在Java 8中，Stream可以容纳不同的数据类型，例如：

Stream<String[]> Stream<Set<String>> Stream<List<String>> Stream<List<Object>>

但是，Stream操作（filter，sum，distinct …）和collectors不支持它，所以我们需要使用flatMap（）进行以下转换：

Stream<String[]> -> flatMap -> Stream<String> Stream<Set<String>> -> flatMap -> Stream<String> Stream<List<String>> -> flatMap -> Stream<String> Stream<List<Object>> -> flatMap -> Stream<Object>

如何flatMap()工作：

{{1,2}，{3,4}，{5,6}}  - > flatMap  - > {1,2,3,4,5,6}

{'a'，'b'}，{'c'，'d'}，{'e'，'f'}}  - > flatMap  - > {'a'，'b'，'c' D”， 'E'， 'F'}

Stream + String [] + flatMap

1.1以下示例将打印一个空的结果，因为filter()不知道如何过滤流String[]。

TestExample1.java

package com.mkyong.java8;  import java.util.Arrays; import java.util.stream.Stream;  public class TestExample1 {  public static void main(String[] args) {  String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};  //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);  }  }

Output

//空...

1.2在上面的例子中，我们应该使用flatMap()转换Stream<String[]>来Stream<String>。

TestExample1.java

package com.mkyong.java8;  import java.util.Arrays; import java.util.stream.Stream;  public class TestExample1 {  public static void main(String[] args) {  String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};  //Stream<String[]> Stream<String[]> temp = Arrays.stream(data);  //Stream<String>, GOOD! Stream<String> stringStream = temp.flatMap(x -> Arrays.stream(x));  Stream<String> stream = stringStream.filter(x -> "a".equals(x.toString()));  stream.forEach(System.out::println);  /*Stream<String> stream = Arrays.stream(data) .flatMap(x -> Arrays.stream(x)) .filter(x -> "a".equals(x.toString()));*/  }  }

Output

a

Stream + Set + flatMap

2.1学生POJO。

Student.java

package com.mkyong.java8;  import java.util.HashSet; import java.util.Set;  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  }

2.2 flatMap()和Set例子。

TestExample2.java

package com.mkyong.java8;  import java.util.ArrayList; import java.util.List; import java.util.stream.Collectors;  public class TestExample2 {  public static void main(String[] args) {  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)); }  }

Output

Spring Boot in Action
Effective Java (2nd Edition)
Java 8 in Action
Learning Python, 5th Edition

尝试评论
flatMap(x -> x.stream())
中
Collectors.toList()
会出现提示编译器错误，因为它不知道如何收集组对象的流。

Stream + Primitive + flatMapToInt

3.1对于原始类型，可以使用flatMapToInt。

TestExample3.java

package com.mkyong.java8; import java.util.Arrays; import java.util.stream.IntStream; import java.util.stream.Stream; public class TestExample3 { public static void main(String[] args) { 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)); } }

Output

1 
2 
3 
4 
5 
6