第十一章对Stream流的聚合函数处理

一、对流中数据进行聚合计算

二、对流中数据进行分组

三、对流中数据进行多级分组

四、对流中数据进行分区

4.1. 使用方式及代码

4.2. 分区于分组的区别

分区（Partitioning）

分组（Grouping）

实际应用场景

五、对流中数据进行拼接

一、对流中数据进行聚合计算

当我们使用Stream流处理数据后，可以像数据库的聚合函数一样对某个字段进行操作。比如获取最大值，获取最小值，求总和，平均值，统计数量。

java">package com.wzx;
import java.util.ArrayList;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;
import java.util.stream.Stream;public class Test {public static void main(String[] args) {Stream<Student> studentStream = Stream.of(new Student("赵丽颖", 58, 95),new Student("杨颖", 56, 88),new Student("迪丽热巴", 56, 99),new Student("柳岩", 52, 77));// 获取最大值Optional<Student> collect1 = studentStream.collect(Collectors.maxBy((o1, o2) ->o1.getScore() - o2.getScore()));System.out.println(collect1.get());// 获取最小值Optional<Student> collect2 = studentStream.collect(Collectors.minBy((o1, o2) ->o1.getScore() - o2.getScore()));System.out.println(collect2.get());// 求总和int sumAge = studentStream.collect(Collectors.summingInt(s -> s.getAge()));System.out.println("sumAge = " + sumAge);// 平均值double avgScore = studentStream.collect(Collectors.averagingInt(s -> s.getScore()));System.out.println("avgScore = " + avgScore);// 统计数量Long count = studentStream.collect(Collectors.counting());System.out.println("count = " + count);}
}class Student {private String name;private int age;private int score;public Student() {}public Student(String name, int age, int score) {this.name = name;this.age = age;this.score = score;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +", score=" + score +'}';}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;}public int getScore() {return score;}public void setScore(int score) {this.score = score;}
}

二、对流中数据进行分组

当我们使用Stream流处理数据后，可以根据某个属性将数据分组：

java">package com.wzx;import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;public class Test {public static void main(String[] args) {Stream<Student> studentStream1 = Stream.of(new Student("赵丽颖", 52, 95),new Student("杨颖", 56, 88),new Student("迪丽热巴", 56, 55),new Student("柳岩", 52, 33));Map<Integer, List<Student>> map1 = studentStream1.collect(Collectors.groupingBy(Student::getAge));map1.forEach((k, v) -> {System.out.println(k + "::" + v);});Stream<Student> studentStream2 = Stream.of(new Student("赵丽颖", 52, 95),new Student("杨颖", 56, 88),new Student("迪丽热巴", 56, 55),new Student("柳岩", 52, 33));// 将分数大于60的分为一组,小于60分成另一组Map<String, List<Student>> map2 = studentStream2.collect(Collectors.groupingBy((s) ->{if (s.getScore() > 60) {return "及格";} else {return "不及格";}}));map2.forEach((k, v) -> {System.out.println(k + "::" + v);});}
}class Student {private String name;private int age;private int score;public Student() {}public Student(String name, int age, int score) {this.name = name;this.age = age;this.score = score;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +", score=" + score +'}';}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;}public int getScore() {return score;}public void setScore(int score) {this.score = score;}
}

运行输出结果：

三、对流中数据进行多级分组

java">package com.wzx;import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;public class Test {public static void main(String[] args) {Stream<Student> studentStream = Stream.of(new Student("赵丽颖", 52, 95),new Student("杨颖", 56, 88),new Student("迪丽热巴", 56, 99),new Student("柳岩", 52, 77));Map<Integer, Map<String, List<Student>>> map =studentStream.collect(Collectors.groupingBy(s -> s.getAge(), Collectors.groupingBy(s -> {if (s.getScore() >= 90) {return "优秀";} else if (s.getScore() >= 80 && s.getScore() < 90) {return "良好";} else if (s.getScore() >= 80 && s.getScore() < 80) {return "及格";} else {return "不及格";}})));map.forEach((k, v) -> {System.out.println(k + " == " + v);});}
}class Student {private String name;private int age;private int score;public Student() {}public Student(String name, int age, int score) {this.name = name;this.age = age;this.score = score;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +", score=" + score +'}';}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;}public int getScore() {return score;}public void setScore(int score) {this.score = score;}
}

运行输出效果：

四、对流中数据进行分区

4.1. 使用方式及代码

Collectors.partitioningBy 会根据值是否为true，把集合分割为两个列表，一个true列表，一个false列表。

java">package com.wzx;import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;public class Test {public static void main(String[] args) {Stream<Student> studentStream = Stream.of(new Student("赵丽颖", 52, 95),new Student("杨颖", 56, 88),new Student("迪丽热巴", 56, 99),new Student("柳岩", 52, 77));// partitioningBy会根据值是否为true，把集合分割为两个列表，一个true列表，一个false列表。Map<Boolean, List<Student>> map = studentStream.collect(Collectors.partitioningBy(s ->s.getScore() > 90));map.forEach((k, v) -> {System.out.println(k + " == " + v);});}
}class Student {private String name;private int age;private int score;public Student() {}public Student(String name, int age, int score) {this.name = name;this.age = age;this.score = score;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +", score=" + score +'}';}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;}public int getScore() {return score;}public void setScore(int score) {this.score = score;}
}

运行输出结果，可以明显看到分区输出结果是一个数组：

4.2. 分区于分组的区别

‌Stream流的分区和分组是两种不同的操作，它们在数据处理上有明显的区别。‌

分区（Partitioning）

分区操作是将流中的元素根据某个条件分成两个或多个子流。分区操作的结果是一个流，而不是一个集合。例如，可以使用分区操作将学生按照分数高低分成两个子流：高分和低分。分区操作通常使用partitioningBy方法实现，该方法返回一个Map，其中键是分区条件，值是对应的流‌1。

分组（Grouping）

分组操作是将流中的元素按照某个分类函数分成不同的组。分组操作的结果是一个Map，其中键是分类函数返回的值，值是具有该分类值的元素的列表。例如，可以使用分组操作将交易记录按照货币类型分成不同的组，每组包含所有相同货币类型的交易记录‌1。分组操作通常使用groupingBy方法实现。

实际应用场景

‌分区‌：适用于需要将数据分成两个或多个子集进行进一步处理的场景。例如，将学生按照分数高低分成两个子流，然后对每个子流进行不同的处理。
‌分组‌：适用于需要将数据按照某个属性进行分类汇总的场景。例如，将交易记录按照货币类型分组，以便进行统计和分析。

五、对流中数据进行拼接

Collectors.joining 会根据指定的连接符，将所有元素连接成一个字符串。

java">package com.wzx;import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;public class Test {public static void main(String[] args) {Stream<Student> studentStream = Stream.of(new Student("赵丽颖", 52, 95),new Student("杨颖", 56, 88),new Student("迪丽热巴", 56, 99),new Student("柳岩", 52, 77));String collect = studentStream.map(Student::getName).collect(Collectors.joining(">_<", "^_^", "^v^"));System.out.println(collect);}
}class Student {private String name;private int age;private int score;public Student() {}public Student(String name, int age, int score) {this.name = name;this.age = age;this.score = score;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +", score=" + score +'}';}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;}public int getScore() {return score;}public void setScore(int score) {this.score = score;}
}

运行输出结果：