java8-stream-flatMap方法简单使用

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简述

在JDK8开始，列表操作新增了一新特性Stream流式操作，Stream中提供了非常多的API供大家使用，灵活的使用这些API，可以非常的方便且优美的实现我们的代码逻辑；

其中，flatMap()方法也是其中的API，flatMap()简单来说就是将多个stream流合并成一个stream，流变得扁平化， 当然你也可以在合并的时候结合其他的API做一些过滤或者转换之类的，如filter，下面详细阐述一下。

案例

1、先来一个简单的使用案例：
String[][]生成流,不用flatMap扁平化处理，依然可以使用 filter,map,collect对流做操作

public class StringArrayFlatMap {
    public static void main(String[] args) {
        String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};
        String[] s = new String[] {"a","b"};
        System.out.println(Arrays.toString(s));

        //Stream<String[]>
        Stream<String[]> temp = Arrays.stream(data);
        //filter a stream of string[], and return a string[]?
        List<String[]> list = temp
                .filter(x -> "a".equals(x[0]))
                .collect(Collectors.toList());

        list.forEach(c-> System.out.println(Arrays.toString(c)));
    }
}

2、List<Set<String>>案例：
Stream<Set<String>>，不用flatMap扁平化处理，依然可以使用 filter,map,collect对流做操作

public class StringSetFlatMap {
    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);
        //filter a stream of string[], and return a string[]?

        List<Set<String>> collect = list.stream()   //Stream<Student>
                .map(Student::getBook)              //Stream<Set<String>>
                .filter(set -> set.contains("Java 8 in Action")) //Stream<Set<String>>
                .distinct()
                .collect(Collectors.toList());
        collect.forEach(System.out::println);
    }
}

3、List<List<String>>案例：
Stream<List<String>>，flatMap扁平化处理流

public class StreamFlatMap {
    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);

        Stream<String> stringStream = list.stream()   //Stream<Student>
                .map(Student::getBook)      //Stream<Set<String>>
                .flatMap(x -> x.stream())   //Stream<String>
                .distinct(); //Stream<String>
        stringStream.forEach(System.out::println);

        List<String> collect =
                list.stream()                       //Stream<Student>
                        .map(Student::getBook)      //Stream<Set<String>>
                        .flatMap(x -> x.stream())   //Stream<String>
                        .distinct()                 //Stream<String>
                        .collect(Collectors.toList());

        collect.forEach(x -> System.out.println(x));

    }
}

4、flatMapToInt 案例

public class StreamIntFlatMap {
    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(Arrays::stream);

        intStream.forEach(System.out::println);

    }
}

参考

https://blog.youkuaiyun.com/Hatsune_Miku_/article/details/73435618

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

{[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']