Java 8 Streams：如何调用Collection.stream（）方法并检索多个聚合值的数组 [英] Java 8 Streams: How to call once the Collection.stream() method and retrieve an array of several aggregate values

问题描述

我从Java 8中的Stream API开始。

I'm starting with the Stream API in Java 8.

这是我使用的Person对象：

Here is my Person object I use:

public class Person {

    private String firstName;
    private String lastName;
    private int age;

    public Person(String firstName, String lastName, int age) {
      this.firstName = firstName;
      this.lastName = lastName;
      this.age = age;
    }

    public String getFirstName() {
      return firstName;
    }

    public String getLastName() {
      return lastName;
    }

    public int getAge() {
      return age;
    }

  }

这是我的代码初始化a对象列表Person，它获取对象的数量，最大年龄和最小年龄，最后创建一个包含这三个值的对象数组：

Here is my code which initializes a list of objects Person and which gets the number of objects, the maximum age and the minimum age, and finally create an array of objects containing these three values:

List<Person> personsList = new ArrayList<Person>();

personsList.add(new Person("John", "Doe", 25));
personsList.add(new Person("Jane", "Doe", 30));
personsList.add(new Person("John", "Smith", 35));

long count = personsList.stream().count();
int maxAge = personsList.stream().mapToInt(Person::getAge).max().getAsInt();
int minAge = personsList.stream().mapToInt(Person::getAge).min().getAsInt();

Object[] result = new Object[] { count, maxAge, minAge };
System.out.println(Arrays.toString(result));

是否可以对流进行一次调用（）方法并直接返回对象数组？

Is it possible to do a single call to the stream() method and to return the array of objects directly ?

Object[] result = personsList.stream()...count()...max()...min()

推荐答案

以下是使用标准JDK 8 API解决此问题的方法：

Here's how to solve this with standard JDK 8 API:

IntSummaryStatistics summary = personsList.stream().collect(
    Collectors.summarizingInt(Person::getAge));
System.out.println("Count: " + summary.getCount());
System.out.println("Max  : " + summary.getMax());
System.out.println("Min  : " + summary.getMin());

结果：

Count: 3
Max  : 35
Min  : 25

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