Java8 : How to filter a map of List value via stream

Question

I'm still a beginner of Java 8. I'm getting stuck on filtering a Map of List values. Here is my code

public class MapFilterList {
    private static Map<String, List<Person>> personMap = new HashMap<>();

    public static void main(String[] args) {
        Person p1 = new Person("John", 22);
        Person p2 = new Person("Smith", 45);
        Person p3 = new Person("Sarah", 27);

        List<Person> group1 = new ArrayList<>();
        group1.add(p1);
        group1.add(p2);

        List<Person> group2 = new ArrayList<>();
        group2.add(p2);
        group2.add(p3);

        List<Person> group3 = new ArrayList<>();
        group3.add(p3);
        group3.add(p1);

        personMap.put("group1", group1);
        personMap.put("group2", group2);
        personMap.put("group3", group3);

        doFilter("group1").forEach(person -> {
            System.out.println(person.getName() + " -- " + person.getAge());
        });

    }

    public static List<Person> doFilter(String groupName) {
        return personMap.entrySet().stream().filter(key -> key.equals(groupName)).map(map -> map.getValue()).collect(Collectors.toList());
    }
}

How can I make to correct doFilter method because the error show me cannot convert from List<List<Person>> to List<Person>.

What you want to achieve? Why not just personMap.get(groupName)? As I understand, doFilter should do exactly this job. — Zefick
– Zefick, Commented Feb 20, 2017 at 5:57
@Zefick I was praticing Java8 lambda expression and Stream API. This program is just an example. I'd like to filter a map of list values via it's stream. — Cataclysm
– Cataclysm, Commented Feb 20, 2017 at 5:59
What do you mean by "correct the doFilter method"? Is something not working? Is there an error? What do you expect to happen? — jannikb
– jannikb, Commented Feb 20, 2017 at 6:04
Even when you are practicing Java 8, you should try to solve tasks that exhibit a benefit of the Stream API over the older APIs, instead of tasks that are simpler without Streams. The entire operation can be expressed as either personMap.get(groupName) or, if collecting into a new list really is required, still as simple as new ArrayList<>(personMap.get(groupName))… — Holger
– Holger, Commented Feb 20, 2017 at 10:29
But possible solutions depend on the actual problem. Even if you weren’t looking for an exact match of the key (which is what get is for), using a predicate that is known to have exactly one match implies that you could use findFirst or similar to get the particular List. Collecting into a new list is only necessary, if you know that you will have to merge multiple lists into one. — Holger
– Holger, Commented Feb 20, 2017 at 12:02

Zefick · Accepted Answer · 2017-02-20 06:14:10Z

12

If I understood correctly you need the following code:

public static List<Person> doFilter(String groupName) {
    return personMap.entrySet().stream()
            .filter(entry -> entry.getKey().equals(groupName))
            .map(entry -> entry.getValue())
            .flatMap(List::stream)
            // as an option to replace the previous two
            // .flatMap(entry -> entry.getValue().stream()) 
            .collect(Collectors.toList());
}

edited Feb 20, 2017 at 6:14

answered Feb 20, 2017 at 6:07

Zefick

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4 Comments

Cataclysm Over a year ago

Thank you. This fixed my error. can you explain me why you use flatMap method ?

Zefick Over a year ago

flatMap converts Stream<List<Person>> to Stream<Person>

Holger Over a year ago

You can fuse .map(entry -> entry.getValue()) .flatMap(List::stream) to a single .flatMap(entry -> entry.getValue().stream())

Cataclysm Over a year ago

@Holger Thank you for more shorter way.

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