map
map() method -> Data Transformation
map() takes Stream as input and return Stream
Stream map(Stream input){}
代码语言：javascript复制 <R> Stream<R> map(Function<? super T, ? extends R> mapper);It's mapper function produces single value for each input value.hence it is also called One-To-One mapping.
这个方法比较好理解，把一个事物映射为另一个事物，是一对一的关系。
在没有stream.map()时，就在使用apache和guava的类似api
apache中的ListUtils
代码语言：javascript复制public static <E> List<E> transformedList(final List<E> list,final Transformer<? super E, ? extends E> transformer)guava中的Lists
代码语言：javascript复制public static <F, T> List<T> transform(List<F> fromList, Function<? super F, ? extends T> function)flatMap
flatMap() -> map() Flattering
flatMap() takes Stream<Stream> as input and return Stream
Stream map(Stream<Stream> input){}
代码语言：javascript复制<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);It's mapper function produces multiple value for each input value.hence it is also called One-To-Many mapping.
flattering
flatMap()其实是两个方法的合并，map()好理解，主要是flattering。
Before Flattening: [[t,u], [v,w,x], [y,x]]
After Flattening: [t,u,v,w,x,y,x]
其实就是把两层数组打平了。
实例
在stackoverflow上找的一个示例：
What's the difference between map() and flatMap() methods in Java 8?[1]
flatMap helps to flatten a Collection<Collection> into a Collection. In the same way, it will also flatten an Optional<Optional> into Optional.
代码语言：javascript复制public class Parcel {
String name;
List<String> items;
public Parcel(final String name, final String... items) {
this.name = name;
this.items = Arrays.asList(items);
}
public List<String> getItems() {
return items;
}
public static void main(final String[] args) {
final Parcel amazon = new Parcel("amazon", "Laptop", "Phone");
final Parcel ebay = new Parcel("ebay", "Mouse", "Keyboard");
final List<Parcel> parcels = Arrays.asList(amazon, ebay);
System.out.println("-------- Without flatMap() ---------------------------");
final List<List<String>> mapReturn = parcels.stream()
.map(Parcel::getItems)
.collect(Collectors.toList());
System.out.println("t collect() returns: " mapReturn);
System.out.println("n-------- With flatMap() ------------------------------");
final List<String> flatMapReturn = parcels.stream()
.map(Parcel::getItems)
.flatMap(Collection::stream)
.collect(Collectors.toList());
System.out.println("t collect() returns: " flatMapReturn);
}
}结果输出：
代码语言：javascript复制-------- Without flatMap() ---------------------------
collect() returns: [[Laptop, Phone], [Mouse, Keyboard]]
-------- With flatMap() ------------------------------
collect() returns: [Laptop, Phone, Mouse, Keyboard]
As you can see, with map() only:
•The intermediate type is Stream<List>•The return type is List<List>
and with flatMap():
•The intermediate type is Stream•The return type is List
References
[1] What's the difference between map() and flatMap() methods in Java 8?: https://stackoverflow.com/questions/26684562/whats-the-difference-between-map-and-flatmap-methods-in-java-8
[2] flatMap() Method in Java 8 : https://www.javatpoint.com/flatmap-method-in-java-8
