Java 8分组、计数和转换为自定义对象

class Person {
    private String name;
    private int age;

    // 构造函数和getter/setter方法省略

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

List<Person> persons = Arrays.asList(
        new Person("Alice", 20),
        new Person("Bob", 30),
        new Person("Charlie", 20),
        new Person("Dave", 30)
);

Map<Integer, List<Person>> personsByAge = persons.stream()
        .collect(Collectors.groupingBy(Person::getAge));

System.out.println(personsByAge);

{20=[Person{name='Alice', age=20}, Person{name='Charlie', age=20}], 30=[Person{name='Bob', age=30}, Person{name='Dave', age=30}]}

List<Integer> numbers = Arrays.asList(5, 10, 15, 20, 25);

long count = numbers.stream()
        .filter(n -> n >= 10)
        .count();

System.out.println(count);

4

List<String> names = Arrays.asList("Alice", "Bob", "Charlie");

List<Person> persons = names.stream()
        .map(name -> new Person(name, 0))
        .collect(Collectors.toList());

System.out.println(persons);

[Person{name='Alice', age=0}, Person{name='Bob', age=0}, Person{name='Charlie', age=0}]