How to build a new list containing all entries from existing list AND a copy of each entry with one field modified?

Question

I have a list of some Object called list. Using list.stream(), I need to create a new list of the same Object where the new list contains all of the original entries AND the new list contains a copy of each entry with one field modified. I know how to do this using list.stream() twice but I'd like to do it under 1 stream.

This is how I've accomplished the task using list.stream() twice

         newList = list.stream().collect(Collectors.toList());
         newList.addAll(list.stream().map(l -> {SomeObject a = new SomeObject(l);
                              a.setField1("New Value");
                              return a;
                              }).collect(Collectors.toList())
                        );

Answer 1

Use flatMap (assuming you don't mind the original and derived values being interleaved):

newList = list.stream()
    .flatMap(l -> {
      SomeObject a = new SomeObject(l);
      a.setField1("New Value");
      return Stream.of(l, a);
    })
    .collect(toList());

If it's just that you don't want to use stream() twice, you could avoid the first one using addAll ; and the unnecessary collect for the second:

newList = new ArrayList<>(list.size() * 2);
newList.addAll(list);
list.stream()
    .map(l -> {
        SomeObject a = new SomeObject(l);
        a.setField1("New Value");
        return a;
    })
    .forEach(newList::add);

Answer 2

If you want objct then processed object and so on, in your new list then, you can try something like this:

List<String> list = List.of("a", "b", "c" );
        
List<String> answer = list.stream()
             .map(s -> List.of(s,s+"1"))
             .flatMap(List::stream).collect(Collectors.toList());
        
System.out.println(answer);

Output:

[a, a1, b, b1, c, c1]