A couple of tabulate/Tabulators.groupBy examples (original) (raw)

Brian Goetz brian.goetz at oracle.com
Wed Dec 26 10:49:26 PST 2012

A simple example.

Before: Map<String, Collection> multimap = this.company.getCustomers().stream().groupBy(Customer::getCity); After: Map<String, Collection> multimap = this.company.getCustomers() .stream() .tabulate(Tabulators.<Customer, String>groupBy(Customer::getCity));

I would think the explicit arguments in this case could be avoided? Usually we've only had problems with inference when there are constructor refs (ArrayList::new). In any case, there's work going on to improve inference here.

With properly working inference and static import we'd get:

         this.company.getCustomers()
             .stream()
             .tabulate(groupBy(Customer::getCity));

Where the Tabulators stuff really shines is the ability to compose these things. The old groupBy/reduceBy did a decent job at what it did well, but fell off a cliff when you asked it to do much more.

A more complex example.

This is kind of a weird example, since the synthetic key is a number.

Another way to accomplish this, that might be more in line with real business usage, is by the equivalent of the old "mapped":

Map<Customer, Double> largestLineItemByByCustomer = customers.stream().tabulate(mappedTo(c -> c.getOrders()...reduce()));

This creates a map from Customer to your max order value reduction. If you then want a "top ten customers by largest transaction", you could do:

largestLineItemByCustomer.entrySet() .stream() .sorted(Comparators.naturalOrderValues()) .limit(10) .map(Map.Entry::getKey) .forEach(...);

Before: Map<Double, Collection> multimap = this.company.getCustomers() .stream() .groupBy(customer -> customer.getOrders() .stream() .flatMap((Block<? super LineItem> sink, Order element) -> {element.getLineItems().forEach(sink);}) .map(LineItem::getValue) .reduce(0.0, (x, y) -> Math.max(x,y)));

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