stream中的map,peek,foreach的区别

﹏ヽ暗。殇╰゛Y 2023-02-19 15:21 5阅读 0赞

在学习java8的stream api时，我们会遇到map,peek以及foreach这三种不同的处理方法，

其中map方法的签名及注释如下

/**
 * Returns a stream consisting of the results of applying the given
 * function to the elements of this stream.
 *
 * 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。

然后再来看一下peek方法的签名及注释

/**
 * 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.
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * 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

再来看一下forEach的签名及注释

/**
 * Performs an action for each element of this stream.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * The behavior of this operation is explicitly nondeterministic.
 * For parallel stream pipelines, this operation does not
 * 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。