Java 8 stream compare two objects and run a function on them

Question

I've a a stream which I want to partition into smaller parts based on matching Id and then apply some proccessing logic on each of the part/element.

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

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

I want to segregate and create a list of Parts of size 10 when the partId of different parts are same.

How to use the filter or reduce or groupingBy function to compare the two elements and put them to a list? I've tried filter like below, doesn't take p1 variable:

big.stream().filter( p -> p.getPartId() == p1.getPartId()) //error

Tried groupingBy like this

big.stream().collect(Collectors.groupingBy(Parts::getPartId) //error

I want to iterate over the filtered/reduced list a another and call another function called abc(). How can I do it using Java Streams?

pseudo:

big.getParts().stream.
//dividing logic logic
for(i < parts.size)
    abc(p)

Thanks

Answer 1

You might use something like this:

 Map<String,List<Parts>> commonId = big.getParts().
                stream().
                collect(
                        Collectors.groupingBy(
                            Parts::getPartId,
                            Collectors.mapping(
                                Function.identity(),
                                Collectors.toList()
                            )
                        )
                ); 

and after it, you will just need to iterate over the map and apply your function. commonId.entrySet().stream().map(entry -> apply(entry))...

Updated We can omit Collectors.mapping(Function.identity(),Collectors.toList()) part, since it is a default behaviour of groupingBy

Map<String,List<Parts>> commonId = big.getParts().
                    stream().
                    collect(
                            Collectors.groupingBy(
                                Parts::getPartId
                            )
                    );