Stream zamiast forEach

Bez użycia streama (trzeba definiować listę obiektów):

private List buildTransactions(AccountNumber accountNumber) {
    List transactions = new ArrayList<>();
    accountNumber
        .getAccountHistoryList()
        .forEach(accountHistory -> transactions.add(transactionBuilder.build(accountHistory)));

    return transactions;
}

Używając streama możemy tworzenie listy obiektów pominąć:

private List buildTransactions(AccountNumber accountNumber) {
    return accountNumber
        .getAccountHistoryList()
        .stream()
        .map(accountHistory -> transactionBuilder.build(accountHistory))
        .collect(Collectors.toList());
}

Update: Można jeszcze bardziej elegancko:

private List buildTransactions(AccountNumber accountNumber) {
    return accountNumber
        .getAccountHistoryList()
        .stream()
        .map(transactionBuilder::build)
        .collect(Collectors.toList());
}

Optional.ofNullable().orElseThrow()

Przykład wykorzystania klasy Optional dostępnej od Java 8.

Bez Optionala

public void test(Task task) throws VerificationException {
    if (task.getCustomer() == null) {
        logger.error(CUSTOMER_NOT_FOUND);
        throw BmVerificationException.badRequest(CUSTOMER_NOT_FOUND);
    }
}

Z Optionalem

public void test(Task task) throws VerificationException {
    Optional.ofNullable(task.getCustomer()).orElseThrow(this::getException);
}

private VerificationException getException() {
    logger.error(CUSTOMER_NOT_FOUND);
    return VerificationException.badRequest(CUSTOMER_NOT_FOUND);
}