source: class Account {
  
      public BigDecimal balance() {
          return activities.stream()
                  .map(AccountActivity::getAmount)
                  .reduce(initialBalance, BigDecimal::add);
      }
  
      public  Account(String number, BigDecimal initialBalance);
  
      public String getNumber();
      public List<AccountActivity> getActivities();
      public void recharge(Recharge recharge);
      public boolean consume(BigDecimal value);
      private boolean hasEnoughBalance(BigDecimal value);
  
  }
  
  class AccountTest {
  
      @Test
      public void should_return_zero_no_recharge() {
  
target:         Account account = new Account("1", BigDecimal.ZERO);
          assertEquals(BigDecimal.ZERO, account.balance());
      }
  
  }