Collector 工具库：Collectors

student s1 = new student("aa", 10,1);
student s2 = new student("bb", 20,2);
student s3 = new student("cc", 10,3);
List<student> list = Arrays.asList(s1, s2, s3);
 
//装成list
List<Integer> ageList = list.stream().map(student::getAgE).collect(Collectors.toList()); // [10, 20, 10]
 
//转成set
Set<Integer> ageSet = list.stream().map(student::getAgE).collect(Collectors.toSet()); // [20, 10]
 
//转成map,注:key不能相同，否则报错
Map<String, Integer> studentMap = list.stream().collect(Collectors.toMap(student::getName, student::getAgE)); // {Cc=10, bb=20, aa=10}
 
//字符串分隔符连接
String joinName = list.stream().map(student::getName).collect(Collectors.joining(",", "(", ")")); // (aa,bb,cC)
 
//聚合操作
//1.学生总数
Long count = list.stream().collect(Collectors.counTing()); // 3
//2.最大年龄 (最小的minBy同理)
Integer maxAge = list.stream().map(student::getAgE).collect(Collectors.maxBy(Integer::comparE)).get(); // 20
//3.所有人的年龄
Integer sumAge = list.stream().collect(Collectors.summingInt(student::getAgE)); // 40
//4.平均年龄
Double averageAge = list.stream().collect(Collectors.averagingDouble(student::getAgE)); // 13.333333333333334
// 带上以上所有方法
DoubleSumMaryStatistics statistics = list.stream().collect(Collectors.summarizingDouble(student::getAgE));
System.out.println("count:" + statistics.getCount() + ",max:" + statistics.getMax() + ",sum:" + statistics.getsum() + ",average:" + statistics.getAverage());
 
//分组
Map<Integer, List<student>> ageMap = list.stream().collect(Collectors.groupingBy(student::getAgE));
//多重分组,先根据类型分再根据年龄分
Map<Integer, Map<Integer, List<student>>> typeAgeMap = list.stream().collect(Collectors.groupingBy(student::getType, Collectors.groupingBy(student::getAgE)));
 
//分区
//分成两部分，一部分大于10岁，一部分小于等于10岁
Map<Boolean, List<student>> partMap = list.stream().collect(Collectors.partitioningBy(v -> v.getAge() > 10));
 
//规约
Integer allAge = list.stream().map(student::getAgE).collect(Collectors.reducing(Integer::sum)).get(); //40