1.修改集合中的值map

传入一个函数，函数的形参即为集合中的每一个值，该函数会对集合中的每一个元素进行函数体中的操作，最后返回一个新的值，修改或代替集合中的元素
分为两种情况：直接修改原来集合中的数据，生成一个新的集合。

高级玩法：可以直接在map中传入Student::getAge或personList.stream().map(p -> p.getAge()) ，获取到集合中所有对象的所有age，然后链式对属性操作，返回值也由集合中的对象类型变为对象的对应的属性类型。

		//直接修改原来的数据 所有学生的年龄加2
        List<Student> list1 = Arrays.asList(new Student("a",1),new Student("b",2),new Student("c",3));
        List<Student> studentAgeAdd1 = list1.stream().map(x -> {
            x.setAge(x.getAge() + 2);
            return x;
        }).collect(Collectors.toList());
        System.out.println("list1 = " + list1);
        System.out.println("studentAgeAdd1 = " + studentAgeAdd1);

		//生成新的集合 所有学生的年龄加2
        List<Student> list2 = Arrays.asList(new Student("a",1),new Student("b",2),new Student("c",3));
        List<Student> studentAgeAdd2 = list2.stream().map(x -> {
            return new Student(x.getUsername(), x.getAge()+2);
        }).collect(Collectors.toList());
        System.out.println("list2 = " + list2);
        System.out.println("studentAgeAdd2 = " + studentAgeAdd2);
//结果
Student(username=null, age=null)
list1 = [Student(username=a, age=3), Student(username=b, age=4), Student(username=c, age=5)]
studentAgeAdd1 = [Student(username=a, age=3), Student(username=b, age=4), Student(username=c, age=5)]
list2 = [Student(username=a, age=1), Student(username=b, age=2), Student(username=c, age=3)]
studentAgeAdd2 = [Student(username=a, age=3), Student(username=b, age=4), Student(username=c, age=5)]

2.过滤集合中符合条件的值filter

传入一个函数，函数的形参即为集合中的每一个值，该函数会对集合中的每一个元素进行函数体中的操作，函数体为一个条件判断，最后返回一个boolean值，为true则保留该元素为，false则去掉。最后生成一个新的集合。不改变原来的集合。

		//保留年龄大于等于3的学生
        List<Student> list1 = Arrays.asList(new Student("a",1),new Student("b",2),new Student("c",3));
        List<Student> studentAgeAdd1 = list1.stream().filter((x) -> {
            return x.getAge() >= 2;
        }).collect(Collectors.toList());
        System.out.println("list1 = " + list1);
        System.out.println("studentAgeAdd1 = " + studentAgeAdd1);

找集合中的最大/小的值，及操作后元素的个数max()/min()/count()

list.stream().max().get();
list.stream().min().get();
list.stream().filter().count();//操作后的个数

		//获取学生年龄最大的那个
        List<Student> list1 = Arrays.asList(new Student("a",1),new Student("b",2),new Student("c",3));
        Student student = list1.stream().max(Comparator.comparing(Student::getAge)).get();
        Student student1 = list1.stream().max((a, b) -> {
            return a.getAge() - b.getAge();
        }).get();
        System.out.println(student);
        System.out.println(student1);

//输出
Student(username=c, age=3)
Student(username=c, age=3)
		
		//统计过滤后的数量
        List<Student> list1 = Arrays.asList(new Student("a",1),new Student("b",2),new Student("c",3));
        long count = list1.stream().filter(x -> x.getAge() >= 2).count();
//输出
2

归约：对集合中所有元素进行自定义计算获取最终值reduce()

reduce 、

		//将所有对象中某元素的值相加
        List<Student> list1 = Arrays.asList(new Student("a",1),new Student("b",2),new Student("c",3));
        Integer sumAge = list1.stream().map(Student::getAge).reduce(Integer::sum).get();
        System.out.println(sumAge);

		//获取集合中所有元素和
        List<Integer> intList = Arrays.asList(1, 2, 3);
        Integer integer = intList.stream().reduce((sum, item) -> {
            sum += item;
            return sum;
        }).get();
        System.out.println(integer);

collect

流不存储数据，那么在流中的数据完成处理后，需要将流中的数据重新归集到新的集合里。
toList/toSet/toMap
例：list->map

        List<Student> list1 = Arrays.asList(new Student("a",1),new Student("b",2),new Student("c",3));
        Map<String, Student> collect = list1.stream().collect(Collectors.toMap(Student::getUsername, p -> p));
        for (Map.Entry<String, Student> studentEntry : collect.entrySet()) {
            System.out.println(studentEntry);
        }
//打印
a=Student(username=a, age=1)
b=Student(username=b, age=2)
c=Student(username=c, age=3)

Collectors提供了一系列用于数据统计的静态方法：
计数：count
平均值：averagingInt、averagingLong、averagingDouble
最值：maxBy、minBy
求和：summingInt、summingLong、summingDouble
统计以上所有：summarizingInt、summarizingLong、summarizingDouble（包括计数，平均值，最值，和）

在这里插入图片描述

分组(partitioningBy/groupingBy)

partitioningBy():条件分组
groupingBy():根据集合内相同的元素进行分组

public class StreamTest {
	public static void main(String[] args) {
		List<Person> personList = new ArrayList<Person>();
		personList.add(new Person("Tom", 8900, "male", "New York"));
		personList.add(new Person("Jack", 7000, "male", "Washington"));
		personList.add(new Person("Lily", 7800, "female", "Washington"));
		personList.add(new Person("Anni", 8200, "female", "New York"));
		personList.add(new Person("Owen", 9500, "male", "New York"));
		personList.add(new Person("Alisa", 7900, "female", "New York"));

		// 将员工按薪资是否高于8000分组
        Map<Boolean, List<Person>> part = personList.stream().collect(Collectors.partitioningBy(x -> x.getSalary() > 8000));
        // 将员工按性别分组
        Map<String, List<Person>> group = personList.stream().collect(Collectors.groupingBy(Person::getSex));
        // 将员工先按性别分组，再按地区分组，嵌套分组
        Map<String, Map<String, List<Person>>> group2 = personList.stream().collect(Collectors.groupingBy(Person::getSex, Collectors.groupingBy(Person::getArea)));
        System.out.println("员工按薪资是否大于8000分组情况：" + part);
        System.out.println("员工按性别分组情况：" + group);
        System.out.println("员工按性别、地区：" + group2);
	}
}


//输出结果
员工按薪资是否大于8000分组情况：{false=[mutest.Person@2d98a335, mutest.Person@16b98e56, mutest.Person@7ef20235], true=[mutest.Person@27d6c5e0, mutest.Person@4f3f5b24, mutest.Person@15aeb7ab]}
员工按性别分组情况：{female=[mutest.Person@16b98e56, mutest.Person@4f3f5b24, mutest.Person@7ef20235], male=[mutest.Person@27d6c5e0, mutest.Person@2d98a335, mutest.Person@15aeb7ab]}
员工按性别、地区：{female={New York=[mutest.Person@4f3f5b24, mutest.Person@7ef20235], Washington=[mutest.Person@16b98e56]}, male={New York=[mutest.Person@27d6c5e0, mutest.Person@15aeb7ab], Washington=[mutest.Person@2d98a335]}}

修改集合中元素之间的连接符join

public class StreamTest {
	public static void main(String[] args) {
		List<Person> personList = new ArrayList<Person>();
		personList.add(new Person("Tom", 8900, 23, "male", "New York"));
		personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
		personList.add(new Person("Lily", 7800, 21, "female", "Washington"));

		String names = personList.stream().map(p -> p.getName()).collect(Collectors.joining(","));
		System.out.println("所有员工的姓名：" + names);
		List<String> list = Arrays.asList("A", "B", "C");
		String string = list.stream().collect(Collectors.joining("-"));
		System.out.println("拼接后的字符串：" + string);
	}
}

提取/组合

public class StreamTest {
	public static void main(String[] args) {
		String[] arr1 = { "a", "b", "c", "d" };
		String[] arr2 = { "d", "e", "f", "g" };

		Stream<String> stream1 = Stream.of(arr1);
		Stream<String> stream2 = Stream.of(arr2);
		// concat:合并两个流 distinct：去重
		List<String> newList = Stream.concat(stream1, stream2).distinct().collect(Collectors.toList());
		// limit：限制从流中获得前n个数据
		List<Integer> collect = Stream.iterate(1, x -> x + 2).limit(10).collect(Collectors.toList());
		// skip：跳过前n个数据
		List<Integer> collect2 = Stream.iterate(1, x -> x + 2).skip(1).limit(5).collect(Collectors.toList());

		System.out.println("流合并：" + newList);
		System.out.println("limit：" + collect);
		System.out.println("skip：" + collect2);
	}
}

//输出
流合并：[a, b, c, d, e, f, g]
limit：[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
skip：[3, 5, 7, 9, 11]