How to return the count, while using nested foreach loops in the stream

Question

I am using java8 streams to iterate two lists, In that one list contains some custom objects and another contains string.

With this, I have to call a method by passing custom object and sting as a input and then I have to get the count.

This is what I tried:

public int returnCode()  {
        /*int count = 0;
         * list.forEach(x -> {
            list2.forEach(p -> {
                count+ = myDao.begin(conn, x.getCode(), p);
            });
            return count;
        });*/   
    }

compiler is giving an error that count should be final.

Can anyone, give me how to do this in a better way.

Trivially, have you tried just declaring final int count = 0 instead of what you currently have? — Tim Biegeleisen
– Tim Biegeleisen, Commented Jun 15, 2018 at 2:51
What is the content of list1 and list2 intern? Which one holds objects and which one holds String ? — Ravindra Ranwala
– Ravindra Ranwala, Commented Jun 15, 2018 at 2:51
@Tim, I tried already, but the compiler is giving quick fiix that, remove final modifer again — Ram
– Ram, Commented Jun 15, 2018 at 3:03

Ousmane D. · Accepted Answer · 2018-06-15 07:09:39Z

1

What you're attempting to do is not possible as local variables accessed from a lambda must be final or effectively final i.e. any variable whose value does not change.

You're attempting to change the value of count in the lambda passed to the forEach hence the compilation error.

To replicate your exact code using the stream API, it would be:

int count = list.stream()
                .limit(1)
                .flatMapToInt(x -> list2.stream().mapToInt(p -> myDao.begin(conn, x.getCode(), p)))
                .sum();

However, if you want to iterate over the entire sequence in list and not just the first then you can proceed with the following:

int count = list.stream()
                .flatMapToInt(x -> list2.stream().mapToInt(p -> myDao.begin(conn, x.getCode(), p)))
                .sum();

answered Jun 15, 2018 at 7:09

Ousmane D.

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5 Comments

Ram Over a year ago

tried your code, giving invalid output 12 instead of 2; question updated.

Ousmane D. Over a year ago

@Creator don't vandalise the post by changing the context completely. if you've got a new problem then you need to put up a new question.

Ousmane D. Over a year ago

@Creator I've reverted the post to how it was initially. the first example snippet in my answer is the exact equivalent of the code in your post but using the stream API.

Ousmane D. Over a year ago

@Creator I didn't say to update your post. instead, I said to post a new question regarding your new problem . your initial problem has already been solved. post reverted again..

Ram Over a year ago

Please find the question here: stackoverflow.com/questions/50888506/…

Ravindra Ranwala · Accepted Answer · 2018-06-15 08:14:52Z

1

Lambdas mainly substitutes anonymous inner classes. Inside an anonymous inner class you can access only final local variables. Hence the same holds true with lambda expressions. Local variable is copied when JVM creates a lambda instance, hence it is counter intuitive to allow any update to them. So declaring the variable as final would solve the issue. But if you make it final you won't be able to do this, leading to another pitfall.

count+ = myDao.begin(conn, x.getCode(), p);

So your solution is not good and does not comply with lambda. So this will be a one way of doing it.

final int count = customObjects.stream()
    .mapToInt(co -> strings.stream().mapToInt(s -> myDao.begin(conn, co.getCode(), s)).sum())
    .sum();

edited Jun 15, 2018 at 8:14

answered Jun 15, 2018 at 2:53

Ravindra Ranwala

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1 Comment

Ram Over a year ago

tried your snippet; it is giving incorrect value as 12. I deally it should give 2 as output. question is updated.

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