.of(1, 2, 3); // 1 year, 2 months, and 3 days

36. What is the OptionalInt class?

The OptionalInt class is a specialized version of Optional for handling int values.

37. How do you create an OptionalInt?

You can create an OptionalInt using the OptionalInt.of() method or OptionalInt.empty().

OptionalInt optionalInt = OptionalInt.of(5);

38. What is the Stream.of() method?

The Stream.of() method creates a stream from a specified set of values.

Stream<String> stream = Stream.of("a", "b", "c");

39. How do you create a parallel stream?

You can create a parallel stream using the parallelStream() method on a collection.

List<String> list = Arrays.asList("a", "b", "c");
Stream<String> parallelStream = list.parallelStream();

40. What is the Collectors.toMap() method?

The Collectors.toMap() method collects elements of a stream into a Map.

Map<String, Integer> map = list.stream()
    .collect(Collectors.toMap(s -> s, String::length));

41. What is the Collectors.counting() method?

The Collectors.counting() method counts the number of elements in a stream.

long count = list.stream().collect(Collectors.counting());

42. What is the Collectors.summingInt() method?

The Collectors.summingInt() method calculates the sum of an integer-valued property of the elements in a stream.

int sum = list.stream()
    .map(String::length)
    .collect(Collectors.summingInt(Integer::intValue));

43. What is the Collectors.averagingInt() method?

The Collectors.averagingInt() method calculates the average of an integer-valued property of the elements in a stream.

double average = list.stream()
    .map(String::length)
    .collect(Collectors.averagingInt(Integer::intValue));

44. What is the Collectors.joining() method's advantage?

The Collectors.joining() method provides a convenient way to concatenate strings with a specified delimiter.

45. What is the Collectors.mapping() method?

The Collectors.mapping() method applies a mapping function before accumulating the results.

List<String> mapped = list.stream()
    .collect(Collectors.mapping(String::toUpperCase, Collectors.toList()));

46. What is the Collectors.toSet() method?

The Collectors.toSet() method collects the elements of a stream into a Set.

Set<String> set = list.stream().collect(Collectors.toSet());

47. What is the Collectors.toCollection() method?

The Collectors.toCollection() method collects the elements of a stream into a specified collection type.

List<String> collectedList = list.stream()
    .collect(Collectors.toCollection(ArrayList::new));

48. What is the Stream.concat() method?

The Stream.concat() method concatenates two streams into a single stream.

Stream<String> combined = Stream.concat(stream1, stream2);

49. What is the Stream.distinct() method?

The Stream.distinct() method returns a stream consisting of the distinct elements of the original stream.

List<String> distinctList = list.stream().distinct().collect(Collectors.toList());

50. What is the Stream.sorted() method?

The Stream.sorted() method returns a stream with the elements sorted in natural order or by a specified comparator.

List<String> sortedList = list.stream().sorted().collect(Collectors.toList());