Public and Private Interfaces in ruby
Your latest client is a bank, and they’ve tasked you with requiring customers to enter their password in order to make withdrawals.
Currently, this is what they’ve got:
class Customer
attr_reader :funds def initialize(funds, password)
@funds = funds
@password = password
end def remove_funds(amount)
@funds -= amount
end
end
Let’s break that apart. You can paste that whole class into irb to follow along.
When a customer is initialized, it receives a specified amount of funds and a password is set.
diego = Customer.new(500, "udacious")
# => #<Customer:0x007fcdb48ca5a8 @funds=500 @password="udacious">
Thanks to the attr_reader
, you can see the value of his current funds.
diego.funds
# => 500
And the remove_funds
method allows funds to be removed from the customer’s account.
Checking on the funds again confirms this.
diego.remove_funds(50)
# => 450
diego.funds
# => 450
These methods, funds
and remove_funds
, are part of the Customer
class’ API, or application programming interface.
An API is, according to Wikipedia, “a set of routines, protocols, and tools for building software applications”.
Well, that’s vague.
“API” is a popular term in recent years, but many people use it without quite understanding what it means. Think of methods like remove_funds
as your way of interfacing with the Customer
class. These methods are the keys to accessing information about a particular customer.
There isn’t currently a way to access the @password
instance variable.
It could be said that the customer’s password can’t be accessed by the customer’s public API.
In this situation, that’s a good thing! You don’t want information like a password to be publicly available to other objects.
Let’s implement a method called withdraw_securely
, which takes two arguments, amount
andpassword
.
If the password entered matches the customer’s password, go ahead and remove the funds. Otherwise, nothing happens.
class Customer
attr_reader :funds def initialize(funds, password)
@password = password
@funds = funds
end def remove_funds(amount)
@funds -= amount
end def withdraw_securely(amount, password)
if password == @password
remove_funds(amount)
end
end
end
Play around with this in irb to see it in action.
diego.withdraw_securely(50, "udacious")
# => 400
diego.withdraw_securely(100, "wrong password")
# => nil
diego.funds
# => 400
✨Hooray. Calling withdraw_securely
using the correct password decreases the total funds by calling remove_funds
,
while using the incorrect password does nothing.
There’s one issue here, can you spot it?
diego.remove_funds(75)
# => 325
diego.funds
# => 325
Malicious users can still withdraw funds directly using the remove_funds
method!
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