java lambda表达式 steam api

案例使用 java版本为 jdk-17

foreach

遍历

public class JavaDemo03 {public static void main(String[] args) {List<Integer> nums = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9);nums.forEach(System.out::println);}
}

运行结果：

map

逐个操作列表中每一个元素

public class JavaDemo04 {public static void main(String[] args) {List<Integer> nums = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9);List<Integer> numbers = nums.stream().map(num -> num * 2).toList();numbers.forEach(System.out::println);}
}

运行结果：

flatMap

先对集合中每个元素进行map，再对map后的每个元素进行flatten

map后的每个元素也是一个集合

public class JavaDemo06 {public static void main(String[] args) {List<Integer> listA = new ArrayList<>();listA.add(1);listA.add(2);List<Integer> listB = new ArrayList<>();listA.add(5);listA.add(6);List<List<Integer>> listC = new ArrayList<>();listC.add(listA);listC.add(listB);List<Integer> numbers = listC.stream().flatMap(Collection::stream).toList();numbers.forEach(System.out::println);}
}

运行结果：

filter

过滤出符合规则的数据生成新的列表

public class JavaDemo07 {public static void main(String[] args) {List<Person> persons = new ArrayList<>();Person person;for (int i = 0; i < 10; i++) {person = new Person("user" + (i + 1), (i + 1) % 2);persons.add(person);}// 获取男性列表List<Person> manList = persons.stream().filter(user -> user.getGender() == 1).toList();// 获取女性列表List<Person> womanList = persons.stream().filter(user -> user.getGender() == 0).toList();manList.forEach(System.out::println);System.out.println("+++++++++++++++++++++++");womanList.forEach(System.out::println);}
}class Person {private String name;private int gender;public Person() {}public Person(String name, int gender) {this.name = name;this.gender = gender;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getGender() {return gender;}public void setGender(int gender) {this.gender = gender;}@Overridepublic String toString() {return "Person{" +"name='" + name + '\'' +", gender=" + gender +'}';}
}class JavaDemo03 {public static void main(String[] args) {List<Integer> nums = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9);nums.forEach(System.out::println);}
}

运行结果：

Person{name='user1', gender=1}
Person{name='user3', gender=1}
Person{name='user5', gender=1}
Person{name='user7', gender=1}
Person{name='user9', gender=1}
+++++++++++++++++++++++
Person{name='user2', gender=0}
Person{name='user4', gender=0}
Person{name='user6', gender=0}
Person{name='user8', gender=0}
Person{name='user10', gender=0}

reduce

相邻两个元素转为一个元素

列表中的所有元素转为一个元素

1, 2, 3, 4, 5, 6, 7, 8, 9
-1, 3, 4, 5, 6, 7, 8, 9
-4, 4, 5, 6, 7, 8, 9
-8, 5, 6, 7, 8, 9
-13, 6, 7, 8, 9
-19, 7, 8, 9
-26, 8, 9
-34, 9
-43

public class JavaDemo08 {public static void main(String[] args) {List<Integer> nums = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9);List<Integer> numbers = nums.stream().reduce((a, b) -> a - b).stream().toList();numbers.forEach(System.out::println);}
}

运行结果：

-43

count

计数器

public class JavaDemo12 {public static void main(String[] args) {List<Integer> nums = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9);long count = nums.stream().filter(num -> num % 2 == 0).count();System.out.println("列表中偶数是数量 >>> " + count);}
}

运行结果：

distinct

去重

public class JavaDemo13 {public static void main(String[] args) {List<Integer> nums = List.of(1, 2, 3, 5, 6, 7, 8, 9, 3, 4, 5, 7, 8, 2, 3);List<Integer> numbers = nums.stream().distinct().toList();numbers.forEach(System.out::println);}
}

运行结果：

sorted

排序类需要实现 Comparable 接口

public class JavaDemo14 {public static void main(String[] args) {List<Dog> dogs = new ArrayList<>();for (int i = 0; i < 10; i++) {dogs.add(new Dog("dog" + (i + 1), ThreadLocalRandom.current().nextInt(1, 10)));}dogs.forEach(System.out::println);List<Dog> newDogs = dogs.stream().sorted((a, b) -> a.getAge() - b.getAge()).toList();System.out.println("++++++++++++++");newDogs.forEach(System.out::println);}
}class Dog {private String name;private int age;public Dog() {}public Dog(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic String toString() {return "Dog{" +"name='" + name + '\'' +", age=" + age +'}';}
}

运行结果：

Dog{name='dog1', age=3}
Dog{name='dog2', age=3}
Dog{name='dog3', age=3}
Dog{name='dog4', age=3}
Dog{name='dog5', age=7}
Dog{name='dog6', age=6}
Dog{name='dog7', age=4}
Dog{name='dog8', age=8}
Dog{name='dog9', age=7}
Dog{name='dog10', age=2}
++++++++++++++
Dog{name='dog10', age=2}
Dog{name='dog1', age=3}
Dog{name='dog2', age=3}
Dog{name='dog3', age=3}
Dog{name='dog4', age=3}
Dog{name='dog7', age=4}
Dog{name='dog6', age=6}
Dog{name='dog5', age=7}
Dog{name='dog9', age=7}
Dog{name='dog8', age=8}

limit

TopN

public class JavaDemo17 {public static void main(String[] args) {int base = 1000;ArrayList<Video> videos = new ArrayList<>();for (int i = 0; i < 10; i++) {videos.add(new Video(base + i + 1, ThreadLocalRandom.current().nextInt(100, 1000)));}videos.forEach(System.out::println);System.out.println("+++++++++++++");videos.sort((a, b) -> b.getGold() - a.getGold());List<Video> topList = videos.stream().limit(3).toList();topList.forEach(System.out::println);}
}class Video {private int vid;private int gold;public Video() {}public Video(int vid, int gold) {this.vid = vid;this.gold = gold;}public int getVid() {return vid;}public void setVid(int vid) {this.vid = vid;}public int getGold() {return gold;}public void setGold(int gold) {this.gold = gold;}@Overridepublic String toString() {return "Video{" +"vid='" + vid + '\'' +", gold=" + gold +'}';}
}

运行结果：

Video{vid='1001', gold=566}
Video{vid='1002', gold=548}
Video{vid='1003', gold=620}
Video{vid='1004', gold=897}
Video{vid='1005', gold=264}
Video{vid='1006', gold=311}
Video{vid='1007', gold=882}
Video{vid='1008', gold=870}
Video{vid='1009', gold=106}
Video{vid='1010', gold=185}
+++++++++++++
Video{vid='1004', gold=897}
Video{vid='1007', gold=882}
Video{vid='1008', gold=870}

groupBy

分组

public class JavaDemo18 {public static void main(String[] args) {List<Animal> animals = new ArrayList<>();Animal animal;for (int i = 0; i < 10; i++) {animal = new Animal("animal" + (i + 1), (i + 1) % 2);animals.add(animal);}// 按照性别分组Map<Integer, List<Animal>> animalGenderGroup = animals.stream().collect(Collectors.groupingBy(Animal::getGender));animalGenderGroup.forEach((key, value) -> System.out.println((key == 1 ? "雄性" : "雌性") + " >>> " + value));}
}class Animal {private String name;private int gender;public Animal() {}public Animal(String name, int gender) {this.name = name;this.gender = gender;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getGender() {return gender;}public void setGender(int gender) {this.gender = gender;}@Overridepublic String toString() {return "Animal{" +"name='" + name + '\'' +", gender=" + gender +'}';}
}

运行结果：

雌性 >>> [Animal{name='animal2', gender=0}, Animal{name='animal4', gender=0}, Animal{name='animal6', gender=0}, Animal{name='animal8', gender=0}, Animal{name='animal10', gender=0}]
雄性 >>> [Animal{name='animal1', gender=1}, Animal{name='animal3', gender=1}, Animal{name='animal5', gender=1}, Animal{name='animal7', gender=1}, Animal{name='animal9', gender=1}]

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java lambda表达式 steam api

java lambda表达式 steam api

foreach

map

flatMap

filter

reduce

count

distinct

sorted

limit

groupBy

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