List of Array of objects to map in java 8

Question

I have a class Person with constructor

Person(Fruit... favoriteFruits) {}

And 3 objects (let's say)

Person A = new Person(Apple);
Person B = new Person(Banana, Papaya);
Person C = new Person(Pineapple, Orange);

and a method in class Person Fruit[] getFavouriteFruits(){}

Using streams, I am trying to convert this into Map

Expected Output: Map of ((Apple, A), (Banana, B), (Papaya, B), (Pineapple, C), (Orange, C))

Not sure how to flatten the Array of Fruits into a stream

Eran · Accepted Answer · 2020-01-22 11:39:17Z

2

I hope I don't have typos. The idea is to transform the List<Person> to a Stream of all the pairs of (Person,Fruit) and group them by Fruit.

Map<Fruit,List<Person>> map =
  List.of(A,B,C)
      .stream()
      .flatMap(p -> Arrays.stream(p.getFavouriteFruits())
                          .map(f -> new SimpleEntry<Person,Fruit>(p,f)))
      .collect(Collectors.groupingBy(Map.Entry::getValue,
                                     Collectors.mapping(Map.Entry::getKey,
                                                        Collectors.toList())));

SimpleEntry is java.util.AbstractMap.SimpleEntry.

edited Jan 22, 2020 at 11:39

answered Jan 22, 2020 at 11:32

Eran

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1 Comment

Naman Over a year ago

Or not swapping <Person,Fruit> to <Fruit, List<Person>, one can create entries such as new SimpleEntry<Fruit,Person>(f,p)` which might read easier.

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