Java Lambda. This method must return a result of type ArrayList<String>

Question

I have a map of employee name and employee salary and I want to return a list of employees' names whose salary is less than or equal to the salary passed as an argument using lambda and functional interface. This is what I am doing but it is showing me an error this method should return a result of the type list.

public static EmployeeAudit findEmployee() {
       return salary ->  
            employeeMap.entrySet()
                       .stream()
                       .filter(s -> s.getValue() <= salary)
                       .collect(Collectors.toList());
}

public interface EmployeeAudit {
    ArrayList<String> fetchEmployeeDetails (double salary);
}

Answer 1

It's better to use an abstraction (interface) over a particular implementation. This satisfies most of the contracts. Note that Collectors.toList() results in List<T> and not ArrayList<T> - the return types are incompatible. Start with:

public interface EmployeeAudit {
    List<String> fetchEmployeeDetails (double salary);
}

Now returning EmployeeAudit as a lambda expression. I assume employeeMap is Map<String, Double> hence you need to map (transform) Stream into the Map's key using Stream#map(Function):

public static EmployeeAudit findEmployee() {
    return salary -> employeeMap.entrySet()
                                .stream()
                                .filter(s -> s.getValue() <= salary)
                                .map(Map.Entry::getKey)
                                .collect(Collectors.toList());
    }
}