How to calculate mask using java stream

You can map your entry to the calculated value and then apply the or within a reduce operator:

map.entrySet().stream()
       .filter(entry -> entry.getValue() && entry.getKey().getValue() > 0)
       .mapToInt(entry -> (1 << (entry.getKey().getValue() - 1)))
       .reduce(0, (r, i) -> r | i)

Edit: Added 0 as identity element to the reduce operation to have a default value of 0 if the map is empty.

Edit2: As suggested in the comments I reversed the filter order to avoid unnecessary method calls

Edit3: As suggested in the comments mapToInt is now used

You can try this , but as Aniket said above here mask is mutating not a good idea to use Strem.

map.entrySet().filter( entry -> entry.getKey().getValue() > 0 && entry.getValue()).forEach(k->{mask=mask | (1 << (k.getKey().getValue() - 1))});

Here is yet another alternative.

int mask = map.entrySet().stream().filter(Entry::getValue)
            .mapToInt(e -> e.getKey().getValue()).filter(a -> a > 0)
            .reduce(0, (a, b) -> a | (1<<(b-1)));