java stream flatmap

心已赠人 2022-10-31 01:42 154阅读 0赞

<table> 
 <thead> 
  <tr> 
   <th>返回</th> 
   <th>调用</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>&lt;R&gt; Stream&lt;R&gt;</code></td> 
   <td><code>flatMap(Function&lt;? super T,? extends Stream&lt;? extends R&gt;&gt; mapper)</code></td> 
  </tr> 
 </tbody> 
</table>

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);
        }