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  1. import java.util.List;
  2. import java.util.Map;
  3. import java.util.stream.Collectors;
  4.  
  5. class Ideone {
  6.     public static void main(String[] args) {
  7.         Department a = new Department("a");
  8.         Department b = new Department("b");
  9.         Department c = new Department("c");
  10.  
  11.         Employee e1 = new Employee("e1", List.of(a, b));
  12.         Employee e2 = new Employee("e2", List.of(c, b));
  13.         Employee e3 = new Employee("e3", List.of(c, a));
  14.         Employee e4 = new Employee("e4", List.of(a, b, c));
  15.  
  16.         List<Employee> employees = List.of(e1, e2, e3, e4);
  17.         Map<Department,List<Employee>> result=employees.stream()
  18.             .flatMap(employee->employee.getDepartments().stream()
  19.                 .map(department->new Pair(department,employee)))
  20.             .collect(Collectors.groupingBy(pair->pair.d,
  21.                                            Collectors.mapping(pair->pair.e,
  22.                                                               Collectors.toList())));
  23.         System.out.println(result);
  24.     }
  25.  
  26.     static class Department{final String name;Department(String name){this.name=name;}public String toString(){return name;}}
  27.     static class Employee{final String name;final List<Department> departments;Employee(String name,List<Department> departments){this.name=name;this.departments=departments;}List<Department> getDepartments(){return departments;}public String toString() {return name;}}
  28.     // this is just a helper class instead of AbstractMap.whatever
  29.     static class Pair{final Department d;final Employee e;Pair(Department d,Employee e){this.d=d;this.e=e;}}
  30. }
Success #stdin #stdout 0.11s 48512KB
comments (?)
stdin 
Standard input is empty
stdout 
{a=[e1, e3, e4], c=[e2, e3, e4], b=[e1, e2, e4]}
https://ideone.com/GxJ91k
language:
Java (HotSpot 12)
created:
4 years ago
visibility:
secret

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