Collection stream使用示例详解

@Data
@Accessors(chain = true)
public static class Person {
    private String name;
    private Integer age;
    private String hobby;
    private LocalDateTime birthday;
}

public List<Person> data() {
    List<Person> data = new ArrayList<>();
    data.add(new Person().setName("张三").setAge(25).setHobby("LOL,Food").setBirthday(LocalDateTime.of(1997, 1, 1, 0, 0)));
    data.add(new Person().setName("李四").setAge(25).setHobby("CS:GO,Swimming").setBirthday(LocalDateTime.of(1997, 2, 1, 0, 0)));
    data.add(new Person().setName("王五").setAge(30).setHobby("RedAlert2,Beer").setBirthday(LocalDateTime.of(1992, 3, 1, 0, 0)));
    data.add(new Person().setName("赵六").setAge(40).setHobby("War3,Journey").setBirthday(LocalDateTime.of(1982, 4, 1, 0, 0)));
    data.add(new Person().setName("孙七").setAge(40).setHobby("DOTA,Jogging").setBirthday(LocalDateTime.of(1982, 5, 1, 0, 0)));
    return data;
}

/**
 * 获取所有成员的爱好集合
 */
@Test
public void test1() {
    // 方式1：
    // 先进行 map 映射单个元素为多值元素
    // 在将多值元素映射为 Stream
    Set<String> hobbySet = data()
            .stream()
            .map(p -> p.getHobby().split(","))
            .flatMap(Stream::of)
            .collect(Collectors.toSet());
    System.out.println(hobbySet);
    // 方式2：直接将单个元素映射为 Stream
    hobbySet = data()
            .stream()
            .flatMap(p -> Stream.of(p.getHobby().split(",")))
            .collect(Collectors.toSet());
    System.out.println(hobbySet);
}

Stream.of("1", "2", "3").map(Integer::parseInt).collect(Collectors.toList());
Stream.of("1", "2", "3").collect(Collectors.mapping(Integer::parseInt, Collectors.toList()));

/**
 * 根据年龄分组，并统计每组下的成员姓名
 */
@Test
public void test2() {
    Map<Integer, Set<String>> ageNameMapping = data()
            .stream()
            .collect(
                    Collectors.groupingBy(
                            Person::getAge,
                            Collectors.mapping(Person::getName, Collectors.toSet())
                    )
            );
    System.out.println(ageNameMapping);
}

/**
 * 根据年龄分组，每组根据生日进行排序
 */
@Test
public void test3() {
    // 通过定制特定数据结构的容器实现排序：正序
    Map<Integer, Collection<Person>> ageMapping = data()
            .stream()
            .collect(
                    Collectors.groupingBy(
                            Person::getAge,
                            Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Person::getBirthday)))
                    )
            );
    printMap(ageMapping);
    // 通过定制特定数据结构的容器实现排序：逆序
    ageMapping = data()
            .stream()
            .collect(
                    Collectors.groupingBy(
                            Person::getAge,
                            Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Person::getBirthday).reversed()))
                    )
            );
    printMap(ageMapping);
    // 通过对聚合元素后返回的容器二次处理，实现排序
    ageMapping = data()
            .stream()
            .collect(
                    Collectors.groupingBy(
                            Person::getAge,
                            Collectors.collectingAndThen(
                                    Collectors.toList(),
                                    l -> {
                                        l.sort(Comparator.comparing(Person::getBirthday).reversed());
                                        return l;
                                    }
                            )
                    )
            );
    printMap(ageMapping);
}
public void printMap(Map<Integer, Collection<Person>> mapping) {
    DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
    mapping.forEach((key, val) -> System.out.println(key + " : " + val.stream().map(p -> p.getName() + " -> " + dateTimeFormatter.format(p.getBirthday())).collect(Collectors.joining(" / "))));
    System.out.println();
}

/**
 * 根据年龄分组，只保留生日最大的成员
 */
@Test
public void test4() {
    // 下面注释代码会抛出异常，因为没有指定当单Key匹配到多值时的 merge 行为
    // 源码中的默认指定的 meger 行为则是抛出异常：throwingMerger()
    // Map<Integer, Person> toMap = data().stream().collect(Collectors.toMap(Person::getAge, Function.identity()));
    Map<Integer, Person> toMap = data()
            .stream()
            .collect(
                    Collectors.toMap(
                            Person::getAge,
                            Function.identity(),
                            (v1, v2) -> Comparator.comparing(Person::getBirthday).compare(v1, v2) > 0 ? v1 : v2
                    )
            );
    toMap.forEach((key, val) -> System.out.println(key + " : " + val.getName() + " -> " + DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss").format(val.getBirthday())));
}