Collect stream with grouping, counting and filtering operations

Question

I\'m trying to collect stream throwing away rarely used items like in this example:

import java.util.*;
import java.util.function.Function;
import static java.ut         


        

Answer 1

There is no way around creating a Map, unless you want accept a very high CPU complexity.

However, you can remove the second collect operation:

Map map = allWords.stream()
    .collect(groupingBy(Function.identity(), HashMap::new, counting()));
map.values().removeIf(l -> l<=2);
Set commonlyUsed=map.keySet();

Note that in Java 8, HashSet still wraps a HashMap, so using the keySet() of a HashMap, when you want a Set in the first place, doesn’t waste space given the current implementation.

Of course, you can hide the post-processing in a Collector if that feels more “streamy”:

Set commonlyUsed = allWords.stream()
    .collect(collectingAndThen(
        groupingBy(Function.identity(), HashMap::new, counting()),
        map-> { map.values().removeIf(l -> l<=2); return map.keySet(); }));