Java 8 lambdas grouping reducing and mapping

Question

Given a List of the following Transaction class, using Java 8 lambdas, I want to obtain a List of ResultantDTO, one per account type.

public class Transaction {

    private final BigDecimal amount;
    private final String accountType;
    private final String accountNumber;

}

public class ResultantDTO {

    private final List<Transaction> transactionsForAccount;

    public ResultantDTO(List<Transaction> transactionsForAccount){
        this.transactionsForAccount = transactionsForAccount;
    }

}

So far, I use the following code to group the List<Transaction> by accountType.

Map<String, List<Transaction>> transactionsGroupedByAccountType = transactions
    .stream()
    .collect(groupingBy(Transaction::getAccountType));

How do I return a List<ResultantDTO>, passing the List from each map key into the constructor, containing one ResultantDTO per accountType?

Answer 1

You can do this in single stream operation:

public List<ResultantDTO> convert(List<Transaction> transactions) {
    return transactions.stream().collect(
            collectingAndThen(
                groupingBy(
                        Transaction::getAccountType,
                        collectingAndThen(toList(), ResultantDTO::new)),
                map -> new ArrayList<>(map.values())));
}

Here collectingAndThen used twice: once for downstream Collector to convert lists to the ResultantDTO objects and once to convert the resulting map to list of its values.