Filter map and return list of keys

Question

We have a Map<String, Student> studentMap, where Student is a class as follows:

class Student{
    String name;
    int age;
}

We need to return a list of all Ids eligibleStudents, where the age > 20.
Why does the following give a compilation error at the Collectors.toList:

HashMap<String, Student> studentMap = getStudentMap();
eligibleStudents = studentMap .entrySet().stream()
        .filter(a -> a.getValue().getAge() > 20)
        .collect(Collectors.toList(Entry::getKey));

Answer 1

Collectors.toList() does not take any argument, you need to map it first:

eligibleStudents = studentMap.entrySet().stream()
    .filter(a -> a.getValue().getAge() > 20)
    .map(Map.Entry::getKey)
    .collect(Collectors.toList());

Answer 2

toList() collector just creates a container to accumulate elements and takes no arguments. You need to do a mapping before it is collected. Here's how it looks.

List<String> eligibleStudents = studentMap.entrySet().stream()
    .filter(a -> a.getValue().getAge() > 20)
    .map(Map.Entry::getKey)
    .collect(Collectors.toList());