How to filter nested loops using Java 8 streams and filters?

Question

I have two simple POJOs:

public class Parent {
    String name;
    List<Child> children = new ArrayList<>();
    void addChild(Integer age) { children.add(new Child(age)); }
}

public class Child {
    Integer age;
}

Then I generate some data, I add for parent 100 children:

List<Parent> parentList = new ArrayList<>();

Parent first = new Parent("first");
for (int i = 0; i < 100; ++i) { first.addChild(i); }
parentList.add(first);

My goal is to remove all children younger than ten:

parentList.stream().forEach(parent -> {
    parent.setChildren(getChildrenOlderThan(parent));
});

List<Child> getChildrenOlderThan10(Parent parent) {
    return parent.getChildren().stream()
        .filter(child -> child.getAge() > 10)
        .collect(Collectors.toList());
}

Finally I have list children older than ten. But I do not want to write a separate method getChildrenOlderThan.

How to get all parents with list chilrden older than ten using only the single stream?

Anatoly Shamov · Accepted Answer · 2017-09-26 08:09:47Z

6

There is nice method Collection.removeIf(Predicate<? super E> filter) method.

For example:

class Example {

    public static void main(String[] args){

        List<Parent> parentList = new ArrayList<>();

        // init parents with children

        parentList.forEach(parent -> 
             parent.getChildren().removeIf(Example::childOlderThan10)
        );

    }

    static boolean childOlderThan10(Child child) {
        return child != null && child.getAge() > 10;
    }
}

See more: Method references, Collection.stream().forEach() vs Collection.forEach()?

edited Sep 26, 2017 at 8:09

Anatoly Shamov

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answered Dec 23, 2016 at 9:55

Andrii Abramov

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2 Comments

Wim Deblauwe Over a year ago

Make sure parent.getChildren() returns a copy of the list in parent or the list of children inside your parent object will have changed.

Andrii Abramov Over a year ago

@WimDeblauwe If parent.getChildren() returns a copy than this cope won't do anything.

Harmlezz · Accepted Answer · 2016-12-23 10:01:28Z

1

You may inline the method getChildreanOlderThan like this:

parentList.stream().forEach(parent -> {
    parent.setChilds(parent.getChildren().stream()
            .filter(child -> child.getAge() > 10)
            .collect(Collectors.toList()));
    });

answered Dec 23, 2016 at 10:01

Harmlezz

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3 Comments

Andrii Abramov Over a year ago

Sure, it will work. But some suggestions: stream() on parentList is useless here, This kind of filtering is harder to read and maintain.

Harmlezz Over a year ago

@Andrii I as well like your solution more than mine. But could you explain why the stream() on parentList is useless? Does it not fulfill the goal? Do I have a bug in the implementation?

Andrii Abramov Over a year ago

See please: stackoverflow.com/questions/23218874/…

Procrastinator · Accepted Answer · 2024-01-17 06:43:01Z

0

You can get the list of child > 10 using Flatmap

List < Child > childList = parentList.stream().flatMap(par - > par.getChildren().stream())
    .filter(child - > child.age > 10).collect(Collectors.toList());

edited Jan 17, 2024 at 6:43

Procrastinator

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answered Jan 15, 2024 at 8:41

aashish pandey

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