java 8 groupingBy

Question

我有一份學生名單，每個學生可以註冊幾門科目。 因此，每個學生將有一個主題列表。我想對使用java 8功能的Subject進行分組。 我無法想出辦法。任何幫助將不勝感激。

Answer 1

這只是一個例子，其中使用了groupBy。你可以在網上找到很多其他的例子。按價格[這個例子是從here採取]

組項目：Collectors.groupingBy和Collectors.mapping例如

public static void main(String[] args) { 

     //3 apple, 2 banana, others 1 
     List<Item> items = Arrays.asList(
       new Item("apple", 10, new BigDecimal("9.99")), 
       new Item("banana", 20, new BigDecimal("19.99")), 
       new Item("orang", 10, new BigDecimal("29.99")), 
       new Item("watermelon", 10, new BigDecimal("29.99")), 
       new Item("papaya", 20, new BigDecimal("9.99")), 
       new Item("apple", 10, new BigDecimal("9.99")), 
       new Item("banana", 10, new BigDecimal("19.99")), 
       new Item("apple", 20, new BigDecimal("9.99")) 
       ); 

     //group by price 
     Map<BigDecimal, List<Item>> groupByPriceMap = 
      items.stream().collect(Collectors.groupingBy(Item::getPrice)); 

     System.out.println(groupByPriceMap); 

     // group by price, uses 'mapping' to convert List<Item> to Set<String> 
     Map<BigDecimal, Set<String>> result = 
       items.stream().collect(
         Collectors.groupingBy(Item::getPrice, 
           Collectors.mapping(Item::getName, Collectors.toSet()) 
         ) 
       ); 

     System.out.println(result); 

    } 

輸出

{ 
    19.99=[ 
      Item{name='banana', qty=20, price=19.99}, 
      Item{name='banana', qty=10, price=19.99} 
      ], 
    29.99=[ 
      Item{name='orang', qty=10, price=29.99}, 
      Item{name='watermelon', qty=10, price=29.99} 
      ], 
    9.99=[ 
      Item{name='apple', qty=10, price=9.99}, 
      Item{name='papaya', qty=20, price=9.99}, 
      Item{name='apple', qty=10, price=9.99}, 
      Item{name='apple', qty=20, price=9.99} 
      ] 
} 

//group by + mapping to Set 
{ 
    19.99=[banana], 
    29.99=[orang, watermelon], 
    9.99=[papaya, apple] 
} 

Answer 2

我的一個朋友的建議這種解決方案，它能正常工作

package grpBy; 

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


class Pair { 
    Subject sub1; 
    Student student; 

    public Pair(Student student, Subject sub1) { 
     this.sub1 = sub1; 
     this.student = student; 
    } 



    public String getSub1() { 
     return sub1.name; 
    } 

    public String getStudent() { 
     return student.name; 
    } 

    static Pair of(Student stu, Subject sub) { 
     return new Pair(stu, sub); 
    } 



} 
public class Test2 { 




    public static void main(String[] args) { 

      Subject maths = new Subject("maths", 1); 
      Subject chemi = new Subject("chemi", 1); 
      Subject phy = new Subject("phy", 1); 
      Subject bio = new Subject("bio", 1); 
      List<Subject> s1 = new ArrayList<>(); 
      s1.add(maths); 
      s1.add(chemi); 

      List<Subject> s2 = new ArrayList<>(); 
      s2.add(maths); 
      s2.add(phy);  

      List<Subject> s3 = new ArrayList<>(); 
      s3.add(bio); 
      s3.add(phy);  

      Student jack = new Student(1, "jack", s1); 
      Student jil = new Student(2, "jil", s2); 
      Student john = new Student(3, "john", s3); 

      List<Student> students = new ArrayList(); 
      students.add(jack); 
      students.add(jil); 
      students.add(john); 

     Map<String, List<String>> m = students.stream(). 
       flatMap(student -> student.subjects.stream().map(subject -> Pair.of(student, subject))). 
       collect(Collectors.groupingBy(e -> e.getSub1(), 
               Collectors.mapping(e -> e.getStudent(), 
                   Collectors.toList()))); 

     System.out.println(m); 
    } 
}