Adding data into a list while streaming another list

Question

Let's say we have some entities, every entity has a list of searchable fields and a type. Is there a better (read more efficient way) to map those field in a list for every different type of entity.

Currently what I am doing is :

final Collection<IndexedField> indexedFields = new ArrayList<>();
for (String type : types) {
    final Class<? extends IndexedEntity> targetClass = indexedEntities.getClassByType(type);
    indexedFields.addAll(indexedEntities.getSearchFieldsFor(targetClass));
}

This works, but is there some better way to achieve this ? Maybe something with stream api.

In your question you say you want to map fields by type. In your code you are using one single list. Either you should use a Map<String, List<IndexedField>> (with the String key being the type), or I don't understand what you're trying to do. Can you make this clearer so we can help you ? — lbndev
– lbndev, Commented Sep 29, 2017 at 12:57

Eugene · Accepted Answer · 2017-09-29 12:59:18Z

4

If I understood correctly:

 types.stream()
     .map(indexedEntities::getClassByType)
     .flatmap(x -> indexedEntities.getSearchFieldsFor(x).stream())
     .collect(Collectors.toList());

answered Sep 29, 2017 at 12:59

Eugene

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1 Comment

mirzak Over a year ago

This is exactly what I looked for. Thank you.

Naman · Accepted Answer · 2017-09-29 12:55:39Z

0

You could shorten that to

types.stream().<Class<? extends IndexedEntity>>map(
            type -> indexedEntities.getClassByType(type)).<Collection<? extends IndexedField>>map(
            targetClass -> indexedEntities.getSearchFieldsFor(targetClass)).forEach(indexedFields::addAll);

answered Sep 29, 2017 at 12:55

Naman

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1 Comment

Anton Balaniuc Over a year ago

It is not recommended to use forEach to populate other collection. Is it no thread safe and is it better to use collect

Beno · Accepted Answer · 2017-09-29 21:16:25Z

0

You also can write by using only method references:

final Collection<IndexedField> indexedFields = types.stream()
                                       .map(indexedEntities::getClassByType)
                                       .map(indexedEntities::getSearchFieldsFor)
                                       .flatMap(Collection::stream)
                                       .collect(Collectors.toList());

answered Sep 29, 2017 at 21:16

Beno

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