Java 8 stream API - is there standard method for processing each value in Map to different type?

Question

I learn Java 8 Lambda Expressions and stream API. In order to understand I try to make expression analog for SQL quesry:

select department, avg(salary) from employee group by department

for:

private static class Employee {
    public String name;
    public String department;
    public int salary;
}

Solution present in official tutorial:

empls.stream().collect(
    Collectors.groupingBy(
        x -> x.department,
        Collectors.averagingInt(x -> x.salary)))

Before I found this solution my strategy to calculate average with grouping:

Map<String, List<Employee>> tmp =
    empls.stream().collect(Collectors.groupingBy(x -> x.department));

and applying functor to each value. But in Map interface there are no method to transform value into different type. In my case reduce List to Double. Standard SE API provide only method replaceAll() that convert value to same type...

What Java 8 style method/trick/one-liner to convert Map value into different type? Worked like pseudo-code:

Map<K, V2> map2 = new HashMap<>();
for (Map.Entry<K, V1> entry : map1.entrySet()) {
     map2.add(entry.getKey(), Function<V1, V2>::apply(entry.getValue()));
 }

Answer 1

You want:

Map<K, V2> map2 = 
    map1.entrySet().stream()
        .collect(toMap(Map.Entry::getKey, 
                       e -> f.apply(e.getValue()));

where f is a function from V to V2.