别告诉我Java8都出来这么久了，你还搞不懂Stream的map和flatmap的区别？

Posted 2021-05-29 c.

文章目录

• • 别告诉我Java8都出来这么久了，你还搞不懂Stream的map和flatmap的区别？
  • • Java8 map相关的方法
    • Java8 flatMap相关的方法
  • 参考

别告诉我Java8都出来这么久了，你还搞不懂Stream的map和flatmap的区别？

Java8 map相关的方法

    /**
     * Returns a stream consisting of the results of applying the given
     * function to the elements of this stream.
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * @param <R> The element type of the new stream
     * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *               <a href="package-summary.html#Statelessness">stateless</a>
     *               function to apply to each element
     * @return the new stream
     */
    <R> Stream<R> map(Function<? super T, ? extends R> mapper);

    /**
     * Returns an {@code IntStream} consisting of the results of applying the
     * given function to the elements of this stream.
     *
     * <p>This is an <a href="package-summary.html#StreamOps">
     *     intermediate operation</a>.
     *
     * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *               <a href="package-summary.html#Statelessness">stateless</a>
     *               function to apply to each element
     * @return the new stream
     */
    IntStream mapToInt(ToIntFunction<? super T> mapper);

    /**
     * Returns a {@code LongStream} consisting of the results of applying the
     * given function to the elements of this stream.
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *               <a href="package-summary.html#Statelessness">stateless</a>
     *               function to apply to each element
     * @return the new stream
     */
    LongStream mapToLong(ToLongFunction<? super T> mapper);

    /**
     * Returns a {@code DoubleStream} consisting of the results of applying the
     * given function to the elements of this stream.
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *               <a href="package-summary.html#Statelessness">stateless</a>
     *               function to apply to each element
     * @return the new stream
     */
    DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);

1. <R> Stream<R> map(Function<? super T, ? extends R> mapper);
2. IntStream mapToInt(ToIntFunction<? super T> mapper);
3. LongStream mapToLong(ToLongFunction<? super T> mapper);
4. DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);

map: 对于Stream中包含的元素使用给定的转换函数进行转换操作，新生成的Stream只包含转换生成的元素。这个方法有三个对于原始类型的变种方法，分别是：mapToInt，mapToLong和mapToDouble。这三个方法也比较好理解，比如mapToInt就是把原始Stream转换成一个新的Stream，这个新生成的Stream中的元素都是int类型。之所以会有这样三个变种方法，可以免除自动装箱/拆箱的额外消耗

最通用和使用最多的就是我们的map方法了。

我们主要就是研究一下这个map方法了。

/**
     * Returns a stream consisting of the results of applying the given
     * function to the elements of this stream.
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * @param <R> The element type of the new stream
     * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *               <a href="package-summary.html#Statelessness">stateless</a>
     *               function to apply to each element
     * @return the new stream
     */
    <R> Stream<R> map(Function<? super T, ? extends R> mapper);

Stream 支持 map 方法，它会接受一个函数作为参数。这个函数会被应用到每个元素上，并将其映 射成一个新的元素。

示例：

     @Test
    void test01(){
        List<String> words = Arrays.asList("Hello", "World");
        List<Integer> wordLens = words.stream()
                .map(String::length) //转为字符串长度的集合
                .collect(Collectors.toList());
        System.out.println(wordLens);
    }

运行结果：

[5, 5]

可以看到原先集合是两个元素，最后通过map之后出来的集合也是两个元素，所以他们是一个一一对应的映射关系。

Java8 flatMap相关的方法

 /**
     * Returns a stream consisting of the results of replacing each element of
     * this stream with the contents of a mapped stream produced by applying
     * the provided mapping function to each element.  Each mapped stream is
     * {@link java.util.stream.BaseStream#close() closed} after its contents
     * have been placed into this stream.  (If a mapped stream is {@code null}
     * an empty stream is used, instead.)
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * @apiNote
     * The {@code flatMap()} operation has the effect of applying a one-to-many
     * transformation to the elements of the stream, and then flattening the
     * resulting elements into a new stream.
     *
     * <p><b>Examples.</b>
     *
     * <p>If {@code orders} is a stream of purchase orders, and each purchase
     * order contains a collection of line items, then the following produces a
     * stream containing all the line items in all the orders:
     * <pre>{@code
     *     orders.flatMap(order -> order.getLineItems().stream())...
     * }</pre>
     *
     * <p>If {@code path} is the path to a file, then the following produces a
     * stream of the {@code words} contained in that file:
     * <pre>{@code
     *     Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);
     *     Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +")));
     * }</pre>
     * The {@code mapper} function passed to {@code flatMap} splits a line,
     * using a simple regular expression, into an array of words, and then
     * creates a stream of words from that array.
     *
     * @param <R> The element type of the new stream
     * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *               <a href="package-summary.html#Statelessness">stateless</a>
     *               function to apply to each element which produces a stream
     *               of new values
     * @return the new stream
     */
    <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);

    /**
     * Returns an {@code IntStream} consisting of the results of replacing each
     * element of this stream with the contents of a mapped stream produced by
     * applying the provided mapping function to each element.  Each mapped
     * stream is {@link java.util.stream.BaseStream#close() closed} after its
     * contents have been placed into this stream.  (If a mapped stream is
     * {@code null} an empty stream is used, instead.)
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *               <a href="package-summary.html#Statelessness">stateless</a>
     *               function to apply to each element which produces a stream
     *               of new values
     * @return the new stream
     * @see #flatMap(Function)
     */
    IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);

    /**
     * Returns an {@code LongStream} consisting of the results of replacing each
     * element of this stream with the contents of a mapped stream produced by
     * applying the provided mapping function to each element.  Each mapped
     * stream is {@link java.util.stream.BaseStream#close() closed} after its
     * contents have been placed into this stream.  (If a mapped stream is
     * {@code null} an empty stream is used, instead.)
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *               <a href="package-summary.html#Statelessness">stateless</a>
     *               function to apply to each element which produces a stream
     *               of new values
     * @return the new stream
     * @see #flatMap(Function)
     */
    LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);

    /**
     * Returns an {@code DoubleStream} consisting of the results of replacing
     * each element of this stream with the contents of a mapped stream produced
     * by applying the provided mapping function to each element.  Each mapped
     * stream is {@link java.util.stream.BaseStream#close() closed} after its
     * contents have placed been into this stream.  (If a mapped stream is
     * {@code null} an empty stream is used, instead.)
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *               <a href="package-summary.html#Statelessness">stateless</a>
     *               function to apply to each element which produces a stream
     *               of new values
     * @return the new stream
     * @see #flatMap(Function)
     */
    DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);

flatMap：和map类似，不同的是其每个元素转换得到的是Stream对象，会把子Stream中的元素压缩到父集合中

示例：我们需要使用flatMap找出单词列表中各不相同的字符，比如我们现在有[Hello, World]这样一个集合，然后我们要找出单词列表中各不相同的字符，最后出来的结果是[H, e, l, o, W, r, d]

    @Test
    void test02() {
        //使用flatMap找出单词列表中各不相同的字符
        List<String> words = Arrays.asList("Hello", "World");
        List<String> wordMap = words.stream()
                .map(word -> word.split(""))
                .flatMap(Arrays::stream)
                .distinct()
                .collect(Collectors.toList());
        System.out.println(wordMap);
    }

运行结果：

[H, e, l, o, W, r, d]

flatmap 方法让你把一个流中的每个值都换成另一个流，然后把所有的流连接起来成为一个流。

参考

Java8初体验（二）Stream语法详解

Java8特性③Stream的使用

【Java 8 新特性】Java 8 Stream flatMapToInt, flatMapToLong和flatMapToDouble使用示例