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  1. import java.util.*;
  2. import java.util.stream.*;
  3. import static java.util.stream.Collectors.*;
  4.  
  5. class Person {
  6. 	final String name, surname;
  7. 	public Person(String n, String s){
  8. 		this.name = n;
  9. 		this.surname = s;
  10. 	}
  11. 	public String getName(){ return name; }
  12. 	public String getSurname(){ return surname; }
  13. }
  14.  
  15. class Ideone {
  16.  
  17. 	public static void main(String[] args) {
  18. 	    List<Person> people = Arrays.asList(
  19. 		    new Person("Sam", "Rossi"),
  20. 		    new Person("Sam", "Verdi"),
  21. 		    new Person("John", "Bianchi"),
  22. 		    new Person("John", "Rossi"),
  23. 		    new Person("John", "Verdi")
  24.     	);
  25.  
  26. 	    Map<String, List<String>> map = people.stream()
  27.             .collect(
  28.                 // function mapping input elements to keys
  29.                 groupingBy(Person::getName, 
  30.                 // function mapping input elements to values,
  31.                 // how to store values
  32.                 mapping(Person::getSurname, toList()))
  33.             );
  34.         System.out.println(map);
  35. 	}
  36. }
Success #stdin #stdout 0.09s 711680KB
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Standard input is empty
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{John=[Bianchi, Rossi, Verdi], Sam=[Rossi, Verdi]}
https://ideone.com/2Ar9IA
language:
Java (HotSpot 12)
created:
7 years ago
visibility:
public

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