Java8 stream 學習3

將結果收集到 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);
    }

分組和分片

groupingBy 會產生一個值為列表的 map 對象。

@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()));// 返回根據國家分組的語言個數的map
        // Map<String, Long> map4 =
        // citys.collect(Collectors.groupingBy(City::getState,
        // Collectors.summingLong(City::getPopulation)));
        // 模擬計算每個州下的城市人口數
        // Map<String, City> map5 =
        // citys.collect(Collectors.groupingBy(City::getState,
        // Collectors.maxBy(Compartor.comparing(City::getPopulation))));
        // 映射每個州人口最多的城市
        Map<String, Set<String>> map6 = stream.collect(
                Collectors.groupingBy(Locale::getDisplayCountry,
                Collectors.mapping(Locale::getDisplayLanguage, Collectors.toSet())));


    System.out.println(map);
}</pre> <h2>元素類型流 </h2>

Stream api 提供了原始類型流： 

@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();// 原生流轉換成對象流

    intStream2.forEach(System.out::println);
}</pre><br />

【參考資料】 


寫給大忙人看的Java SE 8 
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