JAVA1.8--Stream流收集(十一)_list.stream().collect(collectors.reducing(-优快云博客

本文链接：https://blog.youkuaiyun.com/qq_20410965/article/details/106854436

/**
* 收集
*
*            方法                               描述
*    collect(Collector c) 将流转换为其他形式。接收一个Collector接口的实现，用于给Stream中元素做汇总的方法
*
* Collectors
*
* Collector接口中方法的实现决定了如何对流执行收集操作(如收集到List、Set、Map)。但是Collectors实用类
* 提供了很多静态方法，可以方便的创建常见收集器实例，如下表：
*
* 方法返回类型作用示例
* tolist List 把流中元素收集到List List list=list.stream().collect(Collectors .toList());
* toSet Set               把流中元素收集到Set Set  set=list.stream() .collect(Collectors.toSet());
* toCollection Collection 把流中元素收集到创建的集合   Collection maps=list.stream().collect(Collectors
* .toCollection(ArrayList::new));
* counting Long           计算流中元素的个数 long count=list.stream().collect(Collectors.counting());
* summingInt       Integer       对流中元素的整数属性求和       Integer sum=persons.stream().collect(Collectors
* .summingInt(Person::getAge));
* averagingInt   Double       计算流中元素Integer属性的平均值 double avg=list.stream().collect(Collectors
*   .averagingInt(Person::getAge));
* summarizingInt IntSummary-       收集流中Integer属性       IntSummaryStatistics iss= list.stream()
* -Statistics 的统计值.如：平均值               .collect(Collectors.summarizingInt(Person::getAge));
*    joining           String           连接流中每个字符串 String str=list.stream() .map(Person::getName)
* .collect(Collectors.joining());
* maxBy           Optional<T>       根据比较器选择最大值           Optional max=list.stream().collect(Collectors
* .maxBy(comparingInt(Person:getAge)));
* minBy           Optional<T>       根据比较器选择最小值           Optional min=list.stream().collect(Collectors
* .minBy(comparingInt(Person:getAge)));
* reducing       规约产生的类型     从一个作为累加器的初始值开始，   int total=list.stream().collect(Collectors
* 利用BinaryOperator与流中   .reducing(0,Person::getAge,Integer::sum))
* 元素逐个结合，从而规约成单个值
* collecting-       转换函数返回的类型   包裹另一个收集器，对其结果   int how=list.stream().collect(Collectors
* -AndThen    转化函数 .collectingAndThen(Collectors.toList(),List::size)) * groupingBy      Map<K,List<T>>   根据某属性值对流分组，属性为K,   Map<Person.Sex,List<Person>>
* 结果为V map=persons.stream().collect(Collectors
* .groupingBy(Person:;getGender))
* partitioningBy   Map<Boolean,List<T>>    根据true或false进行分区       Map<Boolean,List<Person>>
* map=persons.stream().collect(Collectors
*
* */

public class StramCollectInfo {
	//生成多个Person对象
	List<Person> persons = Person.createRoster();
	
	@Test
	public void test(){
		List<String> list = persons.stream()
				.map(Person::getName)
				.collect(Collectors.toList());
		list.forEach(System.out::println);
	}
	@Test
	public void test1(){
		Optional<Integer> max = persons.stream()
				.map(Person::getAge)
				.collect(Collectors.maxBy(Integer::compare));
		
		System.out.println("最大年龄："+max.get());
		
		
		Optional<Person> op = persons.stream()
				.min(Comparator.comparingInt(Person::getAge));
		
		System.out.println("最小年龄的成员："+op.get());
		
		Integer sum = persons.stream()
				.collect(Collectors.summingInt(Person::getAge));
		
		System.out.println("所有成员的年龄总和："+sum);
		
		IntSummaryStatistics iss=persons.stream()
				.collect(Collectors.summarizingInt(Person::getAge));
		
		System.out.println("最大年龄："+iss.getMax());
		
	}
	//分组
	/**
	 * {
	 *	MALE=[Person [name=Fred, birthday=1980-06-20, gender=MALE, emailAddress=fred@example.com], 
	 *		  Person [name=George, birthday=1983-08-13, gender=MALE, emailAddress=george@example.com], 
	 *		  Person [name=Bob, birthday=2000-09-12, gender=MALE, emailAddress=bob@example.com]], 
	 *	FEMALE=[Person [name=Jane, birthday=1990-07-15, gender=FEMALE, emailAddress=jane@example.com]]
	 *	}
	 * 
	 * */
	@Test
	public void test2(){
		Map<Person.Sex, List<Person>> map = persons.stream()
				.collect(Collectors.groupingBy(Person::getGender));
		
		System.out.println("按性别分组："+map);
	}
	//多级分组
	/**
	 * {
	 *	FEMALE={青年=[Person [name=Jane, birthday=1990-07-15, gender=FEMALE, emailAddress=jane@example.com]]}, 
	 *	MALE={青年=[Person [name=Bob, birthday=2000-09-12, gender=MALE, emailAddress=bob@example.com]], 
	 *		  中年=[Person [name=Fred, birthday=1980-06-20, gender=MALE, emailAddress=fred@example.com], 
	 *		  Person [name=George, birthday=1983-08-13, gender=MALE, emailAddress=george@example.com]]}
	 *	}
	 * */
	@Test
	public void test3(){
		
		Map<Person.Sex, Map<String,List<Person>>> map = persons.stream()
				.collect(Collectors.groupingBy(Person::getGender
						,Collectors.groupingBy(p -> {
					if(p.getAge()>=60){
						return "老年";
					}else if(p.getAge()>=35){
						return "中年";
					}else{
						return "青年";
					}
				})));
		
		System.out.println(map);
		
	}
	
	//分区
	/**
	 * {
	 * false=[Person [name=Fred, birthday=1980-06-20, gender=MALE, emailAddress=fred@example.com], 
	 *		   Person [name=Jane, birthday=1990-07-15, gender=FEMALE, emailAddress=jane@example.com], 
	 *		   Person [name=George, birthday=1983-08-13, gender=MALE, emailAddress=george@example.com], 
	 *		   Person [name=Bob, birthday=2000-09-12, gender=MALE, emailAddress=bob@example.com]], 
	 * true=[]
	 * }
	 * */
	@Test
	public void test4(){
		
		Map<Boolean, List<Person>> map = persons.stream()
				.collect(Collectors.partitioningBy(e -> e.getAge()>=50));
		
		System.out.println(map);
		
	}
	
	//流中元素间添加分隔符并在前后添加标识
	/**
	 * 运行结果：
	 * ----Fred,Jane,George,Bob----
	 * */
	@Test
	public void test5(){
		
		String str = persons.stream()
				.map(Person::getName)
				.collect(Collectors.joining(",","----","----"));
		System.out.println(str);
		
	}
	
	
	//规约
	@Test
	public void test6(){
		
		Optional<Integer> sum = persons.stream()
				.map(Person::getAge)
				.collect(Collectors.reducing(Integer::sum));
		
		System.out.println(sum.get());
		
	}
}

源码下载地址：https://download.youkuaiyun.com/download/qq_20410965/12434193