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Java8中map()方法和flatMap()方法的使用？针对这个问题，这篇文章详细介绍了相对应的分析和解答，希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。
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$3@7699a589
flatMap()方法
public static void main(String[] args) {
List<String> haiList = new ArrayList<>();
haiList.add("hello");
haiList.add("hai");
haiList.add("hehe");
haiList.add("hi");
System.out.println("Output with nested List of List<String>");
List<String> welcomeList = new ArrayList<>();
welcomeList.add("You got it");
welcomeList.add("Don't mention it");
welcomeList.add("No worries.");
welcomeList.add("Not a problem");
List<List<String>> nestedList = Arrays.asList(haiList, welcomeList);
nestedList.stream().flatMap(
list -> list.stream())
.map(value -> value.toUpperCase())
.forEach(value -> System.out.println(value));
}Output
Output with nested List of List<String> HELLO HAI HEHE HI YOU GOT IT DON'T MENTION IT NO WORRIES. NOT A PROBLEM
Java 8 map() vs flatMap()
代码
public static void main(String[] args) {
List<Stream> together = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List<Integer>
.map(List::stream)
.collect(Collectors.toList());
System.out.println("Output with map() -> "+together);
List<Integer> togetherFlatMap = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List<Integer>
.flatMap(List::stream)
.map(integer -> integer + 1)
.collect(Collectors.toList());
System.out.println("Output with flatMap() -> "+togetherFlatMap);
}Output
Output with map() -> [java.util.stream.ReferencePipeline$Head@16b98e56, java.util.stream.ReferencePipeline$Head@7ef20235] Output with flatMap() -> [2, 3, 4, 5]
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