Java Stream Collectors的collectingAndThen()、joining()和counting()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，a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.collectingAndThen()、Collectors.joining()和Collectors.counting()的使用，以及相关的示例代码。

1、Collectors.collectingAndThen()的使用

Collectors.collectingAndThen()添加在集合末尾执行的单个操作，先把流中的所有元素传递给第一个参数，然后把生成的集合元素传递给第二个参数来处理。例如，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     String name;     Person (int age,String name) {         this.age = age;         this.name = name;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }     public String getName() {         return name;     }     public void setName(String name) {         this.name = name;     }  }public static void main(String[] args) {   List pList = Arrays.asList(new Person(11,"mike"),new Person(22,"lynn"), new Person(33,"john"), new Person(44,"mickey"), new Person(55,"fiona"));   String result = pList.stream().collect(Collectors.collectingAndThen(Collectors.maxBy(Comparator.comparing(Person::getAge)), (Optional p) -> { return p.isPresent() ? p.get().getName() : "none";}    ));    System.out.println(result);    System.exit(0); //success  }}

2、Collectors.joining()的使用

Collectors.joining()用来连接stream中的元素，例如，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     String name;     Person (int age,String name) {         this.age = age;         this.name = name;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }     public String getName() {         return name;     }     public void setName(String name) {         this.name = name;     }  }public static void main(String[] args) {   List pList = Arrays.asList(new Person(11,"mike"),new Person(22,"lynn"), new Person(33,"john"), new Person(44,"mickey"), new Person(55,"fiona"));   String result1 = pList.stream().map(p -> p.getName()).collect(Collectors.joining(" "));   System.out.println(result1);   String result2 = pList.stream().map( p -> p.getName())             .collect(Collectors.joining(" ", "prefix "," suffix"));   System.out.println(result2);   String[] strArray = { "C","Java","Py"," : ","https://www.wonhero.com" };   String str= Stream.of(strArray)             .map(arr -> new String(arr))             .collect(Collectors.joining());   System.out.println(str);   System.exit(0); //success  }}

3、Collectors.counting()的使用

Collectors.counting()用来统计stream中元素的个数，如下，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     String name;     Person (int age,String name) {         this.age = age;         this.name = name;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }     public String getName() {         return name;     }     public void setName(String name) {         this.name = name;     }  }public static void main(String[] args) {    List pList = Arrays.asList(new Person(11,"mike"),new Person(22,"lynn"), new Person(33,"john"), new Person(44,"mickey"), new Person(55,"fiona"));    var count = pList.stream().collect(Collectors.counting());    System.out.println("stream() result ：" + count);    System.exit(0); //success  }}