比较两个列表并分配到新的对象列表中

Question

我有一个列表，其中包含 Employee 类的对象，而另一个列表包含 Departments 类的对象，如下所示：-

Class Employee {
String emolpyee_id;
String employee_Name;
}
Class Dapartment {
String emolpyee_id;
String employee_Department;
}

List<Employee> empl =  = new ArrayList<>();
List<Dapartment> depart =  = new ArrayList<>();

Employee e1 = new Employee("12","Testing");
Employee e2 = new Employee("16","TestingOthers");

Dapartment d1 = new Dapartment("12","IT");
Dapartment d2 = new Dapartment("12","Networking");
Dapartment d3 = new Dapartment("12","Financing");
Dapartment d4 = new Dapartment("16","IT");
Dapartment d5 = new Dapartment("16","Networking");

empl.add(e1);
empl.add(e2);

depart.add(d1);
depart.add(d2);
depart.add(d3);
depart.add(d4);
depart.add(d5);

Class ExpectedResult {
String emolpyee_id;
String employee_Name;
List<String> employee_Department;
}

List<ExpectedResult> expResult = {{"12","Testing",{"IT","Networking","Financing"}} ,{"16","TestingOthers",{"IT","Networking"}}}

我知道我们可以使用普通的 for 循环来执行此操作，但我想使用 lambda 和 java 8 的流来实现此输出。

Answer 1

首先使用 groupingBy 将 List<Department> 流式传输到 Map<String,List<String>>，这是员工 ID 作为键的映射，值是部门列表

Map<String, List<String>> groupby = depart.stream().
            collect(Collectors.groupingBy(Department::getEmolpyee_id, Collectors.mapping(Department::getEmployee_Department, Collectors.toList())));

然后对于List<Employee> 中的每个对象，从Map 获取部门

List<ExpectedResult> expected = empl.stream().
            map(emp-> new ExpectedResult(emp.getEmolpyee_id(),emp.getEmployee_Name(),groupby.get(emp.getEmolpyee_id()))).collect(Collectors.toList());

注意如果 groupby Map 不包含员工 ID 和值，则 ExpectedResult 中的 List<String> employee_Department; 可能为 null

旁注：class是小写Class是错误的，部门拼写更正