You just have to refer your original list to be the one that the Collectors.toList() returns.
Here's a demo:
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class Reference {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
System.out.println(list);
// Just collect even numbers and start referring the new list as the original one.
list = list.stream()
.filter(n -> n % 2 == 0)
.collect(Collectors.toList());
System.out.println(list);
}
}
And here's how you can add the newly created elements to your original list in just one line.
List<Integer> list = ...;
// add even numbers from the list to the list again.
list.addAll(list.stream()
.filter(n -> n % 2 == 0)
.collect(Collectors.toList())
);
That's what this Functional Programming Paradigm provides.