How to apply Filtering on groupBy in java streams

Question

How do you group first and then apply filtering using Java streams?

Example: Consider this Employee class: I want to group by Departme

Answer 1

There is no cleaner way of doing this in Java 8: Holger has shown clear approach in java8 here Accepted the Answer.

This is how I have done it in java 8:

Step: 1 Group by Department

Step: 2 loop throw each element and check if department has an employee with salary >2000

Step: 3 update the map copy values in new map based on noneMatch

Map> employeeMap = list.stream().collect(Collectors.groupingBy(Employee::getDepartment));
Map> newMap = new HashMap>();
         employeeMap.forEach((k, v) -> {
            if (v.stream().noneMatch(emp -> emp.getSalary() > 2000)) {
                newMap.put(k, new ArrayList<>());
            }else{
                newMap.put(k, v);
           }

        });

Java 9 : Collectors.filtering

java 9 has added new collector Collectors.filtering this group first and then applies filtering. filtering Collector is designed to be used along with grouping.

The Collectors.Filtering takes a function for filtering the input elements and a collector to collect the filtered elements:

list.stream().collect(Collectors.groupingBy(Employee::getDepartment),
 Collectors.filtering(e->e.getSalary()>2000,toList());