Daily tip #18

Tip of the day #18

Use the groupingBy collector to create a Map<?, List<?>> from a List<?>.

What is it?

The groupingBy collector is a static method in the Collectors class that returns a Collector implementing a “group by” operation on input elements of type T, grouping elements according to a classification function, and returning the results in a Map.

Let’s see how it works:

Imagine that you have an Account class:

class Account {
   UUID id,
    Card card
}

class Card {
    UUID id,
    String number,
    String flag
}

Then you can group by card.getFlag() ( master, visa, others).

Map<String, List<Account>> accountMap = 
        accountList.stream().collect(Collectors.groupingBy(account -> account.getCard().getFlag()));

Easy right?

How to

Take a look in repo-tip-18 to see a real example of today’s tip.