Java lambda expression-Count the number of appearances

Question

So I have the following class and the ArrayList of objects:

class Ticket{
    String destination;
}

ArrayList<Ticket> tickets;

What I want is to get from my list a Map of String and Integer which contains the destinations from tickets and the number of occurences in my list for each destination. I think it would start like this, but I don't know how to continue:

Map<String,Integer> map=tickets.stream().filter(a->a.getDestination()).

Ousmane D. · Accepted Answer · 2018-01-20T12:21:33.020

2

You can use a combination of a groupingBy collector and the summingInt collector like this:

Map<String,Integer> map = 
  tickets.stream()
         .collect(Collectors.groupingBy(Ticket::getDestination, 
                   Collectors.summingInt(e -> 1)));

In this case, I've specifically used the summingInt collector due to the fact that the counting() collector returns a Long type and since you want the map values as Integer's then the summingInt collector is the most appropriate.

edited Jan 20 '18 at 12:21

answered Jan 20 '18 at 11:59

Ousmane D.

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To get `Map` type, this is the nice solution. – sanit Jan 20 '18 at 12:10
@Aominè you shoul explain why not `Collectors.couting` – Eugene Jan 20 '18 at 12:17

Salem · Answer 2 · 2018-01-20T12:00:07.570

2

Use the groupingBy collector.

tickets.stream().collect(Collectors.groupingBy(Ticket::getDestination,
        Collectors.counting()));

This will give you a Map<String, Long>.

If you want a map with Integer values, you can replace counting with a summingInt that simply returns 1 (which is what this answer does)

edited Jan 20 '18 at 12:00

answered Jan 20 '18 at 11:59

Salem

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Java lambda expression-Count the number of appearances

2 Answers2