Use java stream to group by 2 keys on the same type

Question

Using java stream, how to create a Map from a List to index by 2 keys on the same class?

I give here a code Example, I would like the map "personByName" to get all person by firstName OR lastName, so I would like to get the 3 "steves": when it's their firstName or lastname. I don't know how to mix the 2 Collectors.groupingBy.

public static class Person {
    final String firstName;
    final String lastName;

    protected Person(String firstName, String lastName) {
        super();
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

}

@Test
public void testStream() {
    List<Person> persons = Arrays.asList(
            new Person("Bill", "Gates"),
            new Person("Bill", "Steve"),
            new Person("Steve", "Jobs"),
            new Person("Steve", "Wozniac"));

    Map<String, Set<Person>> personByFirstName = persons.stream().collect(Collectors.groupingBy(Person::getFirstName, Collectors.toSet()));
    Map<String, Set<Person>> personByLastName = persons.stream().collect(Collectors.groupingBy(Person::getLastName, Collectors.toSet()));

    Map<String, Set<Person>> personByName = persons.stream().collect(Collectors.groupingBy(Person::getLastName, Collectors.toSet()));// This is wrong, I want bot first and last name

    Assert.assertEquals("we should search by firstName AND lastName", 3, personByName.get("Steve").size()); // This fails

}

I found a workaround by looping on the 2 maps, but it is not stream-oriented.

Answer 1

You can do it like this:

Map<String, Set<Person>> personByName = persons.stream()
       .flatMap(p -> Stream.of(new SimpleEntry<>(p.getFirstName(), p),
                               new SimpleEntry<>(p.getLastName(), p)))
       .collect(Collectors.groupingBy(SimpleEntry::getKey,
                   Collectors.mapping(SimpleEntry::getValue, Collectors.toSet())));

Assuming you add a toString() method to the Person class, you can then see result using:

List<Person> persons = Arrays.asList(
        new Person("Bill", "Gates"),
        new Person("Bill", "Steve"),
        new Person("Steve", "Jobs"),
        new Person("Steve", "Wozniac"));

// code above here

personByName.entrySet().forEach(System.out::println);

Output

Steve=[Steve Wozniac, Bill Steve, Steve Jobs]
Jobs=[Steve Jobs]
Bill=[Bill Steve, Bill Gates]
Wozniac=[Steve Wozniac]
Gates=[Bill Gates]