Mixing
Group by a Collection attribute using Java Streams
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I have an object that contains a Collection of strings, let's say the languages that a person speaks.
public class Person
private String name;
private int age;
private List<String> languagesSpoken;
// ...
Now, creating some instances like this...
Person p1 = new Person("Bob", 21, Arrays.asList("English", "French", "German"));
Person p2 = new Person("Alice", 33, Arrays.asList("English", "Chinese", Spanish"));
Person p3 = new Person("Joe", 43, Arrays.asList("English", "Dutch", "Spanish", "German"));
//put them in list
List<Person> people = Arrays.asList(p1,p2,p3);
... what I want to have is a Map<String, List<Person>>, for every language listing the persons that speak the language:
["English" -> [Bob object, Alice object, Joe object],
"German" -> [Bob object, Joe object],
etc. ]
Of course this can be programmed easily in an imperative way, but how to do it the functional way with Java Streams? I have tried something like people.stream.collect(groupingBy(Person::getLanguagesSpoken)) but that of course gives me a Map<List<String>, List<Person>>. All the examples I could find, are using groupingBy on Primitives or Strings.
java java-8 java-stream
asked 24 mins ago
user3579358
6313
add a comment |Â
up vote
9
down vote
favorite
I have an object that contains a Collection of strings, let's say the languages that a person speaks.
public class Person
private String name;
private int age;
private List<String> languagesSpoken;
// ...
Now, creating some instances like this...
Person p1 = new Person("Bob", 21, Arrays.asList("English", "French", "German"));
Person p2 = new Person("Alice", 33, Arrays.asList("English", "Chinese", Spanish"));
Person p3 = new Person("Joe", 43, Arrays.asList("English", "Dutch", "Spanish", "German"));
//put them in list
List<Person> people = Arrays.asList(p1,p2,p3);
... what I want to have is a Map<String, List<Person>>, for every language listing the persons that speak the language:
["English" -> [Bob object, Alice object, Joe object],
"German" -> [Bob object, Joe object],
etc. ]
Of course this can be programmed easily in an imperative way, but how to do it the functional way with Java Streams? I have tried something like people.stream.collect(groupingBy(Person::getLanguagesSpoken)) but that of course gives me a Map<List<String>, List<Person>>. All the examples I could find, are using groupingBy on Primitives or Strings.
java java-8 java-stream
asked 24 mins ago
user3579358
6313
add a comment |Â
up vote
9
down vote
favorite
up vote
9
down vote
favorite
I have an object that contains a Collection of strings, let's say the languages that a person speaks.
public class Person
private String name;
private int age;
private List<String> languagesSpoken;
// ...
Now, creating some instances like this...
Person p1 = new Person("Bob", 21, Arrays.asList("English", "French", "German"));
Person p2 = new Person("Alice", 33, Arrays.asList("English", "Chinese", Spanish"));
Person p3 = new Person("Joe", 43, Arrays.asList("English", "Dutch", "Spanish", "German"));
//put them in list
List<Person> people = Arrays.asList(p1,p2,p3);
... what I want to have is a Map<String, List<Person>>, for every language listing the persons that speak the language:
["English" -> [Bob object, Alice object, Joe object],
"German" -> [Bob object, Joe object],
etc. ]
Of course this can be programmed easily in an imperative way, but how to do it the functional way with Java Streams? I have tried something like people.stream.collect(groupingBy(Person::getLanguagesSpoken)) but that of course gives me a Map<List<String>, List<Person>>. All the examples I could find, are using groupingBy on Primitives or Strings.
java java-8 java-stream
asked 24 mins ago
user3579358
6313
I have an object that contains a Collection of strings, let's say the languages that a person speaks.
public class Person
private String name;
private int age;
private List<String> languagesSpoken;
// ...
Now, creating some instances like this...
Person p1 = new Person("Bob", 21, Arrays.asList("English", "French", "German"));
Person p2 = new Person("Alice", 33, Arrays.asList("English", "Chinese", Spanish"));
Person p3 = new Person("Joe", 43, Arrays.asList("English", "Dutch", "Spanish", "German"));
//put them in list
List<Person> people = Arrays.asList(p1,p2,p3);
... what I want to have is a Map<String, List<Person>>, for every language listing the persons that speak the language:
["English" -> [Bob object, Alice object, Joe object],
"German" -> [Bob object, Joe object],
etc. ]
Of course this can be programmed easily in an imperative way, but how to do it the functional way with Java Streams? I have tried something like people.stream.collect(groupingBy(Person::getLanguagesSpoken)) but that of course gives me a Map<List<String>, List<Person>>. All the examples I could find, are using groupingBy on Primitives or Strings.
java java-8 java-stream
java java-8 java-stream
asked 24 mins ago
user3579358
6313
asked 24 mins ago
user3579358
6313
asked 24 mins ago
user3579358
6313
asked 24 mins ago
user3579358
6313
asked 24 mins ago
user3579358
6313
6313
add a comment |Â
add a comment |Â
1 Answer
1
active
oldest
votes
up vote
11
down vote
You can break the Person instances into pairs of language and Person (using flatMap) and then group them as required:
Map<String, List<Person>> langPersons =
people.stream()
.flatMap(p -> p.getLanguagesSpoken()
.stream()
.map(l -> new SimpleEntry<>(l,p)))
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.mapping(Map.Entry::getValue,
Collectors.toList())));
answered 19 mins ago
Eran
268k35416503
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
11
down vote
You can break the Person instances into pairs of language and Person (using flatMap) and then group them as required:
Map<String, List<Person>> langPersons =
people.stream()
.flatMap(p -> p.getLanguagesSpoken()
.stream()
.map(l -> new SimpleEntry<>(l,p)))
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.mapping(Map.Entry::getValue,
Collectors.toList())));
answered 19 mins ago
Eran
268k35416503
add a comment |Â
up vote
11
down vote
You can break the Person instances into pairs of language and Person (using flatMap) and then group them as required:
Map<String, List<Person>> langPersons =
people.stream()
.flatMap(p -> p.getLanguagesSpoken()
.stream()
.map(l -> new SimpleEntry<>(l,p)))
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.mapping(Map.Entry::getValue,
Collectors.toList())));
answered 19 mins ago
Eran
268k35416503
add a comment |Â
up vote
11
down vote
up vote
11
down vote
You can break the Person instances into pairs of language and Person (using flatMap) and then group them as required:
Map<String, List<Person>> langPersons =
people.stream()
.flatMap(p -> p.getLanguagesSpoken()
.stream()
.map(l -> new SimpleEntry<>(l,p)))
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.mapping(Map.Entry::getValue,
Collectors.toList())));
answered 19 mins ago
Eran
268k35416503
You can break the Person instances into pairs of language and Person (using flatMap) and then group them as required:
Map<String, List<Person>> langPersons =
people.stream()
.flatMap(p -> p.getLanguagesSpoken()
.stream()
.map(l -> new SimpleEntry<>(l,p)))
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.mapping(Map.Entry::getValue,
Collectors.toList())));
answered 19 mins ago
Eran
268k35416503
edited 16 mins ago
answered 19 mins ago
Eran
268k35416503
answered 19 mins ago
Eran
268k35416503
answered 19 mins ago
Eran
268k35416503
268k35416503
add a comment |Â
add a comment |Â
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