Tell me about Stream API: types of operations, the difference between map and flatMap

Stream API in Java is a powerful tool for working with collections and data sequences, allowing you to write declarative code for data processing.

Types of operations:

• Intermediate — return a new Stream, enabling chaining of operations, for example: filter(), map(), flatMap(), sorted(), distinct().
• Terminal — conclude the chain and return a result or side effect, for example: collect(), forEach(), reduce(), count().

Difference between map and flatMap:

• map transforms each element of the stream into one other element, for example, from a string to its length.
• flatMap transforms each element into a stream of elements, then "flattens" these streams into a single stream.

Example:

List<String> words = Arrays.asList("hello", "world");

// map: transform each word into an array of characters
Stream<char[]> mapped = words.stream().map(word -> word.toCharArray());

// flatMap: transform each word into a Stream<Character> and merge
Stream<Character> flatMapped = words.stream()
    .flatMap(word -> word.chars().mapToObj(c -> (char)c));

// flatMap makes working with nested structures more convenient

Thus, flatMap is useful when you need to obtain multiple elements from a single element for further processing.