Create a Map based on a Stream

Simple case without duplicate keys

Stream<String> characters = Stream.of("A", "B", "C");

Map<Integer, String> map = characters
            .collect(Collectors.toMap(element -> element.hashCode(), element -> element));
// map = {65=A, 66=B, 67=C}

To make things more declarative, we can use static method in Function interface - Function.identity(). We can replace this lambda element -> element with Function.identity().

Case where there might be duplicate keys

The javadoc for Collectors.toMap states:

If the mapped keys contains duplicates (according to Object.equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys may have duplicates, use toMap(Function, Function, BinaryOperator) instead.

Stream<String> characters = Stream.of("A", "B", "B", "C");

Map<Integer, String> map = characters
            .collect(Collectors.toMap(
                element -> element.hashCode(),
                element -> element,
                (existingVal, newVal) -> (existingVal + newVal)));

// map = {65=A, 66=BB, 67=C}

The BinaryOperator passed to Collectors.toMap(...) generates the value to be stored in the case of a collision. It can:

return the old value, so that the first value in the stream takes precedence,
return the new value, so that the last value in the stream takes precedence, or
combine the old and new values

Grouping by value

You can use Collectors.groupingBy when you need to perform the equivalent of a database cascaded “group by” operation. To illustrate, the following creates a map in which people’s names are mapped to surnames:

List<Person> people = Arrays.asList(
    new Person("Sam", "Rossi"),
    new Person("Sam", "Verdi"),
    new Person("John", "Bianchi"),
    new Person("John", "Rossi"),
    new Person("John", "Verdi")
);

Map<String, List<String>> map = people.stream()
        .collect(
                // function mapping input elements to keys
                Collectors.groupingBy(Person::getName, 
                // function mapping input elements to values,
                // how to store values
                Collectors.mapping(Person::getSurname, Collectors.toList()))
        );

// map = {John=[Bianchi, Rossi, Verdi], Sam=[Rossi, Verdi]}

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