How to get map from list of object using stream?

Question

I have class country:

class Country {
private String code;
private String name;
}

I need to do from list of Countries to Map with code and name. I tried to

Map<String, String> countryNames = countries.stream()
            .collect(Collectors.groupingBy(Country::getCode, Country::getName));

But it is not right. How to collect correct?

`countries.stream().collect(Collectors.toMap(Country::getCode, Country::getName));` — Hadi J, May 13 '20 at 08:54
Do you want to use [Collectors.toMap](https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/util/stream/Collectors.html#toMap(java.util.function.Function,java.util.function.Function)) — Rickard Närström, May 13 '20 at 08:54

marstran · Answer 1 · 2020-05-13T09:00:03.490

I assume you want code as the key and name as the value. In that case, you need to use Collectors.mapping as the downstream collector to Collectors.groupingBy. Like this:

Map<String, List<String>> countryNames = countries.stream()
        .collect(Collectors.groupingBy(Country::getCode, 
            Collectors.mapping(Country::getName, Collectors.toList())));

Note that this will return the values as a list of strings, as the names are grouped in a list (by Collectors.toList()) if there are multiple countries with the same code.

If you know that each code only appears once in the list, you can use Collectors.toMap instead.

Map<String, String> countryNames = countries.stream()
        .collect(Collectors.toMap(Country::getCode, Country::getName));

yep, i tried your first version, forget about second arguement and decided to go here — Krazim00da, May 13 '20 at 09:02

How to get map from list of object using stream?

1 Answers1