Java8 Stream流

Posted 2021-03-17 魔有追求

Java8 Stream流

Java8关于map和flatMap的代码片段思考

Java8初体验（二）Stream语法详解

distinct()

/*
    返回一个流包含不同的元素（根据equals方法判断，null值并不会报空指针异常，会保留一个null）。
    对于有序的流保证稳定性，保留最先出现的元素，对于无序的流不保证稳定性。
*/
/**
     * Returns a stream consisting of the distinct elements (according to
     * {@link Object#equals(Object)}) of this stream.
     *
     * <p>For ordered streams, the selection of distinct elements is stable
     * (for duplicated elements, the element appearing first in the encounter
     * order is preserved.)  For unordered streams, no stability guarantees
     * are made.
     *
     * <p>This is a <a href="package-summary.html#StreamOps">stateful
     * intermediate operation</a>.
     *
     * @apiNote
     * Preserving stability for {@code distinct()} in parallel pipelines is
     * relatively expensive (requires that the operation act as a full barrier,
     * with substantial buffering overhead), and stability is often not needed.
     * Using an unordered stream source (such as {@link #generate(Supplier)})
     * or removing the ordering constraint with {@link #unordered()} may result
     * in significantly more efficient execution for {@code distinct()} in parallel
     * pipelines, if the semantics of your situation permit.  If consistency
     * with encounter order is required, and you are experiencing poor performance
     * or memory utilization with {@code distinct()} in parallel pipelines,
     * switching to sequential execution with {@link #sequential()} may improve
     * performance.
     *
     * @return the new stream
     */
Stream<T> distinct();

public static void main(String[] args) {
    List<Integer> integers = Arrays.asList(null, 1, 2, 2, 3, null);
    List<Integer> result = integers.stream().distinct().collect(Collectors.toList());
    //[null, 1, 2, 3] 并不会去除null值和报空指针异常。
    System.out.println(result);

    List<Integer> result2 = integers.stream().filter(Objects::nonNull).distinct().collect(Collectors.toList());
    //[1, 2, 3] 去重同时去除null值。
    System.out.println(result2);
}

flatMap()

<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);

public static void main(String[] args) {
    List<List<String>> lists = new ArrayList<>();
    lists.add(Arrays.asList("tom1", "tom2"));
    lists.add(Arrays.asList("java1", "java2"));
    lists.add(Arrays.asList("web1", "web2"));

    /* 
        将流中的每个元素转换为一个流，会得到多个流，然后将这些流合并成一个流,。
        上面6个字符串被合并到一个流中
    */
    lists.stream().flatMap(Collection::stream).forEach(System.out::println);
}