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[add] add collecting examples of Stream API in java8 module.
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‎java8/README.md

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- Dish: 菜肴类,有一个静态属性的菜肴列表
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- StreamBasic: 指令式和函数式分别挑选低热量食物名
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- StreamOperation: 流操作(中间操作和终端操作)
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- Laziness: 流操作-中间操作
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- BuildingStreams: 构建流,从值序列,数组,文件来创建流。斐波拉切数列的几种生成方法
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使用流
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- Filtering: 筛选(谓词筛选：filter;筛选互异的元素：distinct;忽略头几个元素：limit;截短至指定长度：skip)
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- Mapping: 映射
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- Finding: 查找和匹配
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- Reducing: 归约
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- PuttingIntoPractice: 各种流操作的使用示例
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- NumericStreams: 数值流(原始类型特化,数值范围)
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用流收集数据
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- Summarizing: 归约和汇总
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- Reducing: 收集器的灵活性,以不同的方法执行相同的操作
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- Grouping: 分组
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- Partitioning: 分区
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- GroupingTransactions: 分别使用指令式和函数式进行分组
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- ToListCollector: 自定义 ToListCollector,以及进行自定义收集而不去实现 Collector
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- PartitionPrimeNumbers: 将数字按质数和非质数分区;Collector 详解
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- CollectorHarness: 比较收集器的性能
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‎java8/src/main/java/com/brianway/learning/java8/streamapi/BuildingStreams.java

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import java.util.stream.Stream;
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/**
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* 构建流:从值序列,数组,文件来创建流
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* 斐波拉切数列的几种生成方法
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*/
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public class BuildingStreams {
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public static void main(String... args) throws Exception {
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/**
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* 由值创建流
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*/
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// Stream.of
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Stream<String> stream = Stream.of("Java 8", "Lambdas", "In", "Action");
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stream.map(String::toUpperCase).forEach(System.out::println);
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// Stream.empty
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Stream<String> emptyStream = Stream.empty();
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/**
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* 由数组创建流
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*/
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// Arrays.stream
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int[] numbers = {2, 3, 5, 7, 11, 13};
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System.out.println(Arrays.stream(numbers).sum());
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/**
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* 由函数生成流：创建无限流
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*/
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// Stream.iterate
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Stream.iterate(0, n -> n + 2)
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.limit(10)
@@ -68,6 +78,9 @@ public int getAsInt() {
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};
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IntStream.generate(fib).limit(10).forEach(System.out::println);
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/**
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* 由文件生成流
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*/
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String path = BuildingStreams.class.getResource("/").getPath() + "/data-building.txt";
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long uniqueWords = Files.lines(Paths.get(path), Charset.defaultCharset())
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.flatMap(line -> Arrays.stream(line.split(" ")))

‎java8/src/main/java/com/brianway/learning/java8/streamapi/CollectorHarness.java renamed to ‎java8/src/main/java/com/brianway/learning/java8/streamapi/collect/CollectorHarness.java

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package com.brianway.learning.java8.streamapi;
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package com.brianway.learning.java8.streamapi.collect;
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import java.util.function.Consumer;
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‎java8/src/main/java/com/brianway/learning/java8/streamapi/collect/Grouping.java

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package com.brianway.learning.java8.streamapi.collect;
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import com.brianway.learning.java8.streamapi.Dish;
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import static com.brianway.learning.java8.streamapi.Dish.menu;
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import static java.util.stream.Collectors.collectingAndThen;
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import static java.util.stream.Collectors.counting;
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import static java.util.stream.Collectors.groupingBy;
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import static java.util.stream.Collectors.mapping;
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import static java.util.stream.Collectors.maxBy;
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import static java.util.stream.Collectors.summingInt;
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import static java.util.stream.Collectors.toSet;
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import java.util.Comparator;
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import java.util.List;
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import java.util.Map;
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import java.util.Optional;
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import java.util.Set;
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/**
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* 分组
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*/
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public class Grouping {
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private enum CaloricLevel {
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DIET,
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NORMAL,
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FAT
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}
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public static void main(String... args) {
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System.out.println("Dishes grouped by type: " + groupDishesByType());
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System.out.println("Dishes grouped by caloric level: " + groupDishesByCaloricLevel());
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System.out.println("Dishes grouped by type and caloric level: " + groupDishedByTypeAndCaloricLevel());
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System.out.println("Count dishes in groups: " + countDishesInGroups());
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System.out.println("Most caloric dishes by type: " + mostCaloricDishesByType());
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System.out.println("Most caloric dishes by type: " + mostCaloricDishesByTypeWithoutOprionals());
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System.out.println("Sum calories by type: " + sumCaloriesByType());
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System.out.println("Caloric levels by type: " + caloricLevelsByType());
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}
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private static CaloricLevel getCaloricLevel(Dish dish) {
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if (dish.getCalories() <= 400) {
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return CaloricLevel.DIET;
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} else if (dish.getCalories() <= 700) {
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return CaloricLevel.NORMAL;
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} else {
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return CaloricLevel.FAT;
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}
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}
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private static Map<Dish.Type, List<Dish>> groupDishesByType() {
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return menu.stream().collect(groupingBy(Dish::getType));
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}
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private static Map<CaloricLevel, List<Dish>> groupDishesByCaloricLevel() {
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return menu.stream().collect(
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groupingBy(Grouping::getCaloricLevel));
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}
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/**
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* 多级分组
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*/
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private static Map<Dish.Type, Map<CaloricLevel, List<Dish>>> groupDishedByTypeAndCaloricLevel() {
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return menu.stream().collect(
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groupingBy(Dish::getType,
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groupingBy(Grouping::getCaloricLevel)
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)
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);
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}
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/**
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* 按子数组收集数据
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*/
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private static Map<Dish.Type, Long> countDishesInGroups() {
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return menu.stream().collect(groupingBy(Dish::getType, counting()));
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}
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private static Map<Dish.Type, Optional<Dish>> mostCaloricDishesByType() {
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return menu.stream().collect(
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groupingBy(Dish::getType,
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maxBy(Comparator.comparingInt(Dish::getCalories)))
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);
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}
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/**
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* 按子数组收集数据,把收集器的结果转换为另一种类型
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*/
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private static Map<Dish.Type, Dish> mostCaloricDishesByTypeWithoutOprionals() {
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return menu.stream().collect(
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groupingBy(Dish::getType,
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collectingAndThen(
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maxBy(Comparator.comparingInt(Dish::getCalories)),
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Optional::get)));
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}
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/**
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* 与 groupingBy 联合使用的其他收集器例子
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*/
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private static Map<Dish.Type, Integer> sumCaloriesByType() {
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return menu.stream().collect(groupingBy(Dish::getType,
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summingInt(Dish::getCalories)));
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}
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private static Map<Dish.Type, Set<CaloricLevel>> caloricLevelsByType() {
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return menu.stream().collect(
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groupingBy(Dish::getType, mapping(
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Grouping::getCaloricLevel,
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toSet())));
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}
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}

‎java8/src/main/java/com/brianway/learning/java8/streamapi/GroupingTransactions.java renamed to ‎java8/src/main/java/com/brianway/learning/java8/streamapi/collect/GroupingTransactions.java

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package com.brianway.learning.java8.streamapi;
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package com.brianway.learning.java8.streamapi.collect;
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import static java.util.stream.Collectors.groupingBy;
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‎java8/src/main/java/com/brianway/learning/java8/streamapi/PartitionPrimeNumbers.java renamed to ‎java8/src/main/java/com/brianway/learning/java8/streamapi/collect/PartitionPrimeNumbers.java

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package com.brianway.learning.java8.streamapi;
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package com.brianway.learning.java8.streamapi.collect;
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import static java.util.stream.Collector.Characteristics.IDENTITY_FINISH;
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import static java.util.stream.Collectors.partitioningBy;
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.collect(partitioningBy(PartitionPrimeNumbers::isPrime));
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}
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/**
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* 判断一个数是不是质数
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*/
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public static boolean isPrime(int candidate) {
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return IntStream.rangeClosed(2, candidate - 1)
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.limit((long) Math.floor(Math.sqrt((double) candidate)) - 1)
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return IntStream.rangeClosed(2, n).boxed().collect(new PrimeNumbersCollector());
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}
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/**
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* 仅用质数作除数
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*
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* @param primes 部分结果
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* @param candidate 要判断的数
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* @return
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*/
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public static boolean isPrime(List<Integer> primes, Integer candidate) {
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double candidateRoot = Math.sqrt((double) candidate);
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//return primes.stream().filter(p -> p < candidateRoot).noneMatch(p -> candidate % p == 0);
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return takeWhile(primes, i -> i <= candidateRoot).stream().noneMatch(i -> candidate % i == 0);
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}
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/**
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* 返回元素满足谓词的最长前缀
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*/
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public static <A> List<A> takeWhile(List<A> list, Predicate<A> p) {
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int i = 0;
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for (A item : list) {

‎java8/src/main/java/com/brianway/learning/java8/streamapi/collect/Partitioning.java

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package com.brianway.learning.java8.streamapi.collect;
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import com.brianway.learning.java8.streamapi.Dish;
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import static com.brianway.learning.java8.streamapi.Dish.menu;
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import static java.util.Comparator.comparingInt;
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import static java.util.stream.Collectors.collectingAndThen;
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import static java.util.stream.Collectors.groupingBy;
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import static java.util.stream.Collectors.maxBy;
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import static java.util.stream.Collectors.partitioningBy;
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import java.util.List;
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import java.util.Map;
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import java.util.Optional;
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/**
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* 分区
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*/
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public class Partitioning {
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public static void main(String... args) {
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System.out.println("Dishes partitioned by vegetarian: " + partitionByVegeterian());
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System.out.println("Vegetarian Dishes by type: " + vegetarianDishesByType());
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System.out.println("Most caloric dishes by vegetarian: " + mostCaloricPartitionedByVegetarian());
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}
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private static Map<Boolean, List<Dish>> partitionByVegeterian() {
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return menu.stream().collect(partitioningBy(Dish::isVegetarian));
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}
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private static Map<Boolean, Map<Dish.Type, List<Dish>>> vegetarianDishesByType() {
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return menu.stream().collect(partitioningBy(Dish::isVegetarian, groupingBy(Dish::getType)));
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}
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private static Object mostCaloricPartitionedByVegetarian() {
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return menu.stream().collect(
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partitioningBy(Dish::isVegetarian,
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collectingAndThen(
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maxBy(comparingInt(Dish::getCalories)),
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Optional::get)));
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}
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}
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‎java8/src/main/java/com/brianway/learning/java8/streamapi/Reducing.java renamed to ‎java8/src/main/java/com/brianway/learning/java8/streamapi/collect/Reducing.java

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package com.brianway.learning.java8.streamapi;
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package com.brianway.learning.java8.streamapi.collect;
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import com.brianway.learning.java8.streamapi.Dish;
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import static com.brianway.learning.java8.streamapi.Dish.menu;
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import static java.util.stream.Collectors.reducing;
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/**
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* 收集器的灵活性,以不同的方法执行相同的操作
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*/
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public class Reducing {
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public static void main(String... args) {

‎java8/src/main/java/com/brianway/learning/java8/streamapi/collect/Summarizing.java

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package com.brianway.learning.java8.streamapi.collect;
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import com.brianway.learning.java8.streamapi.Dish;
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import static com.brianway.learning.java8.streamapi.Dish.menu;
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import static java.util.stream.Collectors.averagingInt;
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import static java.util.stream.Collectors.counting;
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import static java.util.stream.Collectors.joining;
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import static java.util.stream.Collectors.reducing;
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import static java.util.stream.Collectors.summarizingInt;
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import static java.util.stream.Collectors.summingInt;
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import java.util.Comparator;
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import java.util.IntSummaryStatistics;
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import java.util.function.BinaryOperator;
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/**
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* 归约和汇总
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*/
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public class Summarizing {
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public static void main(String... args) {
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System.out.println("Nr. of dishes: " + howManyDishes());
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System.out.println("The most caloric dish is: " + findMostCaloricDish());
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System.out.println("The most caloric dish is: " + findMostCaloricDishUsingComparator());
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System.out.println("Total calories in menu: " + calculateTotalCalories());
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System.out.println("Average calories in menu: " + calculateAverageCalories());
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System.out.println("Menu statistics: " + calculateMenuStatistics());
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System.out.println("Short menu: " + getShortMenu());
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System.out.println("Short menu comma separated: " + getShortMenuCommaSeparated());
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}
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/**
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* 统计个数
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*/
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private static long howManyDishes() {
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return menu.stream().collect(counting());
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}
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/**
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* 查找流中最大值
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*/
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private static Dish findMostCaloricDish() {
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return menu.stream().collect(reducing((d1, d2) -> d1.getCalories() > d2.getCalories() ? d1 : d2)).get();
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}
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/**
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* 查找流中最大值
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*/
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private static Dish findMostCaloricDishUsingComparator() {
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Comparator<Dish> dishCaloriesComparator = Comparator.comparingInt(Dish::getCalories);
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BinaryOperator<Dish> moreCaloricOf = BinaryOperator.maxBy(dishCaloriesComparator);
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return menu.stream().collect(reducing(moreCaloricOf)).get();
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}
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/**
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* 汇总-求和
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*/
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private static int calculateTotalCalories() {
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return menu.stream().collect(summingInt(Dish::getCalories));
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}
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/**
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* 汇总-求平均数
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*/
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private static Double calculateAverageCalories() {
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return menu.stream().collect(averagingInt(Dish::getCalories));
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}
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/**
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* 汇总-统计值
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*/
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private static IntSummaryStatistics calculateMenuStatistics() {
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return menu.stream().collect(summarizingInt(Dish::getCalories));
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}
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/**
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* 连接字符串
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*/
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private static String getShortMenu() {
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return menu.stream().map(Dish::getName).collect(joining());
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}
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private static String getShortMenuCommaSeparated() {
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return menu.stream().map(Dish::getName).collect(joining(", "));
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}
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}

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