1 /*
   2  * Copyright (c) 2013 Oracle and/or its affiliates. All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions
   6  * are met:
   7  *
   8  *   - Redistributions of source code must retain the above copyright
   9  *     notice, this list of conditions and the following disclaimer.
  10  *
  11  *   - Redistributions in binary form must reproduce the above copyright
  12  *     notice, this list of conditions and the following disclaimer in the
  13  *     documentation and/or other materials provided with the distribution.
  14  *
  15  *   - Neither the name of Oracle nor the names of its
  16  *     contributors may be used to endorse or promote products derived
  17  *     from this software without specific prior written permission.
  18  *
  19  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
  20  * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
  21  * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  22  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
  23  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  24  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  25  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  26  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
  27  * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
  28  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  29  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  30  */
  31 
  32 /*
  33  * This source code is provided to illustrate the usage of a given feature
  34  * or technique and has been deliberately simplified. Additional steps
  35  * required for a production-quality application, such as security checks,
  36  * input validation and proper error handling, might not be present in
  37  * this sample code.
  38  */
  39 package stream;
  40 
  41 import stream.data.Customer;
  42 import stream.data.Product;
  43 import java.util.Collection;
  44 import java.util.Map;
  45 import java.util.Optional;
  46 import java.util.function.Function;
  47 import static java.util.stream.Collectors.*;
  48 import static java.util.Comparator.*;
  49 import java.util.stream.Stream;
  50 
  51 /**
  52  * This program is demo how to use the conversion operator like toArray,
  53  * collect(Collectors.toList()), collect(Collectors.toCollection())
  54  *
  55  * @author tyan
  56  */
  57 public class Conversion extends DemoHelper {
  58 
  59     /**
  60      * Convert products which is most expensive in the same category into an
  61      * array
  62      *
  63      * @param func Function convert Product to T, T could be any type that
  64      * Product can convert, like price
  65      * @return product array which is in every category, value of T is highest
  66      */
  67 
  68     public static <T extends Comparable<T>> Product[] mostTProductsByCategory(
  69             Function<Product, T> func) {
  70         //Grouping product by category, only select maximal product by comparing
  71         //on func
  72         Map<String, Optional<Product>> categoryToProduct = products.stream().
  73                 collect(groupingBy(Product::getCategory,
  74                                 maxBy(comparing(func))));
  75 
  76         return categoryToProduct.values().stream().
  77                 //get Product object
  78                 map(Optional::get).
  79                 //converting stream to an array
  80                 toArray(Product[]::new);
  81     }
  82 
  83     /**
  84      * Convert customers to a distinct countries list
  85      *
  86      * @param ordered true keep original countries list order false not keep
  87      * original countries list order
  88      * @return a distinct ordered/unordered countries list for all of customers
  89      * by given ordered flag
  90      */
  91     public static Collection<String> countries(boolean ordered) {
  92         Stream<String> countries = customers.stream().map(Customer::getCountry);
  93         if (ordered) {
  94             /**
  95              * Use distinct() follow with collect(Collectors.toCollection) to
  96              * get unique countries linked list. Note this way can generate
  97              * target container
  98              */
  99             return countries.distinct().collect(toList());
 100         } else {
 101             /**
 102              * Use collect(Collectors.toSet) to get unique countries list. Note
 103              * the difference between this and first one is this way doesn't
 104              * guarantee the order
 105              */
 106             return countries.collect(toSet());
 107         }
 108     }
 109 }