How to return List of Object using Stream API

Question

My EmployeeDeatils Object has List of other object inside (employeeData). I am trying to fetch that with some mapping with id

But not sure what I am doing wrong

     private List<EmployeeData> getEmployeeMessage(List<EmployeeDetails> employeeDetailList, Employee employee) {

    return employeeDetailList.stream()
        .filter(employeeDetail -> employeeDetail.getEmployeeId() == employee.getEmployeeId())
        .map(employeeDetail -> employeeDetail.getEmployeeData())
        .collect(Collectors.toList);

}

I guess your problem is a compiler error, so I suggest to pay attention to the compiler error and what it says. Otherwise, include the problem in your question. — Holger
– Holger, Commented Dec 3, 2020 at 20:46
Mostly understanding about map versus flatMap as it sounds like. — Naman
– Naman, Commented Dec 4, 2020 at 2:46

Tasos P. · Accepted Answer · 2020-12-06 16:25:58Z

1

Assuming that your EmployeeDetails class has List<EmployeeData> getEmployeeData() field, you can fetch these using:

List<EmployeeData> getEmployeeMessage(List<EmployeeDetails> employeeDetailList, Employee employee) {

        return employeeDetailList.stream()
                .filter(employeeDetail -> employeeDetail.getEmployeeId() == employee.getEmployeeId())
                .flatMap(employeeDetail -> employeeDetail.getEmployeeData().stream())
                .collect(Collectors.toList());

    }

As @Naman correctly pointed out, you need to pay attention on flatMap vs map operations.

In this case, a flatMap is used because you need a mapping operation for each element of your list (returned by getEmployeeData()).

answered Dec 6, 2020 at 16:25

Tasos P.

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