使用Java对一个对象分组求某一个属性最小的所有数据
问题描述
假设有一个学生类(Student),每个学生有姓名(name)、年龄(age)和分数(score)三个属性。现在有一组学生对象,需要按照年龄进行分组,并求出每组中分数最低的学生。
方案
1. 定义学生类
首先,我们需要定义一个学生类(Student),包含姓名(name)、年龄(age)和分数(score)三个属性。
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;
}
// getter and setter methods
// ...
}
2. 创建学生对象列表
接下来,我们创建一个学生对象列表,并初始化一些学生对象。
import java.util.ArrayList;
import java.util.List;
public class Main {
public static void main(String[] args) {
List<Student> students = new ArrayList<>();
students.add(new Student("Alice", 18, 80));
students.add(new Student("Bob", 20, 85));
students.add(new Student("Charlie", 18, 90));
students.add(new Student("David", 20, 95));
students.add(new Student("Eve", 18, 75));
}
}
3. 对学生对象分组
我们可以使用Java 8的Stream API来对学生对象进行分组。首先,我们需要按照年龄进行分组。
import java.util.Map;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List<Student> students = new ArrayList<>();
students.add(new Student("Alice", 18, 80));
students.add(new Student("Bob", 20, 85));
students.add(new Student("Charlie", 18, 90));
students.add(new Student("David", 20, 95));
students.add(new Student("Eve", 18, 75));
Map<Integer, List<Student>> groupedStudents = students.stream()
.collect(Collectors.groupingBy(Student::getAge));
}
}
4. 求每组中分数最低的学生
接下来,我们需要在每组中找出分数最低的学生。我们可以使用Stream的min方法结合Comparator来实现。
import java.util.*;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List<Student> students = new ArrayList<>();
students.add(new Student("Alice", 18, 80));
students.add(new Student("Bob", 20, 85));
students.add(new Student("Charlie", 18, 90));
students.add(new Student("David", 20, 95));
students.add(new Student("Eve", 18, 75));
Map<Integer, Student> minScoreStudents = students.stream()
.collect(Collectors.groupingBy(Student::getAge,
Collectors.minBy(Comparator.comparing(Student::getScore))))
.entrySet().stream()
.collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue().orElse(null)));
}
}
在上述代码中,我们首先使用Collectors.groupingBy
方法按照年龄对学生对象进行分组,然后使用Collectors.minBy
方法结合Comparator来求每组中分数最低的学生。最后,我们将结果转换为Map,其中Key为年龄,Value为分数最低的学生对象。
5. 输出结果
最后,我们可以遍历分组结果,并输出每组中分数最低的学生信息。
import java.util.*;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List<Student> students = new ArrayList<>();
students.add(new Student("Alice", 18, 80));
students.add(new Student("Bob", 20, 85));
students.add(new Student("Charlie", 18, 90));
students.add(new Student("David", 20, 95));
students.add(new Student("Eve", 18, 75));
Map<Integer, Student> minScoreStudents = students.stream()
.collect(Collectors.groupingBy(Student::getAge,
Collectors.minBy(Comparator.comparing(Student::getScore))))
.entrySet().stream()
.collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue().orElse(null)));
for (Map.Entry<Integer, Student> entry : minScoreStudents.entrySet()) {
System.out.println("Age: " + entry.getKey() + ", Min Score Student: " + entry.getValue());