I have the following code that I'd like to try to write using the java collectors.
Given 2 attributes (firstname and lastname) of a person, I'd like to get a map containing the unique firstname or lastname as a key, and the list of the corresponding persons.
Here's a set of data :
Person person1 = new Person();
person1.setFirstName("john");
person1.setLastName("doe");
person1.setUserId("user1");
Person person2 = new Person();
person2.setFirstName("doe");
person2.setLastName("frank");
person2.setUserId("user2");
Person person3 = new Person();
person3.setFirstName("john");
person3.setLastName("wayne");
person3.setUserId("user3");
List<Person> personList = new ArrayList<>();
personList.add(person1);
personList.add(person2);
personList.add(person3);
Output (as expected) is the following :
frank=[Person{userId='user2', firstName='doe', lastName='frank'}],
john=[Person{userId='user1', firstName='john', lastName='doe'}, Person{userId='user3', firstName='john', lastName='wayne'}],
doe=[Person{userId='user1', firstName='john', lastName='doe'}, Person{userId='user2', firstName='doe', lastName='frank'}],
wayne=[Person{userId='user3', firstName='john', lastName='wayne'}]
And the code to populate the map :
Map<String, List<Person>> mapPersons = new HashMap<String, List<Person>>();
List<Person> listPersons;
for (Person p: personList) {
if (mapPersons.get(p.getFirstName()) == null) {
listPersons = new ArrayList<Person>();
listPersons.add(p);
mapPersons.put(p.getFirstName(), listPersons);
} else {
mapPersons.get(p.getFirstName()).add(p);
}
if (mapPersons.get(p.getLastName()) == null) {
listPersons = new ArrayList<Person>();
listPersons.add(p);
mapPersons.put(p.getLastName(), listPersons);
} else {
mapPersons.get(p.getLastName()).add(p);
}
}
I can't figure out how I can get either the firstname or the lastname as a key (not like in Group by multiple field names in java 8). Do I have to write my own collector?
You can use Stream.flatMap()
and Collectors.groupingBy()
with Collectors.mapping()
:
Map<String, List<Person>> result = personList.stream()
.flatMap(p -> Stream.of(p.getFirstName(), p.getLastName()).map(n -> new AbstractMap.SimpleEntry<>(n, p)))
.collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.mapping(Map.Entry::getValue, Collectors.toList())));
This uses flatMap
to expand all names (first and last) to their Person
object and collects it afterwards.
Alternatively using Java 9 or above you could use Collectors.flatMapping()
:
Map<String, List<Person>> result = personList.stream()
.collect(Collectors.flatMapping(
p -> Stream.of(p.getFirstName(), p.getLastName()).map(n -> new AbstractMap.SimpleEntry<>(n, p)),
Collectors.groupingBy(Map.Entry::getKey,
Collectors.mapping(Map.Entry::getValue, Collectors.toList()))));
But I don't think that this is more readable.
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