I'm not convinced by this particular usage of Optional...

FindPricePossiblyReturningNull.java

    public CommandResponse onBarcode(String barcode) {
        Price price = catalog.findPrice(barcode);
        return price == null
                ? new ProductNotFoundMessage(barcode)
                : new ProductFoundMessage(price);
    }

FindPriceReturningOptionalPrice.java

    public CommandResponse onBarcode(String barcode) {
        return catalog.findPrice(barcode)
                .<CommandResponse> flatMap(price -> Optional.of(new ProductFoundMessage(price)))
                .orElse(new ProductNotFoundMessage(barcode));
    }

FlatMap usage looks a bit weird. Perhaps this is what you're looking for?

    public CommandResponse onBarcode(String barcode) {
        return catalog.findPrice(barcode)
                .map(ProductFoundMessage::new)
                .orElseGet(() -> new ProductNotFoundMessage(barcode));
    }

You can use map instead of flatMap:

    public CommandResponse onBarcode(String barcode) {
        return catalog.findPrice(barcode)
                .<CommandResponse> map(ProductFoundMessage::new)
                .orElse(new ProductNotFoundMessage(barcode));
    }

And if you create a static factory method for ProductFoundMessage that returns an instance of CommandResponse you can get rid of the ugly type specifier.

Thanks, @tumbarumba. Almost. On Java 8 update 112 I have to explicitly specify the type CommandResponse, otherwise Java infers the wrong type for the "or else" branch.

    public CommandResponse onBarcode(String barcode) {
        return catalog.findPrice(barcode)
                .<CommandResponse> map(ProductFoundMessage::new)
                .orElseGet(() -> new ProductNotFoundMessage(barcode));
    }

I didn't understand this when I first tried to write the code, so that might explain how I ended up going down the flatMap() road.

@nicolaiparlog Even better! Thanks. Yes: I might consider a factory method, although that would annoy me, since, in a sense, that's a big part of the point of this class. :)

If I didn't miss anything, it seems that catalog#findPrice returns Optional?
I prefer to not have Optional as a return type (nor as a parameter for that matter) for (at least) these reasons:

• have to change or wrap existing APIs
• the Optional instance can also be null (the Java language enables it ;)) so not really solving the problem

I prefer to use Optional.ofNullable.