Exercise: Law of Demeter
This exercise applies the Law of Demeter. You will build a billing system where one object never reaches through another to touch a third.
Scenario
A Biller charges Customer objects. Each Customer owns a private Wallet. The Biller
calls customer.pay(amount) and receives a boolean result — it never calls
customer.getWallet().deduct(amount) or reads the wallet balance directly. Only Customer
touches its own Wallet. The biller is driven by a sequence of commands.
Commands
The program reads a list of commands, one per line, and prints one line of output per command.
| Command | Behavior | Output |
|---|---|---|
["customer", name, cash] | Register a new customer with the given integer cash balance | "Added <name>" |
["charge", name, amount] | Attempt to charge the named customer | "Charged <amount>, <name> has <remaining>" on success, "<name> cannot afford" if insufficient, or "<name> not found" if unknown |
Example
Input
6 customer Alice 100 customer Bob 30 charge Alice 40 charge Bob 50 charge Alice 80 charge Ghost 10
Output
Added Alice Added Bob Charged 40, Alice has 60 Bob cannot afford Alice cannot afford Ghost not found
Explanation
Alice starts with 100 and Bob with 30. The first charge deducts 40 from Alice, leaving 60. Bob cannot afford 50 so the charge is declined. Alice then tries to pay 80 but has only 60 remaining, so that charge is also declined. The final command names a customer who was never registered.
Your task
Implement Wallet.pay and Wallet.balance, then implement Customer.pay and Customer.balance as delegating wrappers, and finally fill in Biller.charge so it calls only customer methods.