Create Map initialized using keys in given List [duplicate]

Question

I have a list List<String> entries I would like to create a HashMap<String, Deque<Instant>> map with keys being those from entries list.

I could do

for(String s: entries){map.put(s, new Deque<>()} however I'm looking for more elegant solution.

map = Stream.of(entries).collect(Collectors.toMap(x -> (String) x, new Deque<>()));

however I get casting error. Is that fixable, can I constuct a map from list of keys?

entries.stream().collect(Collectors.toMap(Function.identity(), x -> new ArrayDeque())) or Collectors.toMap(Function.identity(), x -> new LinkedList()) should be fine. @YCF_L yes, forgot the x-> — Aman
– Aman, Commented Oct 19, 2020 at 9:32

Youcef LAIDANI · Accepted Answer · 2020-10-19 09:13:58Z

3

I think you need this:

Map<String, Deque<Instant>> map = entries.stream()
        .collect(Collectors.toMap(x -> x, x -> new ArrayDeque<>()));

You can even replace x -> x by Function.identity():

.collect(Collectors.toMap(Function.identity(), x -> new ArrayDeque<>()));

answered Oct 19, 2020 at 9:13

Youcef LAIDANI

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1 Comment

That Function.identity() is a nice math reference, just not sure if programmers think it's a clear code. Thanks.