Java8 stream 学习3
jopen
9年前
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); }【参考资料】