user7950856 :
I have a map like the following:
class Person {
Long personId;
String name;
/*Getters and Setters*/
}
I have populated a map Map<Long, Person> personMap = new HashMap<>();
The key of the map is the personId itself. I have a list of personIds like so,
List<Long> coolPeople = new ArrayList<>();
now I want to iterate through the map and get all the values with the keys corresponding the ids in the list coolPeople, and then store it in a List.
How can I achieve this in Java 8 optimally?
Eran :
It would be more efficient to iterate over the identifiers of the List and look them up in the Map, since search by key in a HashMap takes expected O(1) time, while the search in the List can take O(n) time at the worst case.
List<Person> people =
coolPeople.stream()
.map(id -> personMap.get(id)) // or map(personMap::get)
.filter(Objects::nonNull)
.collect(Collectors.toList());
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