引言
在 java8 中，Stream 可以保存不同的数据类型，例如:
Stream<String[]>
Stream<Set<String>>
Stream<List<String>>
Stream<List<Object>>
但是，流操作(filter、 sum、 distinct…)和收集器不支持它，因此，我们需要 flatMap ()来执行以下转换:
Stream<String[]> -> flatMap -> Stream<String>
Stream<Set<String>> -> flatMap -> Stream<String>
Stream<List<String>> -> flatMap -> Stream<String>
Stream<List<Object>> -> flatMap -> Stream<Object>
Flatmap ()是如何工作的:
{ {1,2}, {3,4}, {5,6} } -> flatMap -> {1,2,3,4,5,6}
1.Stream + String [] + flatMap
下面的示例将打印一个空结果，因为 filter ()不知道如何过滤 String []流。
private static void one() {
//Stream<String[]>
Stream<String[]> temp = Arrays.stream(data);
//filter a stream of string[], and return a string[]?
Stream<String[]> stream = temp.filter(x -> "a".equals(x.toString()));
stream.forEach(System.out::println);
}
在上面的例子中，我们应该使用 flatMap ()将流字符串[]转换为流字符串。
private static void one() {
//Stream<String[]>
Stream<String[]> temp = Arrays.stream(data);
//filter a stream of string[], and return a string[]?
// Stream<String[]> stream = temp.filter(x -> "a".equals(x.toString()));
// stream.forEach(System.out::println);
// Stream<String>, GOOD!
Stream<String> stringStream = temp.flatMap(x -> Arrays.stream(x));
Stream<String> stream1 = stringStream.filter(x -> "a".equals(x.toString()));
stream1.forEach(System.out::println);
}
输出：
a
2.Stream + Set + flatMap
public class Student {
private String name;
private Set<String> book;
public void addBook(String book) {
if (this.book == null) {
this.book = new HashSet<>();
}
this.book.add(book);
}
//getters and setters
}
private static void two() {
Student obj1 = new Student();
obj1.setName("mkyong");
obj1.addBook("Java 8 in Action");
obj1.addBook("Spring Boot in Action");
obj1.addBook("Effective Java (2nd Edition)");
Student obj2 = new Student();
obj2.setName("zilap");
obj2.addBook("Learning Python, 5th Edition");
obj2.addBook("Effective Java (2nd Edition)");
List<Student> list = new ArrayList<>();
list.add(obj1);
list.add(obj2);
List<String> collect = list.stream().map(x -> x.getBook()) //Stream<Set<String>>
.flatMap(x -> x.stream()) //Stream<String>
.distinct()
.collect(Collectors.toList());
collect.forEach(x-> System.out.println(x));
}
输出：
Spring Boot in Action
Effective Java (2nd Edition)
Java 8 in Action
Learning Python, 5th Edition
尝试注释 flatMap (x-x.stream ()) ，Collectors.toList ()将提示一个编译器错误，因为它不知道如何收集 Set 对象流。
3.Stream + Primitive + flatMapToInt
对于基本类型，可以使用 flatMapToInt。
private static void three() {
int[] intArray = {1, 2, 3, 4, 5, 6};
//1. Stream<int[]>
Stream<int[]> streamArray = Stream.of(intArray);
//2. Stream<int[]> -> flatMap -> IntStream
IntStream intStream = streamArray.flatMapToInt(x -> Arrays.stream(x));
intStream.forEach(x -> System.out.println(x));
}
输出：
1
2
3
4
5
6
源码见：java-8-demo
系列文章详见：Java 8 教程
文档信息
- 本文作者：Zhang jinmiao
- 本文链接：http://zhangjinmiao.github.io/java8/2019/08/13/Java-8-flatMap-%E7%A4%BA%E4%BE%8B.html
- 版权声明：自由转载-非商用-非衍生-保持署名（创意共享3.0许可证）