Java 8 extract all keys from matching values in a Map

Question

I'm relatively new to Java8 and I have a scenario where I need to retrieve all the keys from the Map which matched with the objects.

Wanted to know if there is a way to get all keys without iterating them from the list again.

Person.java
private String firstName;
private String lastName;
//setters and getters & constructor


MAIN Class.

String inputCriteriaFirstName = "john";   

Map<String, Person> inputMap = new HashMap<>();
Collection<Person> personCollection = inputMap.values();
List<Person> personList = new ArrayList<>(personCollection);
List<Person> personOutputList = personList.stream()
.filter(p -> p.getFirstName().contains(inputCriteriaFirstName ))
.collect(Collectors.toList());


//IS There a BETTER way to DO Below ??

Set<String> keys = new HashSet<>();
for(Person person : personOutputList) {
    keys.addAll(inputMap.entrySet().stream().filter(entry -> Objects.equals(entry.getValue(), person))
        .map(Map.Entry::getKey).collect(Collectors.toSet()));
}

Answer 1

inputMap.entrySet() 
        .stream()
        .filter(entry -> personOutputList.contains(entry.getValue()))
        .map(Entry::getKey)
        .collect(Collectors.toCollection(HashSet::new))