Update param in list1<object> from list2<object> by using java 8

Question

I have two list of objects accounts and salaries and I need to iterate the list of objects. If the id matches I need to update the account object.

I have list1 and list2 these two objects are different object type. we need to update the object(param) in list1 with list2 object(param).

Example

if(accounts !=null && salaries!=null) { // checking for nulls
    for (Account obj1 : accounts) {// iterating objects
        for (Salary obj2 : salaries) {
            String id = ibj2.getId();
            if (id.equals(obj1.getId())) {// id checks  
                obj1.setxxxx(obj2.getxxxx());// set the value
            }
        }
    }
}

I tried:

list1.stream().flatMap(x -> list2 .stream() .filter(y -> x.getId().equals(y.getId())));

// i have tried this list1.stream().flatMap(x -> list2 .stream() .filter(y -> x.getId().equals(y.getId()))); — Raju Sidda
– Raju Sidda, Commented May 22, 2019 at 6:49
In my opinion, it's no reason to rewrite this iterative code to java 8 streams, because it doesn't get you any performance benefits, but only complicate the readability. — Golov Pavel
– Golov Pavel, Commented May 22, 2019 at 6:58
@GolovPavel, that is unless the lists are big, in which case, parallel streaming will benefit here — Sharon Ben Asher
– Sharon Ben Asher, Commented May 22, 2019 at 7:00
@Sharon Ben Asher, ok, if the list is big, it's may be useful — Golov Pavel
– Golov Pavel, Commented May 22, 2019 at 7:09
@GolovPavel, have a look at my answer and tell me how's the readability — Sharon Ben Asher
– Sharon Ben Asher, Commented May 22, 2019 at 7:11

Eran · Accepted Answer · 2019-05-22 06:56:25Z

4

Your flatMap (suggested in the comment), will produce a Stream<Salary>, which won't allow you do modify the corresponding Account instances.

You can create a Stream of Accounts and their corresponding Salary and run forEach on that Stream:

accounts.stream()
        .flatMap(a->salaries.stream()
                            .filter(s -> s.getID().equals(a.getID())
                            .map(s -> new SimpleEntry<Account,Salary)(a,s)))
        .forEach(e -> e.getKey().setxxxx(e.getValue().getxxxx()));

answered May 22, 2019 at 6:56

Eran

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Comments

Sharon Ben Asher · Accepted Answer · 2019-05-22 07:01:50Z

3

The final operation, obj1.setxxxx(obj2.getxxxx()); requires to have both obj1 and obj2. that dictates the item that is streamed from both lists

list1.stream()
    .forEach(obj1 -> 
        list2.stream()
            .filter(obj2 -> obj1.getId().equals(obj2.getId()))
            .findFirst()
            .ifPresent(obj2 -> obj1.setxxxx(obj2.getxxxx()))
    );

edited May 22, 2019 at 7:01

answered May 22, 2019 at 6:54

Sharon Ben Asher

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Comments

M.F · Accepted Answer · 2019-05-22 07:07:56Z

3

I would always suggest to create a Map since the lookup cost will decrease and it will become more readable.

Map<String, List<Salary>> salaryById = salaries.stream().collect(Collectors.groupingBy(Salary::getId));
accounts.forEach(a -> CollectionUtils.emptyIfNull(salaryById.get(a.getId())).forEach(s -> s.setxxxx(..)));

In case Account Salary <-> Account is One to One you change grouping to Collectors.toMap(..)

answered May 22, 2019 at 7:07

M.F

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