Nested lists with streams in Java8

问题

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;
            }
        }
    }
}

回答1:

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);

回答2:

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]

回答3:

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

回答4:

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.

来源：https://stackoverflow.com/questions/51630352/nested-lists-with-streams-in-java8