Java 8 lambda for selecting top salary employee for each department

by

You can do that with a grouping collector:

Map<String, Employee> topEmployees =
    allEmployees.stream()
                .collect(groupingBy(
                    e -> e.department,
                    collectingAndThen(maxBy(comparingInt(e -> e.salary)), Optional::get) 
                ));

with the static imports 

import static java.util.Comparator.comparingInt;
import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.maxBy;

This code creates a Stream of all the employees and groups them with their department with the help of Collectors.groupingBy. For all the values classified to the same key, we need to keep only the employee with the maximum salary, so we collect them with Collectors.maxBy and the comparator compares the salary with Comparator.comparingInt. Since maxBy returns an Optional<Employee> (to handle the case where there the list is empty), we wrap it with a call to Collectors.collectingAndThen with a finisher that just returns the employee: we know in this case that the optional won’t be empty.

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