CollectorsExamples.java

package fr.univtln.bruno.samples.java101.tp3.functional;

import fr.univtln.bruno.samples.java101.tp3.Book;
import fr.univtln.bruno.samples.java101.tp3.Person;
import lombok.extern.slf4j.Slf4j;

import java.time.LocalDate;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;

/**
 * Collector-focused examples: grouping, partitioning, joining, summarizing and reduction.
 */
@Slf4j
public class CollectorsExamples {
  /**
   * Private constructor to prevent instantiation.
   */
  private CollectorsExamples() {
  }

  /**
   * Showcases terminal stream operations like count/any/all/none/findFirst.
   */
  public static void terminalOperationsExample() {
    log.info("[functional] Terminal operations: count/any/all/none/findFirst");
    List<Integer> nums = List.of(1, 2, 3, 4, 5, 6);
    long evenCount = nums.stream().filter(n -> n % 2 == 0).count();
    boolean anyGt4 = nums.stream().anyMatch(n -> n > 4);
    boolean allPositive = nums.stream().allMatch(n -> n > 0);
    boolean noneNegative = nums.stream().noneMatch(n -> n < 0);
    Optional<Integer> firstGt3 = nums.stream().filter(n -> n > 3).findFirst();
    log.debug("nums: {}", nums);
    log.info("evenCount={} anyGt4={} allPositive={} noneNegative={} firstGt3={}", evenCount, anyGt4, allPositive, noneNegative, firstGt3.orElse(-1));
  }

  /**
   * Demonstrates groupingBy and counting collectors on Person collections.
   */
  public static void groupingAndCountingExample() {
    log.info("[functional] groupingBy and counting");
    List<Person> people = List.of(Person.of("Alice", "Smith", 30), Person.of("Bob", "Jones", 25), Person.of("Charlie", "Brown", 35), Person.of("David", "Smith", 28));
    log.debug("people for grouping: {}", people);
    Map<String, List<Person>> byLast = people.stream().collect(Collectors.groupingBy(Person::lastName));
    byLast.forEach((ln, ps) -> log.info("{} -> {}", ln, ps.stream().map(Person::getFullName).toList()));
    Map<String, Long> counts = people.stream().collect(Collectors.groupingBy(Person::lastName, Collectors.counting()));
    log.debug("group counts: {}", counts);
    log.info("counts: {}", counts);
  }

  /**
   * Demonstrates complex collectors (mapping + grouping) using Book examples.
   */
  public static void complexCollectorsBooksExample() {
    log.info("[functional] complex collectors for Book examples");
    List<Book> books = List.of(
      Book.of("978-0134685991", "Effective Java", "Joshua Bloch", LocalDate.of(2018, 1, 6), 45.99),
      Book.of("978-0596009205", "Head First Java", "Kathy Sierra", LocalDate.of(2005, 2, 9), 39.99),
      Book.of("978-0134685992", "Java Concurrency", "Brian Goetz", LocalDate.of(2006, 5, 19), 42.99)
    );
    log.debug("books: {}", books);
    Map<String, List<String>> titlesByAuthor = books.stream().collect(Collectors.groupingBy(Book::author, Collectors.mapping(Book::title, Collectors.toList())));
    log.debug("titlesByAuthor: {}", titlesByAuthor);
    titlesByAuthor.forEach((a, t) -> log.info("{} -> {}", a, t));

    // Student note: beware of Collectors.toMap when keys might collide. The 2-arg toMap throws
    // IllegalStateException on duplicate keys. Use the 3-arg version to resolve duplicates, for example:
    // Map<String, Book> byIsbn = books.stream().collect(Collectors.toMap(Book::isbn, b -> b, (a,b) -> a)); // keep first
    // Or groupingBy when keys are not unique.
  }
}