Elegantly create map with object fields as key/value from object stream in Java 8

Question

I have the following class

class Person {
    public String name;
    public int age;
    public List<String> hobbies;

    Person(String name, int age, List<String> hobbies)
    {this.name = name; this.age = age; this.hobbies = hobbies;}
}

How do I create a Map of age to hobbies like Map<Integer, Set<String>>?

The Java 8 way I cooked up is:

Map<Integer, Set<String>> collect8 = persons.stream()
    .collect(
        toMap(
            p -> p.age,
            p -> p.hobbies.stream().collect(toSet()),
            (hobbies1, hobbies2) ->
                Stream.concat(hobbies1.stream(), hobbies2.stream()).collect(toSet())
        )
     );

Is there a more idiomatic way of doing this with Collectors.groupingBy() perhaps?

As a related question, I find the version without Java streams to be more readable.

Map<Integer, Set<String>> collect7 = new HashMap<>();

for(Person p: persons) {
    Set<String> hobbies = collect7.getOrDefault(p.age, new HashSet<>());
    hobbies.addAll(p.hobbies);
    collect7.put(p.age, hobbies);
}

Should we go with non streams code if it is easier to read; specially when the streamed version, as seen here, has no intermediate streams with transformations of the data but quickly end in a terminal operation?

Never use streams just because they're fancy - if the imperative version seems easier to read, stick with it! Streams are a tool, and as such they're useful for some problems, not all. — dimo414
– dimo414, Commented Mar 23, 2017 at 5:43

Andrey Koshelev · Accepted Answer · 2017-03-23 05:11:48Z

3

Look at similar example from Java 8 Collectors documentation:

Map<City, Set<String>> namesByCity
     = people.stream().collect(groupingBy(Person::getCity, TreeMap::new,                                           
           mapping(Person::getLastName, toSet())));

You could use the same approach here.

answered Mar 23, 2017 at 5:11

Andrey Koshelev

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Comments

Community · Accepted Answer · 2017-05-23 10:30:09Z

2

As you noted yourself: the Stream solution might not be as readable as your current non-Stream-solution. Solving your problem with groupingBy might not look as good as you might expect as you want to transform your List into a Set.

I constructed a solution with groupingBy, mapping and reducing, but that solution is not that easy to read and did even contain an error. You can read more about that in: Java 8 stream.collect( ... groupingBy ( ... mapping( ... reducing ))) reducing BinaryOperator-usage I really suggest to look up the answer Holger gave as it also contains a simpler solution using a custom Collector and a little Outlook to Java 9's flatMapping, which for me comes close to your non-Stream-solution.

But another solution using groupingBy I came up with and that actually works is the following:

Map<Integer, Set<String>> yourmap;
yourmap = personList.stream()
                    .flatMap(p -> p.hobbies.stream()
                                           .flatMap(hobby -> Stream.of(new SimpleEntry<>(p.age, hobby)))
                            )
                    .collect(Collectors.groupingBy(Entry::getKey,
                             Collectors.mapping(Entry::getValue, Collectors.toSet())));

for that you need the following imports:

import java.util.AbstractMap.SimpleEntry;
import java.util.Map.Entry;

Of course you can take also a Tuple or Pair or what you like the most.

But then again not better in any ways.

I would stay with your current non-Stream-solution. It is more readable and does what it should do.

edited May 23, 2017 at 10:30

CommunityBot

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answered Mar 23, 2017 at 10:16

Roland

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2 Comments

user2624119 Over a year ago

You could not use (strings, strings2) -> {strings.addAll(strings2); return strings;} because addAll() returns a boolean. You'd have to create a more elaborate lambda which actually returns a List.

Roland Over a year ago

Actually that isn't a problem as I returned one of the sets. But I mistakenly treated the operator as if it was a combiner, which it isn't. If you are interested in that problem too you can look up the question or jump directly to Holger's answer, which by the way also contains two good examples for your question.

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