Java 8 how to iterate over a nested list of objects

Question

I am new to Java 8 and i have a response object which has a list of detail object and the detail object contains a list of reason object where the reason object has a reason code. I am trying to iterate to the reason object from the response object and search if there is any reason code equal to a given key. Could you please help in how to do that in java 8, with minimum complexity. Below is sample way in java 7 not the best approach thou.

 if(response != null && CollectionsUtil.isNotEmpty(response.getDetails())) {

            List<Detail> details = response.getDetails();
            for(Detail det : details) {
                if(CollectionsUtil.isNotEmpty(det.getReasons())) {                  
                    for(Reason reason: det.getReasons()) {
                        if(StringUtils.equalsIgnoreCase("ABC", reason.getReasonCode())) {
                            ///////////call an external method
                        }
                    }

                    }
                }
            }

And what does the external method need? Does it need the detail instance? — fps
– fps, Commented Nov 13, 2018 at 19:38
The tests checking if the collections are empty are useless. Just loop through the collections. If they're empty the loop won't execute, whether or not you test if it's empty. — JB Nizet
– JB Nizet, Commented Nov 13, 2018 at 19:38
@JBNizet CollectionUtil.isNotEmpty() might perform a null check. — daniu
– daniu, Commented Nov 13, 2018 at 19:51
@daniu If so, the first thing to do would be to fix the design: a collection shouldn't be null, ever. That should be an invariant of the class. — JB Nizet
– JB Nizet, Commented Nov 13, 2018 at 19:55
@JBNizet I agree. A check still might have its place though in some cases, eg Jaxb deserialization initializes collection members to null. — daniu
– daniu, Commented Nov 13, 2018 at 20:03

GBlodgett · Accepted Answer · 2018-11-13 20:01:03Z

3

Assuming getReasons() returns a List:

details.stream().
     flatMap(e -> e.getReasons().stream()).
     filter(reason -> StringUtils.equalsIgnoreCase("ABC", reason.getReasonCode())).
     forEach(System.out::println);

Where you would replace System.out::println with the method you wanted to invoke. Also note that I removed the check of CollectionsUtil.isNotEmpty(det.getReasons()), as if the list is empty, it won't make any difference

answered Nov 13, 2018 at 20:01

GBlodgett

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4 Comments

Wonko the Sane Over a year ago

I was working on a very similar answer. If OP is just looking to see if there is any matching key to pass along, the filter and forEach could be replaced with a anyMatch call inside the if statement.

LunaticJape Over a year ago

May I know the reason to use flatMap? I think map(r->r.getDetails()).map(d->d.getReasons()) also works?

GBlodgett Over a year ago

@LunaticJape Yes both ways should work. However flatMap flattens it to a Stream of objects, as opposed to a Stream of Stream of objects

LunaticJape Over a year ago

@GBlodgett In this case it should be Stream<List> if I use map instead flatmap, right?

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