分类：简单计算， 收集到映射表中 , 群组和分组, 下游收集器, 约简操作 reduce()

  1. ArrayList<String> arrayList = new ArrayList<>();
  2. arrayList.add("aa");
  3. arrayList.add("aA");
  4. arrayList.add("bbb");
  5. arrayList.add("a1c2");
  6. arrayList.add("a1a");
  7. arrayList.add("c");
  9. Supplier<Stream<String>> supplier = ()->arrayList.stream();

直接输出结果

  1. /***
  2.  * 直接输出结果
  3.  */
  4. //1. iterator()相关		->旧式迭代器
  5. //(1)iterator
  6. System.out.println("----直接输出结果--旧式迭代器-----start--");
  7. Iterator<String> iterator = arrayList.stream().parallel().iterator();
  8. while(iterator.hasNext()){
  9. 	System.out.println(iterator.next());
  10. }
  11. System.out.println();
  13. //forEachRemaining()
  14. arrayList.iterator().forEachRemaining((x)->System.out.println(x.toString()));
  15. System.out.println("----直接输出结果--旧式迭代器-----end--");
  16. System.out.println();
  18. //(2) spliterator()
  19. Spliterator<String> spliterator = arrayList.stream().spliterator();
  20. while(spliterator.tryAdvance((n)->System.out.println(n))){}
  21. System.out.println();
  23. spliterator = arrayList.stream().spliterator();
  24. spliterator.forEachRemaining((n)->System.out.println(n));
  25. System.out.println();
  27. Spliterator<String> spliterator2 = arrayList.stream().spliterator().trySplit();
  28. spliterator2.forEachRemaining((n)->System.out.println(n));
  29. spliterator2.forEachRemaining((n)->System.out.println(n));
  30. System.out.println();
  32. //2. forEach()				->parallel()返回的结果顺序不确定
  33. System.out.println("----直接输出结果--.stream().parallel().forEach()-----start--");
  34. arrayList.stream().parallel().forEach(n->System.out.println(n));
  35. System.out.println("----直接输出结果--.stream().parallel().forEach()-----end--");
  36. System.out.println();
  38. //3. forEachOrdered()		->parallel()返回的结果顺序与流中的顺序相同
  39. System.out.println("----直接输出结果--.stream().parallel().forEachOrdered()-----start--");
  40. arrayList.stream().parallel().forEachOrdered(n->System.out.println(n));
  41. System.out.println("----直接输出结果--.stream().parallel().forEachOrdered()-----end--");
  42. System.out.println();
  43. System.out.println();
  44. System.out.println();
  45. System.out.println();
  46. System.out.println();

简单的计算

  1. //1. count()	数量
  2. System.out.println(supplier.get().count());
  3. System.out.println();
  5. //2. max()		最大数值
  6. System.out.println(supplier.get().max(String::compareToIgnoreCase).orElse(""));
  7. System.out.println();
  9. //3. min()		最小数值
  10. System.out.println(supplier.get().min(String::compareToIgnoreCase).orElse(""));
  11. System.out.println();
  13. //4. findFirst(), 非空集合中第一个
  14. //一般结合filer()==>findFirst==>找到匹配条件中第一个
  15. String findFirstValue = supplier.get().findFirst().orElse("");
  16. System.out.println(findFirstValue);
  18. findFirstValue = supplier.get().filter(x->x.length()>2).findFirst().orElse("");
  19. System.out.println(findFirstValue);
  21. findFirstValue = supplier.get().filter(x->x.length()>2).sorted().findFirst().orElse("");
  22. System.out.println(findFirstValue);
  24. findFirstValue = supplier.get().filter(x->x.length()>2).sorted(Comparator.comparing(String::length).thenComparing(String::compareTo).reversed()).findFirst().orElse("");
  25. System.out.println(findFirstValue);
  26. System.out.println();
  28. //5. findAny(), 非空集合中任意一个
  29. //一般结合filer()==>findAny==>找到匹配任意匹配,表示能够匹配
  30. String findAnyValue = supplier.get().filter(x->x.length()>4).findAny().orElse("");
  31. System.out.println(!findAnyValue.equals(""));
  32. System.out.println();
  34. //6. anyMatch(), 任意元素有无匹配
  35. //直接使用看是否有匹配
  36. boolean anyMatch = supplier.get().anyMatch(x->x.length()>3);
  37. System.out.println(anyMatch);
  38. System.out.println();
  40. //7. allMatch(), 所有元素有无匹配
  41. //直接使用看是否有匹配
  42. boolean allMatch = supplier.get().allMatch(x->x.length()>0);
  43. System.out.println(allMatch);
  44. System.out.println();
  46. //8. noneMatch(), 没有任意元素有无匹配
  47. //直接使用看是否有匹配
  48. boolean noneMatch = supplier.get().noneMatch(x->x.length()>4);
  49. System.out.println(noneMatch);
  50. System.out.println();

收集到映射表中

  1. /***
  2.  * 收集到映射表中
  3.  */
  4. //1.toArray()
  5. System.out.println("----收集到数据结构中--.stream().toArray()-----start--");
  6. String[] stringArray = arrayList.stream().toArray(String[]::new);
  7. System.out.println(Arrays.toString(stringArray));
  8. System.out.println();
  10. System.out.println(Arrays.toString(arrayList.stream().toArray()));
  11. System.out.println("----收集到数据结构中--.stream().toArray()-----end--");
  12. System.out.println();
  14. //2.collect()		收集元素到集合, 总和/平均值/最大/最小值
  15. ArrayList<NamePhoneEmail> myList = new ArrayList<>();
  16. myList.add(new NamePhoneEmail("Larry", "555-5555", "Larry@HerbSchildt.com"));
  17. myList.add(new NamePhoneEmail("James", "555-4444", "James@HerbSchildt.com"));
  18. myList.add(new NamePhoneEmail("Mary", "555-3333", "Mary@HerbSchildt.com"));
  20. //(1) toList()		使用Collectors.toList()
  21. System.out.println("----收集到数据结构中--.stream().map().collect(.toList()).forEach()-----start-------");
  22. myList.stream().map((a) -> new NamePhone(a.name, a.phonenum)).collect(Collectors.toList()).forEach(n->System.out.println(n.name+" "+n.phonenum));
  23. System.out.println();
  25. //在循环之前先做map()处理
  26. myList.stream().map((a) -> a.toString()).collect(Collectors.toList()).forEach(System.out::println);
  27. System.out.println("----收集到数据结构中--.stream().map().collect(.toList()).forEach()-----end-------");
  28. System.out.println();
  29. System.out.println();
  31. //(2) toSet()		使用Collectors.toSet()
  32. System.out.println("----收集到数据结构中--.stream().map().collect(.toSet()).forEach()-----start-------");
  33. myList.stream().map((a) -> new NamePhone(a.name, a.phonenum)).collect(Collectors.toSet()).forEach(n->System.out.println(n.name+" "+n.phonenum));
  34. System.out.println("----收集到数据结构中--.stream().map().collect(.toSet()).forEach()-----end-------");
  35. System.out.println();
  36. System.out.println();
  38. //(3) ->TreeSet		使用Collectors.toCollection()	TreeSet自动排序
  39. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toCollection(TreeSet::new)).forEach()-----start-------");
  40. TreeSet<String> toTreeSet = arrayList.stream().collect(Collectors.toCollection(TreeSet::new));
  41. toTreeSet.forEach(n->System.out.println(n));
  42. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toCollection(TreeSet::new)).forEach()-----end-------");
  43. System.out.println();
  44. System.out.println();
  46. //(4) ->ArrayList	使用Collectors.toCollection()
  47. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toCollection(ArrayList::new)).forEach()-----start-------");
  48. ArrayList<String> toArrayList = arrayList.stream().collect(Collectors.toCollection(ArrayList::new));
  49. toArrayList.forEach(n->System.out.println(n));
  50. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toCollection(ArrayList::new)).forEach()-----end-------");
  51. System.out.println();
  52. System.out.println();
  53. System.out.println();
  54. System.out.println();
  56. //(5) ->toMap		收集到映射表中
  58. //1) 指定keyMapper和valueMapper
  59. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toMap(keyMapper, valueMapper)).forEach()-----start-------");
  60. Map<String, String> toMap = myList.stream().collect(
  61. 		Collectors.toMap(
  62. 				NamePhoneEmail::getName, 					//keyMapper
  63. 				NamePhoneEmail::getPhonenum					//valueMapper
  64. 		));
  65. toMap.entrySet().forEach((n)->System.out.println(n.getKey()+"	"+n.getValue()));
  66. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toMap(keyMapper, valueMapper)).forEach()-----end-------");
  67. System.out.println();
  68. System.out.println();
  70. //2) 指定keyMapper, valueMapper, mergeFunction
  71. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toMap(keyMapper, valueMapper, mergeFunction)).forEach()-----start-------");
  72. Stream<Locale> localesStream = Stream.of(Locale.getAvailableLocales());
  73. //通过指定的mergeFunction, 解决冲突
  74. Map<String, String> languageNames = localesStream.collect(
  75. 		Collectors.toMap(
  76. 			Locale::getDisplayLanguage,						//keyMapper
  77. 			l->l.getDisplayLanguage(l),						//valueMapper
  78. 			(existingValue, newValue)->existingValue		//mergeFunction, 合并两个 keyMapper 相同的值
  79. 		));
  81. languageNames.entrySet().forEach((n)->System.out.println(n.getKey()+"	"+n.getValue()));
  82. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toMap(keyMapper, valueMapper, mergeFunction)).forEach()-----end-------");
  83. System.out.println();
  85. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toMap(keyMapper, valueMapper(Set), mergeFunction)).forEach()-----start-------");
  86. localesStream = Stream.of(Locale.getAvailableLocales());
  87. Map<String, Set<String>> countryLanguageSets = localesStream.collect(
  88.    Collectors.toMap(
  89. 		   Locale::getDisplayCountry,								//keyMapper
  90. 		   l -> Collections.singleton(l.getDisplayLanguage()),		//valueMapper	Returns an immutable set containing only the specified object
  91. 		   (a, b) -> { // union of a and b							//mergeFunction
  92. 			   	Set<String> union = new HashSet<>(a);
  93. 			   	union.addAll(b);
  94. 			   	return union;
  95. 		   }
  96. 	));
  97. countryLanguageSets.entrySet().forEach((n)->System.out.println(n.getKey()+"	"+n.getValue()));
  98. System.out.println("----收集到数据结构中--.stream().collect(Collectors.toMap(keyMapper, valueMapper(Set), mergeFunction)).forEach()-----end-------");
  99. System.out.println();
  100. System.out.println();
  102. //(6) to LinkedList
  103. System.out.println("----收集到数据结构中--.stream().collect(supplier, accumulator, combiner)--to LinkedList---start-------");
  104. LinkedList<NamePhone> nkList = myList.stream().map((a) -> new NamePhone(a.name, a.phonenum)).collect(
  105. 		()->new LinkedList<>(),
  106. 		(list, element) -> list.add(element),
  107. 		(listA, listB)->listA.addAll(listB)
  108. 		);
  109. nkList.forEach(n->System.out.println(n.name+" "+n.phonenum));
  110. System.out.println("----收集到数据结构中--.stream().collect(supplier, accumulator, combiner)--to LinkedList---end-------");
  111. System.out.println();
  112. System.out.println(); 
  114. //(7) to HashSet
  115. System.out.println("----收集到数据结构中--.stream().collect(supplier, accumulator, combiner)--to HashSet---start-------");
  116. HashSet<NamePhone> hsSet = myList.stream().map((a) -> new NamePhone(a.name, a.phonenum)).collect(
  117. 		HashSet::new,
  118. 		HashSet::add,
  119. 		HashSet::addAll
  120. 		);
  121. hsSet.forEach(n->System.out.println(n.name+" "+n.phonenum));
  122. System.out.println("----收集到数据结构中--.stream().collect(supplier, accumulator, combiner)--to HashSet---end-------");
  123. System.out.println();
  124. System.out.println(); 
  126. //(8)总和, 平均值, 最大值, 最小值
  127. ArrayList<Integer> integerList = new ArrayList<>();
  128. integerList.add(23);
  129. integerList.add(66);
  130. integerList.add(888);
  131. integerList.add(999);
  132. integerList.add(1024);
  134. //总和
  135. Integer integerCollect = integerList.stream().collect(Collectors.summingInt(Integer::intValue));
  136. System.out.println(integerCollect);
  138. //总和, 数目, 平均值, 最大值, 最小值
  139. IntSummaryStatistics intSummaryStatistics = integerList.stream().collect(Collectors.summarizingInt(Integer::intValue));
  140. System.out.println(intSummaryStatistics.getSum());
  141. System.out.println(intSummaryStatistics.getCount());
  142. System.out.println(intSummaryStatistics.getAverage());
  143. System.out.println(intSummaryStatistics.getMax());
  144. System.out.println(intSummaryStatistics.getMin());
  145. System.out.println("------intSummaryStatistics-------");
  147. ArrayList<Long> longList = new ArrayList<>();
  148. longList.add(23l);
  149. longList.add(66l);
  150. longList.add(888l);
  151. longList.add(999l);
  152. longList.add(1024l);
  154. Long longCollect = longList.stream().collect(Collectors.summingLong(Long::longValue));
  155. System.out.println(longCollect);
  157. LongSummaryStatistics longSummaryStatistics = longList.stream().collect(Collectors.summarizingLong(Long::longValue));
  158. System.out.println(longSummaryStatistics.getSum());
  159. System.out.println(longSummaryStatistics.getCount());
  160. System.out.println(longSummaryStatistics.getAverage());
  161. System.out.println(longSummaryStatistics.getMax());
  162. System.out.println(longSummaryStatistics.getMin());
  163. System.out.println("------longSummaryStatistics-------");
  165. ArrayList<Double> doubleList = new ArrayList<>();
  166. doubleList.add(23.2);
  167. doubleList.add(66.3);
  168. doubleList.add(888.5);
  169. doubleList.add(999.6);
  170. doubleList.add(1024.8);
  172. Double doubleCollect = doubleList.stream().collect(Collectors.summingDouble(Double::doubleValue));
  173. System.out.println(doubleCollect);
  175. DoubleSummaryStatistics doubleSummaryStatistics = doubleList.stream().collect(Collectors.summarizingDouble(Double::doubleValue));
  176. System.out.println(doubleSummaryStatistics.getSum());
  177. System.out.println(doubleSummaryStatistics.getCount());
  178. System.out.println(doubleSummaryStatistics.getAverage());
  179. System.out.println(doubleSummaryStatistics.getMax());
  180. System.out.println(doubleSummaryStatistics.getMin());
  181. System.out.println("------doubleSummaryStatistics-------");
  183. //(9) joining()		指定连接符
  184. System.out.println("----收集到数据结构中--.stream().collect(Collectors.joining())-----start-------");
  185. String collectJoining = myList.stream().map((a) -> a.email).collect(Collectors.joining());
  186. System.out.println(collectJoining);
  188. //指定连接符
  189. collectJoining = myList.stream().map((a) -> a.email).collect(Collectors.joining(";"));
  190. System.out.println(collectJoining);
  192. //指定连接符, 前缀, 后缀
  193. collectJoining = myList.stream().map((a) -> a.email).collect(Collectors.joining(";","prefix		","		suffix"));
  194. System.out.println(collectJoining);
  195. System.out.println("----收集到数据结构中--.stream().collect(Collectors.joining())-----end-------");
  196. System.out.println();
  197. System.out.println();

群组和分组
  1. /***
  2.  *  群组和分组
  3.  */
  5. //1. groupingBy()
  7. //1)直接使用 groupingBy() 按指定元素分组
  8. System.out.println("----收集到数据结构中--.stream().collect(Collectors.groupingBy(element)).forEach()-----start-------");
  9. localesStream = Stream.of(Locale.getAvailableLocales());
  10. Map<String, List<Locale>> countryToLocalsMap = localesStream.collect(Collectors.groupingBy(Locale::getCountry));
  11. countryToLocalsMap.entrySet().forEach((n)->{
  12.     			System.out.print(n.getKey()+":");
  13.     			n.getValue().forEach(locale->System.out.print(locale.getDisplayLanguage()+","));
  14.     			System.out.println();
  15. 		});
  16. System.out.println("----收集到数据结构中--.stream().collect(Collectors.groupingBy(element)).forEach()-----end-------");
  17. System.out.println();
  18. System.out.println();
  20. //2) groupingBy后toList, 和直接使用.groupingBy(element) 结果相同
  21. System.out.println("----收集到数据结构中--.stream().collect(Collectors.groupingBy(element, toList)).forEach()-----start-------");
  22. localesStream = Stream.of(Locale.getAvailableLocales());
  23. Map<String, List<Locale>> groupingByToList = localesStream.collect(Collectors.groupingBy(Locale::getCountry, Collectors.toList()));
  24. groupingByToList.entrySet().forEach((n)->{
  25. 	System.out.print(n.getKey()+":");
  26. 	n.getValue().forEach(locale->System.out.print(locale.getDisplayLanguage()+","));
  27. 	System.out.println();
  28. });
  29. System.out.println("----收集到数据结构中--.stream().collect(Collectors.groupingBy(element, toList)).forEach()-----end-------");
  30. System.out.println();
  31. System.out.println();
  33. //3) groupingBy后toSet
  34. System.out.println("----收集到数据结构中--.stream().collect(Collectors.groupingBy(element, toSet)).forEach()-----start-------");
  35. localesStream = Stream.of(Locale.getAvailableLocales());
  36. Map<String, Set<Locale>> groupingByToSet = localesStream.collect(Collectors.groupingBy(Locale::getCountry, Collectors.toSet()));
  37. groupingByToSet.entrySet().forEach((n)->{
  38. 	System.out.print(n.getKey()+":");
  39. 	n.getValue().forEach(locale->System.out.print(locale.getDisplayLanguage()+","));
  40. 	System.out.println();
  41. });
  42. System.out.println("----收集到数据结构中--.stream().collect(Collectors.groupingBy(element, toSet)).forEach()-----end-------");
  43. System.out.println();
  44. System.out.println();
  45. System.out.println();
  47. //4) groupingByConcurrent()	得到一个并行映射表
  48. System.out.println("----收集到数据结构中--.stream().collect(Collectors.groupingByConcurrent(element)).forEach()-----start-------");
  49. localesStream = Stream.of(Locale.getAvailableLocales());
  50. countryToLocalsMap = localesStream.collect(Collectors.groupingByConcurrent(Locale::getCountry));
  51. countryToLocalsMap.entrySet().forEach((n)->{
  52.     			System.out.print(n.getKey()+":");
  53.     			n.getValue().forEach(locale->System.out.print(locale.getDisplayLanguage()+","));
  54.     			System.out.println();
  55. 		});
  56. System.out.println("----收集到数据结构中--.stream().collect(Collectors.groupingByConcurrent(element)).forEach()-----end-------");
  57. System.out.println();
  58. System.out.println();
  59. System.out.println();
  61. //a) groupingByConcurrent后toList, 和直接使用.groupingByConcurrent(element) 结果相同
  62. //b) groupingByConcurrent后toSet
  64. //5)使用partitioningBy() 按照指定条件(true or false)分组
  65. System.out.println("----收集到数据结构中--.stream().collect(Collectors.partitioningBy(Predicate<? super T> predicate)).forEach()-----start-------");
  66. localesStream = Stream.of(Locale.getAvailableLocales());
  67. Map<Boolean, List<Locale>> booleanLocalesStream = localesStream.collect(Collectors.partitioningBy((Locale l)->l.getCountry().equals("CN")));
  68. List<Locale> trueList = booleanLocalesStream.get(true);
  69. System.out.println("trueList "+trueList);
  71. List<Locale> falseList = booleanLocalesStream.get(false);
  72. System.out.println("falseList "+falseList); 
  74. System.out.println("----收集到数据结构中--.stream().collect(Collectors.partitioningBy(Predicate<? super T> predicate)).forEach()-----end-------");
  75. System.out.println();
  76. System.out.println();
  77. System.out.println();
  79. //a) partitioningBy后toList, 和直接使用.partitioningBy(element) 结果相同
  80. //b) partitioningBy后toSet
下游收集器
  1. /***
  2.  *  下游收集器
  3.  */
  5. ArrayList<City> cityArrayList = new ArrayList<>();
  6. cityArrayList.add(new City("AAA", "0001", 1234, 1234, 1234.1));
  7. cityArrayList.add(new City("ABA", "0001", 4321, 4321, 4321.2));
  9. cityArrayList.add(new City("BAA", "0002", 1004, 1004, 1004.6));
  10. cityArrayList.add(new City("BBA", "0002", 1034, 1034, 1034.8));
  12. //1. counting()
  14. //在groupingBy(), groupingByConcurrent(), partitioningBy()之后, 处理下游(接下来)的值
  15. //1) Collectors.counting()		计算收集到的元素个数
  16. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), counting()).forEach()-----start-------");
  17. localesStream = Stream.of(Locale.getAvailableLocales());
  18. Map<String, Long> countryToLocalsCountMap = localesStream.collect(Collectors.groupingBy(Locale::getCountry, Collectors.counting()));
  19. countryToLocalsCountMap.entrySet().forEach((n)->System.out.println(n.getKey()+" , counting:"+n.getValue()));
  20. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), counting()).forEach()-----end-------");
  21. System.out.println();
  22. System.out.println();
  24. System.out.println("----收集到数据结构中--.stream().collect(groupingByConcurrent(), counting()).forEach()-----start-------");
  25. localesStream = Stream.of(Locale.getAvailableLocales());
  26. ConcurrentMap<String, Long> countryToLocalsConcurrentCountMap = localesStream.collect(Collectors.groupingByConcurrent(Locale::getCountry, Collectors.counting()));
  27. countryToLocalsConcurrentCountMap.entrySet().forEach((n)->System.out.println(n.getKey()+" , counting:"+n.getValue()));
  28. System.out.println("----收集到数据结构中--.stream().collect(groupingByConcurrent(), counting()).forEach()-----end-------");
  29. System.out.println();
  30. System.out.println();
  32. System.out.println("----收集到数据结构中--.stream().collect(partitioningBy(), counting()).forEach()-----start-------");
  33. localesStream = Stream.of(Locale.getAvailableLocales());
  34. Map<Boolean, Long> booleanLocalesMap = localesStream.collect(Collectors.partitioningBy((Locale l)->l.getCountry().equals("CN"), Collectors.counting()));
  35. System.out.println("trueList count:"+booleanLocalesMap.get(true));
  36. System.out.println("falseList count:"+booleanLocalesMap.get(false)); 
  38. booleanLocalesMap.entrySet().forEach((n)->System.out.println(n.getKey()+" , counting:"+n.getValue()));
  40. System.out.println("----收集到数据结构中--.stream().collect(partitioningBy(), counting()).forEach()-----end-------");
  41. System.out.println();
  42. System.out.println();
  44. //2. Collectors.summing()		计算收集到的元素的和
  46. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), summingInt()).forEach()-----start-------");
  47. Map<String, Integer> collectCitySummingIntMap = cityArrayList.stream().collect(Collectors.groupingBy(City::getState, Collectors.summingInt(City::getIntPopulation)));
  48. collectCitySummingIntMap.entrySet().forEach((n)->System.out.println(n.getKey()+" , summingInt:"+n.getValue()));
  49. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), summingInt()).forEach()-----end-------");
  50. System.out.println();
  51. System.out.println();
  53. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), summingLong()).forEach()-----start-------");
  54. Map<String, Long> collectCitySummingLongMap = cityArrayList.stream().collect(Collectors.groupingBy(City::getState, Collectors.summingLong(City::getLongPopulation)));
  55. collectCitySummingLongMap.entrySet().forEach((n)->System.out.println(n.getKey()+" , summingLong:"+n.getValue()));
  56. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), summingLong()).forEach()-----end-------");
  57. System.out.println();
  58. System.out.println();   	
  60. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), summingDouble()).forEach()-----start-------");
  61. Map<String, Double> collectCitySummingDoubleMap = cityArrayList.stream().collect(Collectors.groupingBy(City::getState, Collectors.summingDouble(City::getDoublePopulation)));
  62. collectCitySummingDoubleMap.entrySet().forEach((n)->System.out.println(n.getKey()+" , summingDouble:"+n.getValue()));
  63. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), summingDouble()).forEach()-----end-------");
  64. System.out.println();
  65. System.out.println();	
  67. //3. Collectors.maxBy()		计算收集到的元素的最大值		传入比较器
  68. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), maxBy()).forEach()-----start-------");
  69. Map<String, Optional<City>> collectCityMaxIntMap = cityArrayList.stream().collect(Collectors.groupingBy(City::getState, Collectors.maxBy(Comparator.comparing(City::getIntPopulation))));
  70. collectCityMaxIntMap.entrySet().forEach((n)->System.out.println(n.getKey()+" , getIntPopulation maxBy:"+n.getValue().get().getIntPopulation()));
  71. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), maxBy()).forEach()-----end-------");
  72. System.out.println();
  73. System.out.println();
  75. //4. Collectors.minBy()		计算收集到的元素的最小值
  76. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), minBy()).forEach()-----start-------");
  77. Map<String, Optional<City>> collectCityMinIntMap = cityArrayList.stream().collect(Collectors.groupingBy(City::getState, Collectors.minBy(Comparator.comparing(City::getIntPopulation))));
  78. collectCityMinIntMap.entrySet().forEach((n)->System.out.println(n.getKey()+" , getIntPopulation minBy:"+n.getValue().get().getIntPopulation()));
  79. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), minBy()).forEach()-----end-------");
  80. System.out.println();
  81. System.out.println();  	
  83. //5. Collectors.mapping()		** 两层操作，第一遍groupby(), 第二遍再mapping()
  84. //两层操作，第一层，按照getState分组后；第二层，拿getName得最大
  85. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), mapping()).forEach()-----start-------");
  86. Map<String, Optional<String>> collectCityMaxStatusMap = cityArrayList.stream().collect(Collectors.groupingBy(City::getState, Collectors.mapping(City::getName, Collectors.maxBy(String::compareTo))));
  87. collectCityMaxStatusMap.entrySet().forEach((n)->System.out.println(n.getKey()+" , collectCityMaxStatusMap maxBy:"+n.getValue().get()));
  88. System.out.println("----收集到数据结构中--.stream().collect(groupingBy(), mapping()).forEach()-----end-------");
  89. System.out.println();
  90. System.out.println();  	
  92. //可以调用min()
约简操作 reduce()

  1. /***
  2.  * 约简操作	reduce()	从流中计算值
  3.  *
  4.  * V1 op V2 op V3 ... op VN
  5.  *
  6.  * 约简操作中, 流中元素的顺序不能有限制,
  7.  * 可以应用于约简操作的: 求和， 乘积， 字符串连接，求最大值， 求最小值， 求集合的并和交
  8.  * 不可应用于约简操作的: 减法， 除法
  9.  */
  11. List<Integer> integerValues = Arrays.asList(11,22,33,44,55,66);
  13. //1. 简单的示例
  14. //求和， 最大值， 最小值
  15. Optional<Integer> sumOptional = integerValues.stream().reduce(Integer::sum);//(x,y) -> (x+y)
  16. if(sumOptional.isPresent()){
  17. 	System.out.println(sumOptional.get());
  18. }
  20. Optional<Integer> maxOptional = integerValues.stream().reduce(Integer::max);
  21. if(maxOptional.isPresent()){
  22. 	System.out.println(maxOptional.get());
  23. }
  25. Optional<Integer> minOptional = integerValues.stream().reduce(Integer::min);
  26. if(minOptional.isPresent()){
  27. 	System.out.println(minOptional.get());
  28. }
  30. Optional<Integer> multiplyOptional = integerValues.stream().reduce((x,y)->(x*y));
  31. if(multiplyOptional.isPresent()){
  32. 	System.out.println(multiplyOptional.get());
  33. }
  35. //2. 传入初始值(幺元值), 作为计算的起点		stream()
  36. //T reduce(T identity, BinaryOperator<T> accumulator)
  37. //identity只在第一次使用
  38. /**
  39. 	a:1 		b:11
  40. 	a:11 		b:22
  41. 	a:242 		b:33
  42. 	a:7986 		b:44
  43. 	a:351384 	b:55
  44. 	a:19326120 	b:66
  45. 	1275523920
  46.  */
  47. Integer reduce = integerValues.stream().reduce(1, (a,b)->{
  48. 	System.out.println("a:"+a+" b:"+b);
  49. 		return a+b;
  50. });
  51. System.out.println(reduce);
  53. //3. 传入初始值(幺元值), 作为计算的起点; 		parallelStream()
  54. //<U> U reduce(U identity, BiFunction<U, ? super T, U> accumulator, BinaryOperator<U> combiner)
  55. //identity每次都会使用, 和accumulator结合使用时
  56. //combiner用来处理accumulator的结果
  57. //使用stream()时combiner不起作用; 只有parallelStream() 并行流 时combiner才起作用
  58. Integer reduce2 = integerValues.parallelStream().reduce(
  59. 		1,
  60. 		(a,b)->{
  61. 			System.out.println("a+b		a:"+a+" b:"+b);
  62. 			return a+b;
  63. 		},
  64. 		(a,b)->{
  65. 			System.out.println("a*b		a:"+a+" b:"+b);
  66. 			return a*b;
  67. 		}
  68. );
  69. System.out.println(reduce2);

辅助类

  1. class City{
  2. 	private String name;
  3. 	private String state;
  4. 	private int intPopulation;
  5. 	private long longPopulation;
  6. 	private double doublePopulation;
  8. 	public City(String name, String state, int intPopulation, long longPopulation, double doublePopulation) {
  9. 		this.name = name;
  10. 		this.state = state;
  11. 		this.intPopulation = intPopulation;
  12. 		this.longPopulation = longPopulation;
  13. 		this.doublePopulation = doublePopulation;
  14. 	}
  16. 	public String getName() {
  17. 		return name;
  18. 	}
  20. 	public void setName(String name) {
  21. 		this.name = name;
  22. 	}
  24. 	public String getState() {
  25. 		return state;
  26. 	}
  28. 	public void setState(String state) {
  29. 		this.state = state;
  30. 	}
  32. 	public int getIntPopulation() {
  33. 		return intPopulation;
  34. 	}
  36. 	public void setIntPopulation(int intPopulation) {
  37. 		this.intPopulation = intPopulation;
  38. 	}
  40. 	public long getLongPopulation() {
  41. 		return longPopulation;
  42. 	}
  44. 	public void setLongPopulation(long longPopulation) {
  45. 		this.longPopulation = longPopulation;
  46. 	}
  48. 	public double getDoublePopulation() {
  49. 		return doublePopulation;
  50. 	}
  52. 	public void setDoublePopulation(double doublePopulation) {
  53. 		this.doublePopulation = doublePopulation;
  54. 	}
  56. }
  58. class NamePhoneEmail {
  59. 	String name;
  60. 	String phonenum;
  61. 	String email;
  63. 	NamePhoneEmail(String n, String p, String e) {
  64. 		name = n;
  65. 		phonenum = p;
  66. 		email = e;
  67. 	}
  69. 	public String getName() {
  70. 		return name;
  71. 	}
  73. 	public void setName(String name) {
  74. 		this.name = name;
  75. 	}
  77. 	public String getPhonenum() {
  78. 		return phonenum;
  79. 	}
  81. 	public void setPhonenum(String phonenum) {
  82. 		this.phonenum = phonenum;
  83. 	}
  85. 	public String getEmail() {
  86. 		return email;
  87. 	}
  89. 	public void setEmail(String email) {
  90. 		this.email = email;
  91. 	}
  93. 	@Override
  94. 	public String toString() {
  95. 		return "name:"+name+",phonenum:"+phonenum+",email:"+email;
  96. 	}
  97. }
  99. class NamePhone {
  100. 	String name;
  101. 	String phonenum;
  103. 	NamePhone(String n, String p) {
  104. 		name = n;
  105. 		phonenum = p;
  106. 	}
  107. }











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