Nested lists with streams in Java8

Question

I have a list of objects A. Each object A in this list contains list of object B and the object B contains list of Object C. The object C contains an attribute name that i want to use to filter using java 8.

how to write the code below in java 8 using streams to avoid nested loop :

C c1 = null;
String name = "name1"
for (A a: listOfAObjects) {
    for (B b: a.getList()) {
        for (C c: b.getPr()) {
            if (c.getName().equalsIgnoreCase(name)) {
                c1= c;
                break;
            }
        }
    }
}

Youcef LAIDANI · Accepted Answer · 2018-08-01 10:00:05Z

67

You can use two flatMap then a filter then you can pick the first one or if no result return null :

C c1 = listOfAObjects.stream()
        .flatMap(a -> a.getList().stream())
        .flatMap(b -> b.getPr().stream())
        .filter(c -> c.getName().equalsIgnoreCase(name))
        .findFirst()
        .orElse(null);

answered Aug 1, 2018 at 10:00

Youcef LAIDANI

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7 Comments

Eugene Over a year ago

flatMap is eager until java-10, unlike the for loop

Holger Over a year ago

@Eugene it still stops at the first matching A, only the processing of the sublists is not short-circuiting.

Eugene Over a year ago

@sk555 just search flatMap eager, run a few examples and you will see for yourself; but long story short: flatMap will traverse the entire source, your for loops will not

sk555 Over a year ago

@YCF_L i have tested it in simple example but id t doesn't work with the list i want to use i get always c1 null

Eugene Over a year ago

@sk555 well if you get a null it probably means that orElse was triggered, in which case it means that there isn't such an element in your list, simple as that.

|

Arnaud Denoyelle · Accepted Answer · 2018-08-01 10:03:14Z

18

You can do it with flatMap.

I made an example with Company which contains a list of Person :

public static void main(String[] args) {
    List<Company> companies = Arrays.asList(
            new Company(Arrays.asList(new Person("Jon Skeet"), new Person("Linus Torvalds"))),
            new Company(Arrays.asList(new Person("Dennis Ritchie"), new Person("Bjarne Stroustrup"))),
            new Company(Arrays.asList(new Person("James Gosling"), new Person("Patrick Naughton")))
    );

    List<String> persons = companies.stream()
            .flatMap(company -> company.getPersons().stream())
            .map(Person::getName)
            .collect(Collectors.toList());

    System.out.println(persons);
}

Output :

[Jon Skeet, Linus Torvalds, Dennis Ritchie, Bjarne Stroustrup, James Gosling, Patrick Naughton]

answered Aug 1, 2018 at 10:03

Arnaud Denoyelle

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2 Comments

GDC Over a year ago

This is the correct approach, however the result is different: at the end of all nested loops, you should be left with just the last "c" object containing "name" in all the nested lists.

Arnaud Denoyelle Over a year ago

@GDC I agree. To my understanding, the OP's question was "How to make one Stream from 2 nested collections", so I just answered that.

Eugene · Accepted Answer · 2018-08-01 10:00:38Z

7

 listOfObjectsA.stream()
               .flatMap(a -> a.getListOfObjectsB.stream())
               .flatMap(b -> b.getListOfObjectsC().stream())
               .filter(c -> name.equals(c.getName()))
               .findAny()
               .orElse(....)

answered Aug 1, 2018 at 10:00

Eugene

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2 Comments

sk555 Over a year ago

whatbis the diffrence with the one specified above ?

Eugene Over a year ago

@sk555 none really... the difference here is that I've used findAny instead of findFirst - under the current implementation for a sequential stream they will act the same; but a parallel one - you want findFirst probably - as it will get the first element in encounter order. The thing is - if you don't care which to take (as long as it matches your condition) - you probably want findAny

Olexander Yushko · Accepted Answer · 2019-07-27 19:17:29Z

I had same task but i had one nested class. And I had to filter the objects with filter in the nested collection. As a result, I had to get items that have matches in the collection.

for example:

public class RootElement {
    private String name;
    private List<String> nestedElements;

    //getters / setters and constructors
}

init collection with elements:

List<RootElement> elements = Arrays.asList(
                new RootElement("first", Arrays.asList("one", "two", "three")),
                new RootElement("second", Arrays.asList("four", "one", "two")));

filter example:

String filterParam = "four";
        List<RootElement> filtered = elements.stream()
                .flatMap(root -> root.getNestedElements()
                        .stream()
                        .filter(nested -> nested.equalsIgnoreCase(filterParam))
                        .map(filteredElement -> new RootElement(root.getName(), root.getNestedElement())))
                .collect(Collectors.toList());

Hope it will help someone.

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