Calculate sum using Java Stream API grouping by field

Question

You have two classes:

Account: number: String, balance: Long
Transaction: uuid: String, sum: Long, account: Account

Both classes have getters for all fields with the corresponding names (getNumber(), getSum(), getAccount() and so on).

I need to calculate sum of transaction for every account, but exactly not by Account, but grouping by Account.number

I make it like this:

Map<Account, Long> totalSumOfTransByEachAccount =
            transactions.stream()
                    .collect(Collectors.groupingBy(Transaction::getAccount, Collectors.reducing(0, Transaction::getSum, Long::sum)));

But I need map with String key - Account.getNumber()

Map<String, Long> totalSumOfTransByEachAccount =
            transactions.stream()
                    .collect(Collectors.  ??????)

Can anyone help me?

IvanSPb · Accepted Answer · 2018-02-13 21:27:54Z

4

One more variant, my decision:

 Collectors.groupingBy(t -> t.getAccount().getNumber(),
                    Collectors.reducing(0L, Transaction::getSum, Long::sum))

answered Feb 13, 2018 at 21:27

IvanSPb

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1 Comment

Ousmane D. Over a year ago

The Collectors.reducing factory method is a generalization of pretty much majority of the collectors, however, I'd go with Collectors.summingLong(Transaction::getSum) as suggested in my answer instead of Collectors.reducing(0L, Transaction::getSum, Long::sum) for readability and maybe has better performance as opposed to the long-winded reducing. anyway +1 for coming up with your own answer ;-)

Ousmane D. · Accepted Answer · 2018-02-13 21:50:40Z

2

You can do so with:

Map<String, Long> resultSet = transactions.stream()
                .collect(Collectors.groupingBy(t -> t.getAccount().getNumber(),
                               Collectors.summingLong(Transaction::getSum)));

edited Feb 13, 2018 at 21:50

answered Feb 13, 2018 at 21:12

Ousmane D.

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2 Comments

IvanSPb Over a year ago

Thank you very much ) Maybe it is possible to do it with reduce?

Ousmane D. Over a year ago

@IvanSPb if the solution has solved your question then please feel free to accept the answer.

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