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 *
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 *     notice, this list of conditions and the following disclaimer.
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 *   - Redistributions in binary form must reproduce the above copyright
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 *
 *   - Neither the name of Oracle nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
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/*
 * This source code is provided to illustrate the usage of a given feature
 * or technique and has been deliberately simplified. Additional steps
 * required for a production-quality application, such as security checks,
 * input validation and proper error handling, might not be present in
 * this sample code.
 */
package stream;

import stream.data.Customer;
import stream.data.Vendor;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;
import static java.util.stream.Collectors.*;
import java.util.stream.Stream;

/**
 * A demo shows how does Stream's flatMap works. Demo shows tabulate stream
 * element to single element stream or empty stream
 *
 * @author tyan
 */
public class Tabulate extends DemoHelper {
    /**
     * Join vendor and customer that in same country. note not all vendor
     * has customer
     * @return  a list of customer-vendor pair which in same country
     */
    public static Map<Customer, List<Vendor>> sameCountryCustomerAndVendors() {
        Map<String, List<Vendor>> countryToVendors = vendors.stream().
                // group vendors by their country
                collect(groupingBy(Vendor::getCountry));
        //Find customer in countryToVendors with key as customer's country
        Function<Customer, List<Vendor>> vendorsInSameCustomerCountry
                = customer-> countryToVendors.getOrDefault(
                        customer.getCountry(), new ArrayList<>());
        //Map customer to vendor who are same country as customer's
        return customers.stream().
                filter(c -> countryToVendors.containsKey(c.getCountry())).
                collect(toMap(Function.identity(), vendorsInSameCustomerCountry));
    }

    /**
     * flatMap can generate empty stream, sometimes it's helpful for ruling
     * out data
     * @return   a string set who is either customer and vendor both.
     */
    public static Set<String> bothCustomerAndVendor() {
        Set<String> vendorsName = vendors.stream().
                map(Vendor::getVendorName).
                collect(toSet());
        return customers.stream().
                //map customer to its name
                map(Customer::getCompanyName).
                //query vendor set with customer's name, if exist, create
                //a stream with single element customer name, if doesn't, create
                //an empty stream
                flatMap(c -> vendorsName.contains(c)
                        ? Stream.of(c) : Stream.empty()).
                //convert all match name into a set
                collect(Collectors.toSet());
    }
}