Collecting lists from an object list using Java 8 Stream API

Question

I have a class like this

public class Example {
    private List<Integer> ids;

    public getIds() {
        return this.ids; 
    }
}

If I have a list of objects of this class like this

List<Example> examples;

How would I be able to map the id lists of all examples into one list? I tried like this:

List<Integer> concat = examples.stream().map(Example::getIds).collect(Collectors.toList());

but getting an error with Collectors.toList()

What would be the correct way to achive this with Java 8 stream api?

Possible duplicate of What's the difference between map and flatMap methods in Java 8?. And even it is not a perfect duplicate, the answers there are useful. — Robin Topper
– Robin Topper, Commented Apr 28, 2017 at 14:05

Andy Turner · Accepted Answer · 2017-04-28 14:04:19Z

58

Use flatMap:

List<Integer> concat = examples.stream()
    .flatMap(e -> e.getIds().stream())
    .collect(Collectors.toList());

answered Apr 28, 2017 at 14:04

Andy Turner

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8 Comments

CraigR8806 Over a year ago

Can you explain the difference between flatMap and map

Robin Topper Over a year ago

@CraigR8806 Extensive explanations here

gabriel Over a year ago

What would happen if e.getIds() returns an empty list or null? How could a test for empty list or null be included into the statement?

Andy Turner Over a year ago

@gabriel "empty list" well, there's nothing in the list, but it would work just the same. "null" a NullPointerException would be thrown. How would you want to handle null; and why would you want to (specially) handle an empty list?

Andy Turner Over a year ago

Well you could do something like { List<Integer> ids = e.getIds(); if (ids == null) ids = Collections.emptyList(); return ids.stream(); }. But consider that the null might itself be a bug; so it might be better to fix it rather than work around it.

|

holi-java · Accepted Answer · 2017-04-28 15:13:08Z

10

Another solution by using method reference expression instead of lambda expression:

List<Integer> concat = examples.stream()
                               .map(Example::getIds)
                               .flatMap(List::stream)
                               .collect(Collectors.toList());

answered Apr 28, 2017 at 15:13

holi-java

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Comments

bric3 · Accepted Answer · 2024-01-23 09:04:48Z

2

Here is an example of how to use Stream and Map to collect the object list:

List<Integer> integerList = productsList.stream().map(x -> x.getId()).collect(Collectors.toList());

edited Jan 23, 2024 at 9:04

bric3

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answered Jan 20, 2024 at 14:22

chandra shekhar Sharma

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