java stream foreach 怎么累加 java stream map foreach

「 Java 8 新特性 」Stream 中的 map、peek、foreach 方法的区别

一、概述

在学习java 8的stream api时，我们会遇到map,peek以及foreach这三种不同的处理方法，到底它们之间有些什么样的区别呢？本篇文章讲为你揭晓。

Map

/**
     * 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);

总结一下，就是应用一个函数型的接口，返回一个新流，是一个中间操作。

peek

/**
     * Returns a stream consisting of the elements of this stream, additionally
     * performing the provided action on each element as elements are consumed
     * from the resulting stream.
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * <p>For parallel stream pipelines, the action may be called at
     * whatever time and in whatever thread the element is made available by the
     * upstream operation.  If the action modifies shared state,
     * it is responsible for providing the required synchronization.
     *
     * @apiNote This method exists mainly to support debugging, where you want
     * to see the elements as they flow past a certain point in a pipeline:
     * <pre>{@code
     *     Stream.of("one", "two", "three", "four")
     *         .filter(e -> e.length() > 3)
     *         .peek(e -> System.out.println("Filtered value: " + e))
     *         .map(String::toUpperCase)
     *         .peek(e -> System.out.println("Mapped value: " + e))
     *         .collect(Collectors.toList());
     * }</pre>
     *
     * @param action a <a href="package-summary.html#NonInterference">
     *                 non-interfering</a> action to perform on the elements as
     *                 they are consumed from the stream
     * @return the new stream
     */
    Stream<T> peek(Consumer<? super T> action);

总结一下:接收一个消费型（Consumer）的接口，是一个中间操作，主要是用于debug的，可以进行二次的流处理

ForEach

/**
     * Performs an action for each element of this stream.
     *
     * <p>This is a <a href="package-summary.html#StreamOps">terminal
     * operation</a>.
     *
     * <p>The behavior of this operation is explicitly nondeterministic.
     * For parallel stream pipelines, this operation does <em>not</em>
     * guarantee to respect the encounter order of the stream, as doing so
     * would sacrifice the benefit of parallelism.  For any given element, the
     * action may be performed at whatever time and in whatever thread the
     * library chooses.  If the action accesses shared state, it is
     * responsible for providing the required synchronization.
     *
     * @param action a <a href="package-summary.html#NonInterference">
     *               non-interfering</a> action to perform on the elements
     */
    void forEach(Consumer<? super T> action);

总结一下，接收一个消费型的接口，然后无返回值，是一个终止操作，注意线程安全问题及集合遍历的顺序问题。

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二、示例

我们先创建一个集合用于测试

private List<String> languageList = new ArrayList<String>() {{
    add("java");
    add("python");
    add("c++");
    add("php");
    add("go");
}};

peek 方法中的函数式接口参数不能有返回值：

意味着它不能像 map 一样处理流中的元素然后形成新流：

peek 不能修改流中的元素，只能对元素进行打印输出或者其他外部处理操作。

但流元素如果是引用类型，peek 却可以达到 map 的效果：

private List<User> userList = new ArrayList<User>() {{
    add(new User("张三"));
    add(new User("李四"));
    add(new User("王五"));
    add(new User("赵六"));
}};

@Test
public void peekTest3() {
    userList.stream()
            .peek(user -> user.setName("peek: " + user.getName()))
            .forEach(System.out::println);
}

输出结果：

SteamPeekTest.User(name=peek: 张三)
SteamPeekTest.User(name=peek: 李四)
SteamPeekTest.User(name=peek: 王五)
SteamPeekTest.User(name=peek: 赵六)

虽然不能有返回值形成新的流，但却可以修改引用类型字段的值。

这也是建议的为什么要把 map 换成 peek 了，因为是引用类型，使用 peek 就没必要 set 之后还要进行 return 了。

List<Menu> children = all.stream().filter(...).map(
    (m) -> {
        m.setChildList(getChildrens(m, all));
        return m;
    }
).collect(Collectors.toList());

修改为：

List<Menu> children = all.stream().filter(...).peek(
    m -> m.setChildList(getChildrens(m, all))
).collect(Collectors.toList());

是不是优雅多了？

peek 和 foreach 有什么区别？

Foreach 和 peek 一样也是接收 Consumer 参数，不同是 foreach 没有返回参数，意味着 foreach 会中断流操作，只能用来遍历，不能再进行后续的流处理。

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三、小结

根据文中的示例，大家应该都搞清楚了 map、peek、foreach 的区别和用法了，现在再来总结下吧！

• map：用于对流中的每个元素进行映射处理，然后再形成新的流；
• peek：用于 debug 调试流中间结果，不能形成新的流，但能修改引用类型字段的值；
• foreach：用于遍历，会中断流操作；