Java 用 Stream 將 List<obj> 變成 Map<key, List<obj>>

例如我手邊有 class Student

class Student{
	private int studentID, teacherID;
	private String studentName;
	public Student (int studentID, int teacherID, String studentName){
		this.studentID = studentID;
		this.teacherID = teacherID;
		this.studentName = studentName;
	}
	
	public int getStudentID(){return studentID;}
	public int getTeacherID(){return teacherID;}
	public String getStudentName(){return studentName;}
	
	public String toString(){
		return String.format("[Student %02d: %s]", studentID, studentName);
	}
	
}

假設我手邊有學生清單（ List<Student>）

但我想看每個老師底下有哪些學生，換言之，想將之轉為 Map< int (teacherID), List<Student>>

作法如下：

import java.util.*;
import java.util.stream.*;
public class Main{
	public static void main(String[] args) {
		List<Student> list = Arrays.asList(
			new Student(1, 100, "andy"),
			new Student(2, 100, "bella"),
			new Student(3, 200, "Can"),
			new Student(4, 100, "Donny")
		);
		
		System.out.println(list);
		System.out.println(
			list.stream()
				.collect(
					Collectors.groupingBy(
						s -> s.getTeacherID(),
						Collectors.toList()
					)
				)
		);
	}
}

參考資料

Eran, (2018, February 28). java - Grouping by and map value. StackOverflow. https://stackoverflow.com/a/49029481

Fabian, (2019, September 11). java - Grouping by and map value. Stackoverflow. https://stackoverflow.com/a/57897640