Algoritmi di collector

modern-java/src/main/java/org/gym/fp/moderjava/StreamTest.java

@@ -1,11 +1,13 @@
 package org.gym.fp.moderjava;
 
 import java.util.*;
-import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 import java.util.stream.Stream;
 
 import static java.lang.System.out;
+import static java.util.Arrays.asList;
+import static java.util.Comparator.comparingInt;
+import static java.util.stream.Collectors.*;
 
 public class StreamTest {
 
@@ -14,13 +16,14 @@ public class StreamTest {
         doStreamMappingDemo();
         doStreamFindOrMatchingDemo();
         doStreamReducingDemo();
+        doStreamCollectingDemo();
     }
 
     private static void doStreamFilterDemo() {
         out.println("FILTER EXAMPLE");
         getDishes().stream()
                    .filter(Dish::isVegetarian)
-                   .collect(Collectors.toList())
+                   .collect(toList())
                    .forEach(out::println);
         out.println("----------------------------------------");
         out.println("DISTINCT EXAMPLE");
@@ -42,26 +45,26 @@ public class StreamTest {
     }
 
     private static void doStreamMappingDemo() {
-        List<String> words = Arrays.asList("Hello", "World");
+        List<String> words = asList("Hello", "World");
         List<String> uniqueCharacters = words.stream()
                                              .map(word -> word.split(""))
                                              .flatMap(Arrays::stream)
                                              .distinct()
-                                             .collect(Collectors.toList());
+                                             .collect(toList());
         out.println(uniqueCharacters);
         out.println("----------------------------------------");
         out.println("Permutations");
-        List<Integer> numbers1 = Arrays.asList(1, 2, 3);
+        List<Integer> numbers1 = asList(1, 2, 3);
-        List<Integer> numbers2 = Arrays.asList(3, 4);
+        List<Integer> numbers2 = asList(3, 4);
         List<int[]> pairs1 = numbers1.stream()
                                      .flatMap(i -> numbers2.stream().map(j -> new int[]{i, j}))
-                                     .collect(Collectors.toList());
+                                     .collect(toList());
         pairs1.forEach(pair -> out.println(Arrays.toString(pair)));
         out.println("----------------------------------------");
         List<int[]> pairs2 = numbers1.stream()
                                      .flatMap(i -> numbers2.stream().map(j -> new int[]{i, j}))
                                      .filter(pair -> (pair[0] + pair[1]) % 3 == 0)
-                                     .collect(Collectors.toList());
+                                     .collect(toList());
         pairs2.forEach(pair -> out.println(Arrays.toString(pair)));
         out.println("----------------------------------------");
     }
@@ -89,7 +92,7 @@ public class StreamTest {
         dish.ifPresent(out::println);
         out.println("----------------------------------------");
         out.println("FINDFIRST EXAMPLE");
-        List<Integer> someNumbers = Arrays.asList(1, 2, 3, 4, 5);
+        List<Integer> someNumbers = asList(1, 2, 3, 4, 5);
         someNumbers.stream()
                    .map(n -> n * n)
                    .filter(n -> n % 3 == 0)
@@ -118,7 +121,7 @@ public class StreamTest {
         Trader mario = new Trader("Mario", "Milan");
         Trader alan = new Trader("Alan", "Cambridge");
         Trader brian = new Trader("Brian", "Cambridge");
-        List<Transaction> transactions = Arrays.asList(
+        List<Transaction> transactions = asList(
                 new Transaction(brian, 2011, 300),
                 new Transaction(raoul, 2012, 1000),
                 new Transaction(raoul, 2011, 400),
@@ -130,14 +133,14 @@ public class StreamTest {
         transactions.stream()
                     .filter(t -> t.getYear() == 2011)
                     .sorted(Comparator.comparing(Transaction::getValue))
-                    .collect(Collectors.toList())
+                    .collect(toList())
                     .forEach(out::println);
         out.println("2)");
         transactions.stream()
                     .map(Transaction::getTrader)
                     .map(Trader::getCity)
                     .distinct()
-                    .collect(Collectors.toList())
+                    .collect(toList())
                     .forEach(out::println);
         out.println("3)");
         transactions.stream()
@@ -145,7 +148,7 @@ public class StreamTest {
                     .filter(trader -> "Cambridge".equalsIgnoreCase(trader.getCity()))
                     .distinct() // anche se sul testo non è specificato che dovevano essere diversi
                     .sorted(Comparator.comparing(Trader::getName))
-                    .collect(Collectors.toList())
+                    .collect(toList())
                     .forEach(out::println);
         out.println("4)");
         String result = transactions.stream()
@@ -191,8 +194,96 @@ public class StreamTest {
         out.println("----------------------------------------");
     }
 
+    private static void doStreamCollectingDemo() {
+        List<Dish> menu = getDishes();
+        long howManyDishes = menu.stream().count();
+        out.println(howManyDishes);
+        out.println("----------------------------------------");
+        Comparator<Dish> dishCaloriesComparator = comparingInt(Dish::getCalories);
+        menu.stream()
+            .collect(maxBy(dishCaloriesComparator))
+            .ifPresent(out::println);
+        out.println("----------------------------------------");
+        int totalCalories = menu.stream().collect(summingInt(Dish::getCalories));
+        out.println(totalCalories);
+        out.println("----------------------------------------");
+        double avgCalories = menu.stream().collect(averagingInt(Dish::getCalories));
+        out.println(avgCalories);
+        out.println("----------------------------------------");
+        IntSummaryStatistics menuStatistics = menu.stream().collect(summarizingInt(Dish::getCalories));
+        out.println(menuStatistics);
+        out.println("----------------------------------------");
+        String shortMenu = menu.stream()
+                               .map(Dish::getName)
+                               .collect(joining(", "));
+        out.println(shortMenu);
+        out.println("----------------------------------------");
+        Map<Dish.Type, List<Dish>> dishesByType = menu.stream().collect(groupingBy(Dish::getType));
+        out.println(dishesByType);
+        out.println("----------------------------------------");
+        Map<CaloricLevel, List<Dish>> dishesByCaloricLevel = menu.stream().collect(groupingBy(dish -> {
+            if (dish.getCalories() <= 400) return CaloricLevel.DIET;
+            else if (dish.getCalories() <= 700) return CaloricLevel.NORMAL;
+            else return CaloricLevel.FAT;
+        }));
+        out.println(dishesByCaloricLevel);
+        out.println("----------------------------------------");
+        Map<Dish.Type, List<Dish>> caloricDishesByType =
+                menu.stream()
+                    .collect(groupingBy(Dish::getType,
+                            filtering(dish -> dish.getCalories() > 500,
+                                    toList())));
+        out.println(caloricDishesByType);
+        out.println("----------------------------------------");
+        Map<Dish.Type, List<String>> dishNamesByType =
+                menu.stream()
+                    .collect(groupingBy(Dish::getType,
+                            mapping(Dish::getName, toList())));
+        out.println(dishNamesByType);
+        out.println("----------------------------------------");
+        Map<String, List<String>> dishTags = Map.of(
+                "pork", asList("greasy", "salty"),
+                "beef", asList("salty", "roasted"),
+                "chicken", asList("fried", "crisp"),
+                "french fries", asList("greasy", "fried"),
+                "rice", asList("light", "natural"),
+                "season fruit", asList("fresh", "natural"),
+                "pizza", asList("tasty", "salty"),
+                "prawns", asList("tasty", "roasted"),
+                "salmon", asList("delicious", "fresh")
+        );
+        Map<Dish.Type, Set<String>> dishNamesByType2 =
+                menu.stream()
+                    .collect(groupingBy(Dish::getType,
+                            flatMapping(dish -> {
+                                // FIXME: The example of book is not safety
+                                List<String> tags = dishTags.get(dish.getName());
+                                if (tags != null) {
+                                    return tags.stream();
+                                }
+                                return Stream.empty();
+                            }, toSet())));
+        out.println(dishNamesByType2);
+        out.println("----------------------------------------");
+        Map<Dish.Type, Map<CaloricLevel, List<Dish>>> dishesByTypeCaloricLevel =
+                menu.stream()
+                    .collect(groupingBy(Dish::getType, groupingBy(dish -> {
+                        if (dish.getCalories() <= 400)
+                            return CaloricLevel.DIET;
+                        else if (dish
+                                .getCalories() <= 700)
+                            return CaloricLevel.NORMAL;
+                        else
+                            return CaloricLevel.FAT;
+                    })));
+        out.println(dishesByTypeCaloricLevel);
+        out.println("----------------------------------------");
+    }
+
+    enum CaloricLevel {DIET, NORMAL, FAT}
+
     private static List<Dish> getDishes() {
-        List<Dish> dishes = Arrays.asList(
+        List<Dish> dishes = asList(
                 new Dish("pork", 300, Dish.Type.MEAT),
                 new Dish("salad", 50, Dish.Type.VEGETARIAN),
                 new Dish("chicken", 100, Dish.Type.MEAT),