关于Java8中map()和flatMap()的一些事
时间：2023-02-17 15:51:40|栏目：JAVA代码|点击： 次
两个方法的背景
这两个方法看起来做着同样的事情，但实际上又有些不一样。看源码部分是这样的
package java.util.stream;
map()方法
/** * @param <R> The element type of the new stream * @param mapper a <a href="package-summary.html#NonInterference" rel="external nofollow" rel="external nofollow" >non-interfering</a>, * <a href="package-summary.html#Statelessness" rel="external nofollow" rel="external nofollow" >stateless</a> * function to apply to each element * @return the new stream */ <R> Stream<R> map(Function<? super T, ? extends R> mapper);
flatMap()方法
/** * @param <R> The element type of the new stream * @param mapper a <a href="package-summary.html#NonInterference" rel="external nofollow" rel="external nofollow" >non-interfering</a>, * <a href="package-summary.html#Statelessness" rel="external nofollow" rel="external nofollow" >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);
Stream map() Method
看源码做推测，map是一种中间操作，返回的是Stream
代码测试
map()方法
public static void main(String[] args) {
System.out.println("Output with simple list");
List<String> vowels = Arrays.asList("A", "E", "I", "O", "U");
vowels.stream().map(vowel -> vowel.toLowerCase())
.forEach(value -> System.out.println(value));
List<String> haiList = new ArrayList<>();
haiList.add("hello");
haiList.add("hai");
haiList.add("hehe");
haiList.add("hi");
System.out.println("Output with nested List of List<String>");
List<String> welcomeList = new ArrayList<>();
welcomeList.add("You got it");
welcomeList.add("Don't mention it");
welcomeList.add("No worries.");
welcomeList.add("Not a problem");
List<List<String>> nestedList = Arrays.asList(haiList, welcomeList);
nestedList.stream().map(list -> {
return list.stream().map(value -> value.toUpperCase());
}).forEach(value -> System.out.println(value));
}
Output
Output with simple list a e i o u Output with nested List of List<String> java.util.stream.ReferencePipeline$3@3b9a45b3 java.util.stream.ReferencePipeline$3@7699a589
flatMap()方法
public static void main(String[] args) {
List<String> haiList = new ArrayList<>();
haiList.add("hello");
haiList.add("hai");
haiList.add("hehe");
haiList.add("hi");
System.out.println("Output with nested List of List<String>");
List<String> welcomeList = new ArrayList<>();
welcomeList.add("You got it");
welcomeList.add("Don't mention it");
welcomeList.add("No worries.");
welcomeList.add("Not a problem");
List<List<String>> nestedList = Arrays.asList(haiList, welcomeList);
nestedList.stream().flatMap(
list -> list.stream())
.map(value -> value.toUpperCase())
.forEach(value -> System.out.println(value));
}
Output
Output with nested List of List<String> HELLO HAI HEHE HI YOU GOT IT DON'T MENTION IT NO WORRIES. NOT A PROBLEM
Java 8 map() vs flatMap()
- map（）和flatMap（）方法都可以应用于Stream <T>和Optional <T>。 并且都返回Stream <R>或Optional <U>。
- 区别在于，映射操作为每个输入值生成一个输出值，而flatMap操作为每个输入值生成任意数量（零个或多个）的值。 在flatMap（）中，每个输入始终是一个集合，可以是List或Set或Map。 映射操作采用一个函数，该函数将为输入流中的每个值调用，并生成一个结果值，该结果值将发送到输出流。 flatMap操作采用的功能在概念上想消耗一个值并产生任意数量的值。 但是，在Java中，方法返回任意数量的值很麻烦，因为方法只能返回零或一个值。
代码
public static void main(String[] args) {
List<Stream> together = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List<Integer>
.map(List::stream)
.collect(Collectors.toList());
System.out.println("Output with map() -> "+together);
List<Integer> togetherFlatMap = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List<Integer>
.flatMap(List::stream)
.map(integer -> integer + 1)
.collect(Collectors.toList());
System.out.println("Output with flatMap() -> "+togetherFlatMap);
}
Output
Output with map() -> [java.util.stream.ReferencePipeline$Head@16b98e56, java.util.stream.ReferencePipeline$Head@7ef20235] Output with flatMap() -> [2, 3, 4, 5]
总结
上一篇：详解Jenkins 实现Gitlab事件自动触发Jenkins构建及钉钉消息推送
栏 目：JAVA代码
本文标题：关于Java8中map()和flatMap()的一些事
本文地址：http://www.codeinn.net/misctech/225897.html
