Java 8 flatMap示例

妖狐艹你老母 2023-02-15 05:05 38阅读 0赞

在Java 8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，Stream操作（过滤器，求和，不同…）和收集器不支持它，因此，我们需要[flatMap（）][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>

`flatMap()`如何工作：

{ {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'}

## 1.流+字符串\[\] + flatMap ##

1.1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。

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);
    
        }
    
    }

输出量

//empty...

1.2在上面的示例中，我们应该使用`flatMap()`将`Stream<String[]>`转换为`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()));*/
    
        }
    
    }

输出量

## 2.流+设置+ 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`示例。

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));
        }
    
    }

输出量

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

尝试注释一下`flatMap(x -> x.stream())` `Collectors.toList()`将提示编译器错误，因为它不知道如何收集Set对象的流。

## 3. Stream + Primitive + flatMapToInt ##

3.1对于基本类型，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));
    
        }
    
    }

输出量

1
    2
    3
    4
    5
    6

## 参考文献 ##

1.  [Stream＃flatMap JavaDoc][flatMap]
2.  [Stackoverflow – Java 8中的map和flatMap方法之间的区别][Stackoverflow _ Java 8_map_flatMap]
3.  [Java 8 –如何打印数组][Java 8]
4.  [Java 8 –收集器分组依据和映射示例][Java 8 1]

标签： [flatMap][flatMap 1] [java8][] [流][Link 1]

> 翻译自: [https://mkyong.com/java8/java-8-flatmap-example/][https_mkyong.com_java8_java-8-flatmap-example]

[flatMap]: http://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#flatMap-java.util.function.Function-
[Stackoverflow _ Java 8_map_flatMap]: http://stackoverflow.com/questions/26684562/difference-between-map-and-flatmap-methods-in-java-8
[Java 8]: http://www.mkyong.com/java/java-how-to-print-an-array/
[Java 8 1]: http://www.mkyong.com/java8/java-8-collectors-groupingby-and-mapping-example/
[flatMap 1]: https://mkyong.com/tag/flatmap/
[java8]: https://mkyong.com/tag/java8/
[Link 1]: https://mkyong.com/tag/stream/
[https_mkyong.com_java8_java-8-flatmap-example]: https://mkyong.com/java8/java-8-flatmap-example/