class Account {

    public BigDecimal balance() {
        return activities.stream()
                .map(AccountActivity::getAmount)
                .reduce(initialBalance, BigDecimal::add);
    }

}

class AccountTest {

    @Test
    public void should_return_zero_no_recharge() {

        Account account = new Account("1", BigDecimal.ZERO);
        assertEquals(BigDecimal.ZERO, account.balance());
    }

}