Java Stream Collectors的summarizingInt()、averagingInt()、summingInt()的使用

Stream是Java 8的新特性，基于lambda表达式，是对集合对象功能的增强，它专注于对集合对象进行各种高效、方便聚合操作或者大批量的数据操作，提高了编程效率和代码可读性。Collectors通常在Stream处理后，返回转换成集合类时使用，本文主要介绍Java Stream中Collectors.summarizingDouble/Long/Int()、Collectors.averagingDouble/Long/Int()和Collectors.summingDouble/Long/Int()的使用,，以及相关的示例代码。

1、Collectors.summarizingDouble/Long/Int()的使用

SummarizingDouble()、SummarizingLong() 和SummarizingInt()为stream中的元素生成了统计信息，返回的结果是一个统计类，例如，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     Person (int age) {         this.age = age;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }}public static void main(String[] args) {    List pList = Arrays.asList(new Person(11),new Person(22), new Person(33), new Person(44), new Person(55));    List result = pList.stream()                                .map(o -> {                            o.setAge(18);                            return o;                        }).collect(Collectors.toList());    IntSummaryStatistics intResult = pList.stream()                .collect(Collectors.summarizingInt(Person::getAge));    System.out.println("stream() result ：" + intResult);    LongSummaryStatistics longResult = pList.stream()                .collect(Collectors.summarizingLong(Person::getAge));    System.out.println("stream() result ：" + longResult);    DoubleSummaryStatistics doubleResult = pList.stream()                .collect(Collectors.summarizingDouble(Person::getAge));    System.out.println("stream() result ：" + doubleResult);    System.exit(0); //success  }}

2、Collectors.averagingDouble/Long/Int()的使用

averagingDouble()、averagingLong()和averagingInt()对stream中的元素计算平均，例如，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     Person (int age) {         this.age = age;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }}public static void main(String[] args) {    List pList = Arrays.asList(new Person(11),new Person(22), new Person(33), new Person(44), new Person(55));    List result = pList.stream()                                .map(o -> {                            o.setAge(18);                            return o;                        }).collect(Collectors.toList());    Double intResult = pList.stream()                .collect(Collectors.averagingInt(Person::getAge));    System.out.println("stream() result ：" + intResult);    Double longResult = pList.stream()                .collect(Collectors.averagingLong(Person::getAge));    System.out.println("stream() result ：" + longResult);    Double doubleResult = pList.stream()                .collect(Collectors.averagingDouble(Person::getAge));    System.out.println("stream() result ：" + doubleResult);    System.exit(0); //success  }}

3、Collectors.summingDouble/Long/Int()的使用

summingDouble()、summingLong()和summingInt()对stream中的元素做sum计算，如下，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     Person (int age) {         this.age = age;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }}public static void main(String[] args) {    List pList = Arrays.asList(new Person(11),new Person(22), new Person(33), new Person(44), new Person(55));    List result = pList.stream()                                .map(o -> {                            o.setAge(18);                            return o;                        }).collect(Collectors.toList());    Integer intResult = pList.stream()                .collect(Collectors.summingInt(Person::getAge));    System.out.println("stream() result ：" + intResult);    Long longResult = pList.stream()                .collect(Collectors.summingLong(Person::getAge));    System.out.println("stream() result ：" + longResult);    Double doubleResult = pList.stream()                .collect(Collectors.summingDouble(Person::getAge));    System.out.println("stream() result ：" + doubleResult);    System.exit(0); //success  }}