java stream flatmap

返回	调用
<R> Stream<R>	flatMap(Function<? super T,? extends Stream<? extends R>> mapper)

参数类型是T
返回值是Stream 且 Stream的泛型为R(在spark中flatmap的返回值是iterator)

1.内部类实现方式,写法一:匿名内部类
这里Function的泛型<T,R>变成了<String,String>
重写了apply方法,参数类型T 变成了String,返回值类型R也变成了String

    public static void main(String[] args) throws IOException {
    
    
        List<String> list1 = Arrays.asList("1","2","3");
        list1.stream().flatMap(
                new Function<String, Stream<? extends String>>() {
    
    
                    @Override
                    public Stream<? extends String> apply(String s) {
    
    
                        return Arrays.asList("a", s).stream();
                    }
                }
        ).forEach(System.out::println);
    }

2.内部类实现方式,写法二:内部类

    public static void main(String[] args) throws IOException {
    
    
        List<String> list1 = Arrays.asList("1","2","3");
        list1.stream().flatMap(
            new Fun()
        ).forEach(System.out::println);
    }

    static class Fun implements Function<String, Stream<? extends String>>{
    
    //实现接口时,泛型是写在接口上
        @Override
        public Stream<? extends String> apply(String s) {
    
    
            return Arrays.asList("a", s).stream();
        }
    }

3.使用lambda
lambda的本质就是Function内部类

    public static void main(String[] args) throws IOException {
    
    
        List<String> list1 = Arrays.asList("1","2","3");
        list1.stream().flatMap(
            x -> Arrays.asList("a",x).stream()
        ).forEach(System.out::println);
    }