Using Streams, see if a list contains a property of an object from another list-CodePudding

Jeez, I almost need another bit of help in how to phrase this question! New Java II student here, thanks in advance for your time.

I have a list of employees that look like this:

public class Employee {
    private String name;
    private String department;
}

And a list of Companies that look like this:

public class Company {  
    private String name;    
    List<Department> departments;
}

Department is just:

public class Department{    
    private String name;
    private Integer totalSalary;
}

So I'm tasked with streaming a list of employees that work for the same company. (Sorry for not saying before: the company is passed in to a fuction. It's the lone argument) It seemed easy when I first read it, but because of how the classes are set up, (Company with only a list of departments, and Employee with a single department, but no link between employee and company) I can stream to make a list of all Departments in a Company, but just don't know how to bring that back and match the employee's department string with any string from the departments that belong to the Company in question...

List<Department> deptsInCompany = companies.stream()
                .filter(s -> s.getName().equals(passedInCompany))
                .flatMap(s -> s.getDepartments().stream())              
                .collect(Collectors.toList());

I'm just not sure how to use that list of departments to backtrack and find the Employees in those departments. I think my ROOKIE mind can't get past wanting there to be a list of Employees in each department object, but there's not!

Any little nudge would be greatly appreciated! I promise to pay it forward when i've got some skill!!

CodePudding user response：

Assuming that you have a list of all employees and that all your model classes have getters for their properties, you can do the following:

public static void main(String[] args) {
    List<Company> companies = // Your list of Companies
    String passedInCompany = "Company";
    
    List<String> deptsNameInCompany = companies.stream()
            .filter(s -> s.getName().equals(passedInCompany))
            .flatMap(s -> s.getDepartments().stream())
            .map(Department::getName)
            .collect(Collectors.toList());

    List<Employee> employees = // All Employees
    List<Employee> employeesInCompanyDepts = employees.stream()
            .filter(employee -> deptsNameInCompany.contains(employee.getDepartment()))
            .collect(Collectors.toList());
}

Basically you need to collect all the Departments names and then find the Employees that have such Department name in its department property.

CodePudding user response：

Collect the department names of the (single) company with the given name into a Set (which is faster for lookup than a List).

Set<String> departmentNames = companies.stream()
    .filter(c -> c.getName().equals(companyName))
    .findFirst().get().getDepartments().stream()
    .map(Department::getName)
    .collect(Collectors.toSet());

Then remove all employees that aren't in those departments from the list.

employees.removeIf(e -> !departmentNames.contains(e.getDepartment()));

If you want to preserve the list of employees, filter and collect:

List<Employee> employeesInCompany = employees.stream()
    .filter(e -> departmentNames.contains(e.getDepartment()))
    .collect(Collectors.toList());