Java 8 collecting the list that is already present in object

问题

I was just searching for better way to handle this scenario using java 8 streams. Object A has list of object b. What I get is a list of object A (List). I need to stream through list of object A and get all the listB's in each of the object A as a one single list.

class A {
    List<B> listB
}

I have tried the below way it throws compilation

List<A> as = someObject.getAs();
List<B> listofBs = as.stream().map(in -> in.getListB()).collect(Collectors.toList());

回答1:

To get a single list of all B's, you should use flatMap as:

List<B> listOfBs = listOfAs.stream()
        .flatMap(a -> a.getListB().stream())
        .collect(Collectors.toList());

回答2:

Class A{
  List<B> listB
};
List<A> listA;
listA.stream().map(
 a->{
  //some code for A
  a.ListB.stream().map(
   b->{
    // some code for B
  })
});

may be help you

来源：https://stackoverflow.com/questions/56014710/java-8-collecting-the-list-that-is-already-present-in-object