Convert a List of objects to a Map using Streams

Question

I have a list of objects of class A:

List<A> list;
class A {
 String name;
 String lastname;
 //Getter and Setter methods
}

I want to convert this list to a map from name to a set of lastnames:

Map<String, Set<String>> map;

For example, for the following list:

John Archer, John Agate, Tom Keinanen, Tom Barren, Cindy King

The map would be:

John -> {Archer, Agate}, Tom -> {Keinanen, Barren}, Cindy -> {King}

I tried the following code, but it returns a map from name to objects of class A:

list.stream.collect(groupingBy(A::getFirstName, toSet()));

Answer 1

Map< String, Set<String>> map = list.stream()
 .collect(
 Collectors.groupingBy(
 A::getFirstName, Collectors.mapping(
 A::getLastName, Collectors.toSet())));

You were on the right track you need to use:

• Collectors.groupingBy to group by the firstName.

• And then use a downstream collector like Collectors.mappping as a second parameter of Collectors.groupingBy to map to the lastName .

• And then finally collect that in a Set<String> by invoking Collectors.toSet:

Answer 2

You never told the collector to extract last names.

I suppose you need something like

list.stream
 .collect(groupingBy(
 A::getFirstName, // The key is extracted.
 mapping( // Map the stream of grouped values.
 A::getLastName, // Extract last names.
 toSet() // Collect them into a set.
)));