Split a collection of objects based on a condition

问题

I have a list of objects to be added in a bag and bags capacity is 100 qty

The Object and Bag look like below

public class MyObject{
    String id;
    int qty;
}

public class MyBag{
    String id;
    int qty;
}

Is there any way to split MyObject in multiple MyBags grouping on the qty limit using Java 8 streams

For example: myObjects are

[myObject1:{id1, 150}, 
 myObject2:{id2, 30}, 
 myObject3:{id3, 150}]

Since bag has a capacity of 100. Bags should be grouped as

[ bag1:[{id1, 100}], 
  bag2:[{id1, 50},{id2, 30},{id3, 20}], 
  bag3:[{id3, 100}], 
  bag4:[{id3, 30}]]

回答1:

As it was maintained in comments the solution won't be easy. Below is an example that is close to what you want. First I used only first class MyObject and change it a bit.

public class MyObject{
    String id;
    int qty;

    public MyObject(String id, int qty) {
        this.id = id;
        this.qty = qty;
    }

    @Override
    public String toString() {
        return "MyObject{" +
                "id='" + id + '\'' +
                ", qty=" + qty +
                '}';
    }
}

Init code:

List<MyObject> myObjects = Arrays.asList(
        new MyObject("id1", 150),
        new MyObject("id2", 30),
        new MyObject("id3", 150)
);

And the main code. In flatMap there is splitting MyObject with large qty. After that objects are collected to lists with given limit. The output isn't exactly as you want. The third list includes two objects with identifier id3.

[MyObject{id='id1', qty=100}]
[MyObject{id='id1', qty=50}, MyObject{id='id2', qty=30}, MyObject{id='id3', qty=20}]
[MyObject{id='id3', qty=80}, MyObject{id='id3', qty=20}]
[MyObject{id='id3', qty=30}]

LinkedList<List<MyObject>> firstCollection = myObjects.stream()
    .flatMap(o -> o.qty < 100 ? Stream.of(o) : IntStream.range(0, (o.qty / 100 + 1))
        .mapToObj(x -> new MyObject(o.id, x < o.qty / 100 ? 100 : o.qty % 100))
    )
    .collect(
        Collector.of(
            LinkedList::new,
            (a, b) -> Optional.of(a).filter(Predicate.not(List::isEmpty))
                    .map(l -> l.getLast().stream().map(o -> o.qty).reduce(Integer::sum).get())
                    .filter(lastSum -> lastSum < 100)
                    .ifPresentOrElse(lastSum -> {
                        int maxQty = 100 - lastSum;
                        if (maxQty >= b.qty) {
                            a.getLast().add(b);
                        } else {
                            a.getLast().add(new MyObject(b.id, maxQty));
                            a.add(new ArrayList<>(List.of(new MyObject(b.id, b.qty - maxQty))));
                        }
                    }, () -> a.add(new ArrayList<>(List.of(b))))
            ,
            (a, b) -> { throw new UnsupportedOperationException();}
        )
    );
firstCollection.forEach(System.out::println);

来源：https://stackoverflow.com/questions/59701676/split-a-collection-of-objects-based-on-a-condition