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?

• Robin Topper: 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.
• devang: Your method getIds does not have a return type.

Answer 1

Use flatMap:

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

• CraigR8806: Can you explain the difference between flatMap and map
• Robin Topper: @CraigR8806 Extensive explanations here
• gabriel: 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: @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: 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.

Answer 2

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());

Answer 3

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());