目录
1.筛选与切片
filter:过滤流中的某些元素；
limit(n):获取n个元素；
skip(n)：跳过n个元素，配合limit可实现分页；
distinct：通过流中元素的hashCode()和equals()去除重复元素；
Stream<Integer> stream=Stream.of(4,3,1,8,65,2,8,4,67,9);
Stream<Integer> stream1=stream.filter(s->s>3) //4,8,65,8,4,67,9
.distinct() //4,8,65,67,9
.skip(1) //8,65,67,9
.limit(2); //8,65
stream1.forEach(System.out::println);
2.映射map
map：接收一个函数作为参数，该函数会被应用到每个元素上，并将其映射成一个新的元素；
flatMap：接收一个行数作为参数，将流中的每个值都换成另一个流，然后把所有流连城一个流；
List<String> list= Arrays.asList("z,h,a,o","y,a,n");
List<String> list1=list.stream()
.map(s->s.replaceAll(",",""))
.collect(Collectors.toList());
System.out.println(list1);
// [zhao, yan]
List<String> list= Arrays.asList("z,h,a,o","y,a,n");
List<String> list2=list.stream().flatMap(s->{
String[] arr=s.split(",");
Stream<String> s1=Arrays.stream(arr);
return s1;
}).collect(Collectors.toList());
System.out.println(list2);
// [z, h, a, o, y, a, n]3.peek
peek，和map相似，能得到流中的每一个元素。但map接收的是一个Function表达式，有返回值；而peek接收的是Consumer表达式，无返回值。
Student s1=new Student("haha",1);
Student s2=new Student("hihi",2);
List<Student> list= Arrays.asList(s1,s2);
List<Student> list1=list.stream()
.peek(s->s.setAge(4))
.collect(Collectors.toList());
for (int i = 0; i < list1.size(); i++) {
System.out.println(list1.get(i).getAge());
}4.Collector工具库：Collectors
Student s1=new Student("haha",1);
Student s2=new Student("hihi",2);
List<Student> list= Arrays.asList(s1,s2);
// 将age转成List
List<Integer> list2=list.stream()
.map(Student::getAge)
.collect(Collectors.toList());
System.out.println(list2);
// 转成set
Set<Integer> set2=list.stream()
.map(Student::getAge)
.collect(Collectors.toSet());
System.out.println(set2);
// 转成map key不能相同，否则报错
Map<String,Integer> map2=list.stream()
.collect(Collectors.toMap(Student::getName,Student::getAge));
System.out.println(map2);字符串分隔符连接：
Student s1=new Student("haha",1);
Student s2=new Student("hihi",2);
List<Student> list= Arrays.asList(s1,s2);
String str=list.stream()
.map(Student::getName)
.collect(Collectors.joining(",","(",")"));
System.out.println(str);
// (haha,hihi)聚合操作：
Student s1=new Student("haha",1);
Student s2=new Student("hihi",2);
List<Student> list= Arrays.asList(s1,s2);
// 求学生总数
Long count=list.stream().collect(Collectors.counting());
// 求学生最大年纪
Integer max=list.stream().map(Student::getAge).collect(Collectors.maxBy(Integer::compare)).get();
// 求最小年纪
Integer min=list.stream().map(Student::getAge).collect(Collectors.minBy(Integer::compare)).get(); // 求所有年纪之和
Double dou=list.stream().collect(Collectors.summingDouble(Student::getAge));
// 求平均年纪
Double dou1=list.stream().collect(Collectors.averagingDouble(Student::getAge));?分组groupingBy
Student s1=new Student("haha",1);
Student s2=new Student("hihi",2);
Student s3=new Student("hei",1);
List<Student> list= Arrays.asList(s1,s2,s3);
// 根据age分组
Map<Integer,List<Student>> listMap=list.stream().collect(Collectors.groupingBy(Student::getAge));
// 多重分组，先根据age分，在根据name分
Map<Integer,Map<String,List<Student>>> listMap1=list.stream().collect(Collectors.groupingBy(Student::getAge,Collectors.groupingBy(Student::getName)));
//分区
Map<Boolean, List<Student>> listMap2=list.stream().collect(Collectors.partitioningBy(s->s.getAge()>1));
