根据不同的参数将Collection（使用Java 8流）拆分为较小的组件

Question

This is a continuation of this question Java 8 stream compare two objects and run a function on them

Class BigRequest{
    String bId;
    List<Parts> parts;
    //getters and setter here
    }

Class Parts{
String pId;
String partId;
//getters and setter here
}

My question is to split a Collection(using java 8 streams) into smaller components based on some dynamic parameters. The Dynamic params can change based on different conditions (here, Ive given example as getPartId() being the same) passed to the method splitParts, whose return type is List of Set of Parts

List<Set<Parts>> splitParts(BigRequest big){}

I've tried the following:

 //groupingBy function but that returns a Map 
   Map<String, List<Parts>> parts = big.getParts().stream().collect(Collectors.groupingBy(Parts::getPartId));

//doesn't do it accurately
    big.getParts().stream().filter(p -> p.getPartId.equals(p.getPartId())).collect();

How do I split a stream based on the dynamic params(maybe by calling a functional interface over reduce method) and then run a method over it?

My function's signature is

List<Set<Parts>> splitParts(BigObject b){}

I assume doing something like this might help to keep the splitting condition dynamic, but Im not sure how to do it?

 big.getParts().stream().reduce(MyFuncInterface::divide);

and then using forEach(), I think i can iterate over the List and call the abc() method.

How can I do it better?

Any help here is greatly appreciated.

Answer 1

If you want a List<Set<Parts>> , first change your code to get a Map<String, Set<Parts>> :

Map<String, Set<Parts>> parts = big.getParts().stream().collect(
        Collectors.groupingBy(Parts::getPartId, Collectors.toSet()));

Calling parts.values() will return a Collection<Set<Parts>> , so convert the Collection to a List :

List<Set<Parts>> list = new ArrayList<>(parts.values());

Or you could just re-stream it, so you can do it in a single call chain:

List<Set<Parts>> list = big.getParts()
                           .stream()
                           .collect(Collectors.groupingBy(Parts::getPartId,
                                                          Collectors.toSet()))
                           .values()
                           .stream()
                           .collect(Collectors.toList());