How to group and count items in a List using the Java stream API

Question

I have the following OrderItems class:

public class OrderItems {
    public int id;
    public double portion;
}

and the following List<OrderItems>:

List<OrderItems> orderItems = new ArrayList<>();
orderItems.add(new OrderItems(1,0.5));
orderItems.add(new OrderItems(2,1));
orderItems.add(new OrderItems(1,0.5));
orderItems.add(new OrderItems(1,1.5));
orderItems.add(new OrderItems(2,1));
orderItems.add(new OrderItems(2,0.5));

How can I group this list and convert to the following NewOrderItems class with the stream API?

public class NewOrderItems {
   public int id;
   public double portion;
   public long amount;
}

The result should be like this:

NewOrderItems{id=1, portion=0.5, amount=2}
NewOrderItems{id=1, portion=1.5, amount=1}
NewOrderItems{id=2, portion=0.5, amount=1}
NewOrderItems{id=2, portion=1.0, amount=2}

Answer 1

My solution like this :

Map<Integer, Map<Double, Long>> collect = orderItems.stream()
                    .collect(Collectors.groupingBy(OrderItems::getId, Collectors.groupingBy(OrderItems::getPortion, Collectors.counting())));
            System.out.println(collect);

            List<NewOrderItems> newOrderItems = new ArrayList<>();

            collect.forEach((k,v)->{
                    v.forEach((x,y)->{
                           NewOrderItems newOrderitems=new NewOrderItems(k,x,y);
                           newOrderItems.add(newOrderitems);
                    });
            });
            newOrderItems.forEach(System.out::println);

Console:

NewItems{id=1, portion=0.5, amount=2}
NewItems{id=1, portion=1.5, amount=1}
NewItems{id=2, portion=0.5, amount=1}
NewItems{id=2, portion=1.0, amount=2}