java 8 steam groupby 使用示例

假设我们有一个Student类，包含姓名、年龄和成绩等属性，我们需要按照年龄分组，然后统计每个年龄段的人数和总成绩。

我们可以使用Java 8的Stream API中的groupBy方法来实现这个需求。具体示例代码如下：

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Student {
    private String name;
    private int age;
    private int score;

    public Student(String name, int age, int score) {
        this.name = name;
        this.age = age;
        this.score = score;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public int getScore() {
        return score;
    }

    public static void main(String[] args) {
        List<Student> students = Arrays.asList(
                new Student("Alice", 18, 90),
                new Student("Bob", 19, 80),
                new Student("Charlie", 18, 95),
                new Student("David", 19, 85),
                new Student("Ella", 20, 92),
                new Student("Frank", 20, 88)
        );

        Map<Integer, List<Student>> groupByAge = students.stream()
                .collect(Collectors.groupingBy(Student::getAge));

        Map<Integer, Integer> countByAge = groupByAge.entrySet().stream()
                .collect(Collectors.toMap(
                        Map.Entry::getKey,
                        e -> e.getValue().size()
                ));

        Map<Integer, Integer> sumByAge = groupByAge.entrySet().stream()
                .collect(Collectors.toMap(
                        Map.Entry::getKey,
                        e -> e.getValue().stream().mapToInt(Student::getScore).sum()
                ));

        System.out.println("Group by age: " + groupByAge);
        System.out.println("Count by age: " + countByAge);
        System.out.println("Sum by age: " + sumByAge);
    }
}

运行结果如下：

Group by age: {18=[Student{name='Alice', age=18, score=90}, Student{name='Charlie', age=18, score=95}], 19=[Student{name='Bob', age=19, score=80}, Student{name='David', age=19, score=85}], 20=[Student{name='Ella', age=20, score=92}, Student{name='Frank', age=20, score=88}]}
Count by age: {18=2, 19=2, 20=2}
Sum by age: {18=185, 19=165, 20=180}

从运行结果可以看出，我们成功地按照年龄分组，并统计了每个年龄段的人数和总成绩。