Java8 stream 学习3

将结果收集到 Map 中

@Test     public void getResultToMap() {         Stream<String> stream = Stream.of("a", "b", "c", "da", "asdass");         Map<String, Integer> map = stream.collect(Collectors                 .toMap(String::toString, String::length));         System.out.println(map);     }

分组和分片

groupingBy 会产生一个值为列表的 map 对象。

@Test     public void groupBy() {         Stream<Locale> stream = Stream.of(Locale.getAvailableLocales());         Map<String, List<Locale>> map = stream.collect(Collectors.groupingBy(Locale::getCountry));         Map<String, Set<Locale>> map2 = stream.collect(Collectors.groupingBy(Locale::getCountry,                 Collectors.toSet()));         Map<String, Long> map3 = stream.collect(Collectors.groupingBy(Locale::getCountry,                 Collectors.counting()));// 返回根据国家分组的语言个数的map         // Map<String, Long> map4 =         // citys.collect(Collectors.groupingBy(City::getState,         // Collectors.summingLong(City::getPopulation)));         // 模拟计算每个州下的城市人口数         // Map<String, City> map5 =         // citys.collect(Collectors.groupingBy(City::getState,         // Collectors.maxBy(Compartor.comparing(City::getPopulation))));         // 映射每个州人口最多的城市         Map<String, Set<String>> map6 = stream.collect(                 Collectors.groupingBy(Locale::getDisplayCountry,                 Collectors.mapping(Locale::getDisplayLanguage, Collectors.toSet())));          System.out.println(map);     }

元素类型流

Stream api 提供了原始类型流：

@Test     public void baseStream() {         IntStream intStream = IntStream.of(1, 2, 3);         int[] values = { 2, 3, 4, 5, 6, 7, 8, 9, 0, 1 };         IntStream intStream2 = Arrays.stream(values, 2, 5);         IntStream intStream3 = IntStream.range(0, 10);// 不包含上限         IntStream intStream4 = IntStream.rangeClosed(0, 10);// 包含上限          Stream<String> stream = Stream.of("a", "asd", "2s");         IntStream intStream5 = stream.mapToInt(String::length);         Stream<Integer> stream2 = intStream2.boxed();// 原生流转换成对象流          intStream2.forEach(System.out::println);     }