Java Stream 的遍历/筛选/去重/统计/求和/平均值/排序/累计/跳过

1.遍历（forEach）

    public static void main(String[] args) {

        List<Person> userList = new ArrayList<>();
        userList.add(new Person("段誉",25));
        userList.add(new Person("萧峰",40));
        userList.add(new Person("虚竹",30));
        userList.add(new Person("无涯子",100));
        userList.add(new Person("慕容复",35));
        userList.add(new Person("云中鹤",45));

        System.out.println(userList);
        System.out.println("---------------");

        userList.stream().forEach(u -> {
            u.setName("天龙八部-" + u.getName());
        });

        System.out.println(userList);
    }
控制台输出：
[Person(name=段誉, age=25), Person(name=萧峰, age=40), Person(name=虚竹, age=30), Person(name=无涯子, age=100), Person(name=慕容复, age=35), Person(name=云中鹤, age=45)]
---------------
[Person(name=天龙八部-段誉, age=25), Person(name=天龙八部-萧峰, age=40), Person(name=天龙八部-虚竹, age=30), Person(name=天龙八部-无涯子, age=100), Person(name=天龙八部-慕容复, age=35), Person(name=天龙八部-云中鹤, age=45)]

2.筛选（filte）

        List<Person> userList = new ArrayList<>();
        userList.add(new Person("段誉",25));
        userList.add(new Person("萧峰",40));
        userList.add(new Person("虚竹",30));
        userList.add(new Person("无涯子",100));
        userList.add(new Person("慕容复",35));
        userList.add(new Person("云中鹤",45));

        List<Person> collect = 
                userList.stream().filter(user -> (user.getAge() > 30 && user.getName().length() >2)).collect(Collectors.toList());

        System.out.println(collect);

控制台输出：
[Person(name=无涯子, age=100), Person(name=慕容复, age=35), Person(name=云中鹤, age=45)]

3.查找（findAny/findFirst）

    public static void main(String[] args) {

        List<Person> userList = new ArrayList<>();
        userList.add(new Person("段誉",25));
        userList.add(new Person("萧峰",41));
        userList.add(new Person("萧峰",42));
        userList.add(new Person("萧峰",43));
        userList.add(new Person("虚竹",30));
        userList.add(new Person("无涯子",100));
        userList.add(new Person("慕容复",35));
        userList.add(new Person("云中鹤",45));

        Person user = userList.stream().filter(u -> u.getName().equals("阿紫")).findAny().orElse(null);
        System.out.println(user);

        Person user2 = userList.stream().filter(u -> u.getName().equals("萧峰")).findAny().orElse(null);
        System.out.println(user2);

        Person user3 = userList.stream().filter(u -> u.getName().equals("萧峰")).findFirst().orElse(null);
        System.out.println(user3);
    }

控制台输出：
null
Person(name=萧峰, age=41)
Person(name=萧峰, age=41)

1 .findFirst() 方法根据命名可以大致知道是获取Optional流中的第一个元素。
2 .findAny() 方法是获取Optional 流中任意一个，存在随机性，其实里面也是获取元素中的第一个。

使用findAny()是为了更高效的性能。如果是数据较少，串行地情况下，一般会返回第一个结果，如果是并行的情况，那就不能确保是第一个。

4.转换/去重（map/flatMap/distinct）

        List<Person> userList = new ArrayList<>();
        userList.add(new Person("段誉",25));
        userList.add(new Person("萧峰",41));
        userList.add(new Person("萧峰",42));
        userList.add(new Person("虚竹",30));
        userList.add(new Person("无涯子",100));
        userList.add(new Person("慕容复",35));
        userList.add(new Person("云中鹤",45));

        List<String> nameList = userList.stream().map(Person::getName).collect(Collectors.toList());
        System.out.println(nameList);
        List<String> nameList2 = userList.stream().map(Person::getName).distinct().collect(Collectors.toList());
        System.out.println(nameList2);

控制台输出：
[段誉, 萧峰, 萧峰, 虚竹, 无涯子, 慕容复, 云中鹤]
[段誉, 萧峰, 虚竹, 无涯子, 慕容复, 云中鹤]

map的作用：是将输入一种类型，转化为另一种类型，通俗来说：就是将输入类型变成另一个类型。
flatMap的作用：是扁平化的，比如有对象List<List<Student>> userLists, 这属于套娃现象，而最终只想得到List<Student> userList对象，即将多维集合，变成一个集合，相当于压缩，俗称扁平化。

对于处理嵌套list，flatMap更合适

    public static void main(String[] args) {
        List<Student> studentList1 = new ArrayList<>();
        studentList1.add(new Student("枯荣大师",22));
        studentList1.add(new Student("游坦之",44));
        List<Student> studentList2 = new ArrayList<>();
        studentList2.add(new Student("天山童姥",66));
        studentList2.add(new Student("苏星河",88));

        List<Person> userList = new ArrayList<>();
        userList.add(new Person(studentList1));
        userList.add(new Person(studentList2));

        List<String> collect = userList.stream()
                .map(Person::getStudentList)
                .flatMap(Collection::stream)
                .map(Student::getName)
                .collect(Collectors.toList());
        System.out.println(collect);

        List<String> studentList = new ArrayList<>();
        userList.stream().forEach(person -> {
            person.getStudentList().forEach(people -> {
                studentList.add(people.getName());
            });
        });
        System.out.println(studentList);
    }

控制台输出：
[枯荣大师, 游坦之, 天山童姥, 苏星河]
[枯荣大师, 游坦之, 天山童姥, 苏星河]

5.跳过（limit/skip）

    public static void main(String[] args) {
        List<Person> userList = new ArrayList<>();
        userList.add(new Person("段誉",25));
        userList.add(new Person("萧峰",41));
        userList.add(new Person("虚竹",30));
        userList.add(new Person("无涯子",100));
        userList.add(new Person("慕容复",35));

        List<Person> collect = userList.stream().skip(1).collect(Collectors.toList());
        System.out.println(collect);
        List<Person> collect2 = userList.stream().limit(2).collect(Collectors.toList());
        System.out.println(collect2);
        List<Person> collect3 = userList.stream().skip(4).limit(3).collect(Collectors.toList());
        System.out.println(collect3);
    }

控制台输出：
[Person(name=萧峰, age=41), Person(name=虚竹, age=30), Person(name=无涯子, age=100), Person(name=慕容复, age=35)]
[Person(name=段誉, age=25), Person(name=萧峰, age=41)]
[Person(name=慕容复, age=35)]

6.最大值/最小值/平均值（reduce）

    public static void main(String[] args) {
        List<Person> userList = new ArrayList<>();
        userList.add(new Person("段誉",25));
        userList.add(new Person("萧峰",41));
        userList.add(new Person("虚竹",30));
        userList.add(new Person("无涯子",100));
        userList.add(new Person("慕容复",35));
        
        System.out.println("求和");
        Integer sum1 = userList.stream().map(Person::getAge).reduce(0, Integer::sum);
        System.out.println(sum1);

        int sum2 = userList.stream().mapToInt(Person::getAge).sum();
        System.out.println(sum2);

        Integer sum3 = userList.stream().collect(Collectors.summingInt(Person::getAge));
        System.out.println(sum3);
        
        System.out.println("最大值");
        Optional<Integer> max1 = userList.stream().map(Person::getAge).collect(Collectors.toList())
                .stream().reduce((v1, v2) -> v1 > v2 ? v1 : v2);
        System.out.println(max1.get());

        Optional<Integer> max2 = userList.stream().map(Person::getAge).collect(Collectors.toList())
                .stream().reduce(Integer::max);
        System.out.println(max2.get());

        int max3 = userList.stream().mapToInt(Person::getAge).max().getAsInt();
        System.out.println(max3);
        
        System.out.println("最小值");
        Optional<Integer> min = userList.stream().map(Person::getAge).collect(Collectors.toList())
                .stream().reduce(Integer::min);
        System.out.println(min.get());
        
        System.out.println("平均值");
        Double averaging1 = userList.stream().collect(Collectors.averagingDouble(Person::getAge));
        System.out.println(averaging1);

        double averaging2 = userList.stream().mapToInt(Person::getAge).average().getAsDouble();
        System.out.println(averaging2);
    }

控制台输出：
求和
231
231
231
最大值
100
100
100
最小值
25
平均值
46.2
46.2

7.统计（count/counting）

    public static void main(String[] args) {
        List<Person> userList = new ArrayList<>();
        userList.add(new Person("段誉",25));
        userList.add(new Person("萧峰",41));
        userList.add(new Person("虚竹",30));
        userList.add(new Person("无涯子",100));
        userList.add(new Person("虚竹",30));
        userList.add(new Person("慕容复",35));

        Long collect = userList.stream().filter(p -> p.getName() == "虚竹").collect(Collectors.counting());
        System.out.println(collect);

        long count = userList.stream().filter(p -> p.getAge() >= 50).count();
        System.out.println(count);
    }

控制台输出：
2
1

8.排序（sorted）

    public static void main(String[] args) {
        List<Person> userList = new ArrayList<>();
        userList.add(new Person("1",25));
        userList.add(new Person("2",41));
        userList.add(new Person("33",30));
        userList.add(new Person("11",100));
        userList.add(new Person("55",30));
        userList.add(new Person("22",35));

        List<Person> collect =
                userList.stream().sorted(Comparator.comparing(Person::getAge).reversed()).collect(Collectors.toList());
        System.out.println(collect);

        List<Person> collect2 =
                userList.stream().sorted(Comparator.comparing(Person::getAge)).collect(Collectors.toList());
        System.out.println(collect2);

        List<Person> collect3 =
                userList.stream().sorted(Comparator.comparing(Person::getName)).collect(Collectors.toList());
        System.out.println(collect3);

        List<Person> collect4 = userList.stream().sorted((e1, e2) -> {
            return Integer.compare(Integer.parseInt(e1.getName()), Integer.parseInt(e2.getName()));
        }).collect(Collectors.toList());
        System.out.println(collect4);
    }

控制台输出：
[Person(name=11, age=100), Person(name=2, age=41), Person(name=22, age=35), Person(name=33, age=30), Person(name=55, age=30), Person(name=1, age=25)]
[Person(name=1, age=25), Person(name=33, age=30), Person(name=55, age=30), Person(name=22, age=35), Person(name=2, age=41), Person(name=11, age=100)]
[Person(name=1, age=25), Person(name=11, age=100), Person(name=2, age=41), Person(name=22, age=35), Person(name=33, age=30), Person(name=55, age=30)]
[Person(name=1, age=25), Person(name=2, age=41), Person(name=11, age=100), Person(name=22, age=35), Person(name=33, age=30), Person(name=55, age=30)]

字符串排序可以自定义排序规则文章来源地址https://uudwc.com/A/oyA0