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()));

Amardeep Bhowmick · Accepted Answer · 2019-09-18 21:47:59Z

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:

9000 · Accepted Answer · 2019-09-18 21:42:28Z

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. )));

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