Java Language Streams Create a Map based on a Stream
Example
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)), anIllegalStateExceptionis thrown when the collection operation is performed. If the mapped keys may have duplicates, usetoMap(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]}
