Java List<String> to Map<String, Integer> convertion

Question

I'd like to convert a Map <String, Integer> from List<String> in java 8 something like this:

Map<String, Integer> namesMap = names.stream().collect(Collectors.toMap(name -> name, 0));

because I have a list of Strings, and I'd like to to create a Map, where the key is the string of the list, and the value is Integer (a zero).

My goal is, to counting the elements of String list (later in my code).

I know it is easy to convert it, in the "old" way;

Map<String,Integer> namesMap = new HasMap<>();
for(String str: names) {
  map1.put(str, 0);
}

but I'm wondering there is a Java 8 solution as well.

Just change 0 to name -> 0: Map<String, Integer> namesMap = names.stream().collect(Collectors.toMap(name -> name, name -> 0)); but this will fail if you have duplicates. If you want to count occurrences, do it right in the first place: Map<String, Long> namesMap = names.stream().collect(Collectors.groupingBy(name -> name, Collectors.counting())); Instead of name -> name, you can also use Function.identity(). — Holger
– Holger, Commented Feb 27, 2017 at 13:25

tobias_k · Accepted Answer · 2017-02-27 13:45:43Z

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<String, Integer> 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<String, Long> counts = names.stream().collect(
        Collectors.groupingBy(s -> s, Collectors.counting()));

Mureinik · Accepted Answer · 2017-02-27 13:39:59Z

9

the toMap collector receives two mappers - one for the key and one for the value. The key mapper could just return the value from the list (i.e., either name -> name like you currently have, or just use the builtin Function.Identity). The value mapper should just return the hard-coded value of 0 for any key:

namesMap = 
    names.stream().collect(Collectors.toMap(Function.identity(), name -> 0));

edited Feb 27, 2017 at 13:39

answered Feb 27, 2017 at 13:28

Mureinik

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