How to iterate through nested java collections using lambda

Question

I read through several threads about operating on the nested java collections using Lambda but none addressed my specific situation, although this one came close and then took off to a different direction (flatMap). Please show me how to write the following code in Lambda.

 for(AppUser user : users){
     List<CustomerOrder> orders = user.getOrders();
     for(CustomerOrder order : orders){
             order.setConsumer(user);
             List<LineItem> items = order.getLineItems();
             for (LineItem item : items){
                  item.setOrder(order);
             }
     }
 }

Thanks

Your loop is modifying the elements it loops through. I'd say it's not really a good candidate for streams. IMHO it's ok as it is. — Federico klez Culloca, Mar 05 '21 at 13:22
@TonyHimself if you found a solution you could help future readers by answering your own question. — Federico klez Culloca, Mar 05 '21 at 13:31
First I need to test it then I will publish the final solution — Tony Himself, Mar 05 '21 at 13:33

score 0 · Accepted Answer · answered Mar 05 '21 at 13:35

If all you want to do is iterate, the forEach method on either the Stream.java or the Iterable.java should be enough. Here is the "streamy" way of doing what you are trying to do with the for loops

users.stream().forEach( user ->
        user.getOrders().stream().forEach( order -> {
            order.setConsumer(user);
            order.getLineItems().stream().forEach(
                    lineItem -> lineItem.setOrder(order)
            );
        }
));

Although you don't really need to convert the iterable returned by by those getters to streams.

Smarth, since you took the time to work on the solution, I will check yours as an answer — Tony Himself, Mar 05 '21 at 13:42

score 0 · Answer 2 · answered Mar 05 '21 at 13:40

This is a solution I came up with but I see that Smarth Ktyal already posted an answer.

users
                    .stream()
                    .forEach(e->e.getOrders()
                            .stream()
                            .forEach(k->{
                                k.setConsumer(e);
                                k.getLineItems()
                                    .stream()
                                    .forEach(l->l.setOrder(k));
                                }
                            )
                    );

How to iterate through nested java collections using lambda

2 Answers2