Java 8 lambda specifies the map type and makes it immutable

I have the following code, generated using Lambdas for several months

Map<Integer,String> tempMap = new LinkedHashMap<>();

 EnumSet.allOf(Month.class).forEach(m -> {
        String formattedMonth = DateTimeFormatter.ofPattern("MMM").format(m);
        tempMap.put(m.getValue(),formattedMonth);
    });

 MONTHS_MAP = Collections.unmodifiableMap(tempMap);

I wonder if I can use Lambdas to do all this at once?

return EnumSet.allOf(Month.class).stream()
                .collect(Collectors.collectingAndThen(Collectors.toMap(
                        Month::getValue,m -> DateTimeFormatter.ofPattern("MMM").format(m)
                ),Collections::unmodifiableMap));

It doesn't work Where do I specify that I want to use LinkedHashMap?

Solution

You need to use the collectors toMap overload that accepts a Supplier

Collectors.toMap(Month::getValue,m -> DateTimeFormatter.ofPattern("MMM").format(m)),(v1,v2) -> // whatever,LinkedHashMap::new)