Convert two for loops to Java Stream?

Question

I have the following nested for loop that I want to convert to use a stream instead as I am currently learning to use streams, how can I do so?

I have added my current attempt below but it is currently incomplete.

Part part = getPart();
List<Machines> machines = new ArrayList<>();
List<String> identities = getMachineIdentities();
Set<MachinePart> machineParts = new HashSet<>();

    //create machines
    for (String identity : identities) {
               Machine machine = getMachine(identity);
               machines.add(machine);
           }

    //map to MachineParts
    for (Machine machines : machines) {
               MachinePart machinePart = MachinePartCreator.new(machines, part);
               machineParts.add(machinePart);
           }

Stream attempt:

Set<MachinePart > machineParts = identities.stream()
    .map(identity-> ??? ).collectors.collect(Collectors.toSet()));

Answer 1

Your first loop creates the input for the second loop. This can be achieved with two map() calls:

Set<MachinePart> machineParts =
    identities.stream()
              .map(id -> getMachine(id))
              .map(m -> MachinePartCreator.new(m, part))
              .collect(Collectors.toSet());

Or even one:

Set<MachinePart> machineParts =
    identities.stream()
              .map(id -> MachinePartCreator.new(getMachine(id),part))
              .collect(Collectors.toSet());

Of course, you can also write the original code with a single for loop and skip the intermediate List<Machines> machines.