Java List to Map convertion

Question

I\'d like to convert a Map from List in java 8 something like this:

Map

        

Answer 1

As already noted, the parameters to Collectors.toMap have to be functions, so you have to change 0 to name -> 0 (you can use any other parameter name instead of name).

Note, however, that this will fail if there are duplicates in names, as that will result in duplicate keys in the resulting map. To fix this, you could pipe the stream through Stream.distinct first:

Map namesMap = names.stream().distinct()
                                     .collect(Collectors.toMap(s -> s, s -> 0));

Or don't initialize those defaults at all, and use getOrDefault or computeIfAbsent instead:

int x = namesMap.getOrDefault(someName, 0);
int y = namesMap.computeIfAbsent(someName, s -> 0);

Or, if you want to get the counts of the names, you can just use Collectors.groupingBy and Collectors.counting:

Map counts = names.stream().collect(
        Collectors.groupingBy(s -> s, Collectors.counting()));