java streams - 如何使用键上的条件来平移集合映射中的所有值

Question

I have a Map. Let's say

Map<Long, List<MyObj>> 

I want to create a long array where of all MyObj where the key (long) is found in another set()

anotherSet.contains(long)

using java stream.

I tried

map.entrySet()
   .stream()
   .filter(e->anotherSet(e.getKey()))
   .flatMap(e.getValue)
   .collect(Collectors.toList);

But it doesnt even compile

Answer 1

You had a few syntax errors.

This should produce your desired List :

List<MyObj> filteredList = 
    map.entrySet()
       .stream()
       .filter(e->anotherSet.contains(e.getKey())) // you forgot contains
       .flatMap(e-> e.getValue().stream()) // flatMap requires a Function that 
                                           // produces a Stream
       .collect(Collectors.toList()); // you forgot ()

If you want to produce an array instead of a List , use :

MyObj[] filteredArray = 
    map.entrySet()
       .stream()
       .filter(e->anotherSet.contains(e.getKey()))
       .flatMap(e-> e.getValue().stream())
       .toArray(MyObj[]::new);