java - 无法使用 Stream 和 Lambda 从另一个类导入的方法

Question

有些奇怪:我无法使用在导入的类中实现的方法。 我有一个名为“Person.java”的类，其中包含一个名为“isFrom()”的实现方法，如下所示:

import java.util.List;
import java.util.function.Function;

public class Person implements Comparable<Person> {
  private String name;
  private String role;
  private String city;
  private LocalDate birthdate;

  public Person(String name, String role, String city, LocalDate birthdate) {
    super();
    this.name = name;
    this.role = role;
    this.city = city;
    this.birthdate = birthdate;
  }

  public boolean isFrom(String name) {
    return city.equals(name);
  }

  public String getRole() {
    return role;
  }

  @Override
  public String toString() {
    return "Stakeholder [name=" + name + ", role=" + role + ", city=" + city + ", birthdate=" + birthdate + "]";
  }

  public boolean isOlderThan(Person other) {
    return birthdate.isBefore(other.birthdate);
  }

  public boolean isYoungerThan(Person other) {
    return birthdate.isAfter(other.birthdate);
  }

  /** 
   * Questa è una funzione di ordine superiore (higher-order function) perché restituisce una funzione
   * 
   */
  public static Function<Person, LocalDate> toBirthdate() {
    return p -> p.birthdate;
  }

  public LocalDate getBirthdate() {
    return birthdate;
  }

  public String getName() {
    return name;
  }

  @Override
  public int compareTo(Person o) {
    return name.compareTo(o.name);
  }

  public String getCity() {
    return city;
  }

  public static List<Person> makeList() {
    List<Person> persons = new ArrayList<>();
    persons.add(new Person("Paolo Bianchi", "Tecnico", "Milano", LocalDate.of(1960, 4, 15)));
    persons.add(new Person("Mario Rossi", "Tecnico", "Milano", LocalDate.of(1981, 3, 25)));
    persons.add(new Person("Lucia Verdi", "Tecnico", "Milano", LocalDate.of(1955, 5, 1)));
    persons.add(new Person("Marzia Gentile", "Commerciale", "Milano", LocalDate.of(1969, 6, 3)));
    persons.add(new Person("Anna De Rosa", "Responsabile", "Milano", LocalDate.of(1974, 4, 22)));
    persons.add(new Person("Paola De Blasi", "Tecnico", "Torino", LocalDate.of(1967, 8, 11)));
    persons.add(new Person("Maria Abate", "Tecnico", "Torino", LocalDate.of(1977, 10, 21)));
    persons.add(new Person("Federico Cacciari", "Commerciale", "Torino", LocalDate.of(1962, 12, 17)));
    persons.add(new Person("Arturo Pace", "Commerciale", "Roma", LocalDate.of(1979, 10, 19)));
    persons.add(new Person("Giovanni Tomassini", "Commerciale", "Genova", LocalDate.of(1974, 9, 8)));
    return persons;
  }

}

因此，我创建了另一个名为“InternalIterator.java”的类，目的是尝试沿着前一个类的方法“makeList()”创建的集合使用 Streams。在流内部，我尝试调用“isFrom()”方法，但就像找不到该方法一样:

Cannot resolve method 'isFrom(java.lang.String)

这很奇怪，因为我已经在 Person 类中实现了该方法。第二类的代码如下所示:

import java.util.List;
import ardea.project.Person;
public class InternalIterator {
    public static void main(String[] args) {
        List persons = Person.makeList();

        long count = persons.stream()
                .filter(p -> p.isFrom("Milano"))
                .count();
        System.out.println("Persone di Milano: " + count);

    }
}

我不明白，我也尝试过 Person.isFrom() 但无事可做

Answer 1

原因是您使用的是原始类型。

List persons = Person.makeList();

因此，当您从 persons 创建流时，类型会被推断为对象。

相反，这样做

List<Person> persons = Person.makeList();

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What is a raw type and why shouldn't we use it?