Java8 transform a [List<Object>, String] in a Map<Object, String>

Question

I have a List<Computer>. Every Computer has a list of CPU and a hostname. So,suppose I have:

List<Computer> computers

I can call

List<CPU> CPUs = computer.getCPUs();

and I can call

String hostname = computer.getHostName();

What I want to do is, using Streams, obtain a Map that contains as key the CPU and as String the hostname. Same CPU inside the same Computers will replicate the hostname.

How can I do that?

Pre Java8 code would be this:

public Map<CPU, String> getMapping(List<Computer> computers) {
    Map<CPU, String> result = new HashMap<>();
    for (Computer computer : computers) {
        for (CPU cpu : computer.getCPUs()) {
            result.put(cpu, computer.getHostname());
        }
    }

    return result;
}

Answer 1

If your CPU class has a back-reference to it's Computer instance, then you can do this easily. First stream over all the computers, and flat-map with getCPUs, this will give you a Stream<CPU> of all CPUs. Then you can use Collectors.toMap to collect into a Map<CPU, String> using Function.identity for the key and a lambda extracting first the Computer and then the hostname from the CPU for the value. In code:

computers.stream()
    .flatMap(computer -> computer.getCPUs().stream())
    .collect(Collectors.toMap(Function.identity(), cpu -> cpu.getComputer().getHostname()));