How to apply Filtering on groupBy in java streams

Question

How do you group first and then apply filtering using Java streams?

Example: Consider this Employee class: I want to group by Departme

Answer 1

Use Map#putIfAbsent(K,V) to fill in the gaps after filtering

Map> map = list.stream()
              .filter(e->e.getSalary() > 2000)
              .collect(Collectors.groupingBy(Employee::getDepartment, HashMap::new, toList()));
list.forEach(e->map.putIfAbsent(e.getDepartment(), Collections.emptyList()));

Note: Since the map returned by groupingBy is not guaranteed to be mutable, you need to specify a Map Supplier to be sure (thanks to shmosel for pointing that out).

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Another (not recommended) solution is using toMap instead of groupingBy, which has the downside of creating a temporary list for every Employee. Also it looks a bit messy

Predicate filter = e -> e.salary > 2000;
Map> collect = list.stream().collect(
        Collectors.toMap(
            e-> e.department, 
            e-> new ArrayList(filter.test(e) ? Collections.singleton(e) : Collections.emptyList()) , 
            (l1, l2)-> {l1.addAll(l2); return l1;}
        )
);