Java 8 List into Map

Question

I want to translate a List of objects into a Map using Java 8\'s streams and lambdas.

This is how I would write it in Java 7 and below.

private Map&l         


        

Answer 1

If you don't mind using 3rd party libraries, AOL's cyclops-react lib (disclosure I am a contributor) has extensions for all JDK Collection types, including List and Map.

ListX<Choices> choices;
Map<String, Choice> map = choices.toMap(c-> c.getName(),c->c);

Answer 2

You can create a Stream of the indices using an IntStream and then convert them to a Map :

Map<Integer,Item> map = 
IntStream.range(0,items.size())
         .boxed()
         .collect(Collectors.toMap (i -> i, i -> items.get(i)));

Answer 3

Based on Collectors documentation it's as simple as:

Map<String, Choice> result =
    choices.stream().collect(Collectors.toMap(Choice::getName,
                                              Function.identity()));

Answer 4

This can be done in 2 ways. Let person be the class we are going to use to demonstrate it.

public class Person {

    private String name;
    private int age;

    public String getAge() {
        return age;
    }
}

Let persons be the list of Persons to be converted to the map

1.Using Simple foreach and a Lambda Expression on the List

Map<Integer,List<Person>> mapPersons = new HashMap<>();
persons.forEach(p->mapPersons.put(p.getAge(),p));

2.Using Collectors on Stream defined on the given List.

 Map<Integer,List<Person>> mapPersons = 
           persons.stream().collect(Collectors.groupingBy(Person::getAge));