You can imagine that as our program grows larger and larger in OOP, you will define more objects and classes. Some of the classes will share similar characteristics. For example, you might have a box class, a safety-deposit-box class, and a locked-box class. They all need to know similar methods like adding items to it and removing items from it. It will be redundant to recode it for every single box-like class. What we want is to define a generic class (like a box class) that knows the general methods like opening and then let the more specific classes (like the safe-deposit-box class) inherit from the general box class.

Parents and Children

Let's say we want to create a checking-account class. Checking accounts are just like regular bank accounts, except that you can write checks as well as withdrawing money in person. But you're charged ten cents every time you write a check.

> (define Hal-Account (instantiate checking-account 1000))
> (ask Hal-Account 'balance)
> (ask Hal-Account 'deposit 100)
> (ask Hal-Account 'withdraw 50)
> (ask Hal-Account 'write-check 30)

One way to implement a checking-account is to copy all of the code we have for the account class but then if we need to make a change in our account then we need to remember to change our checking-account.

It is very common in object-oriented programming that one class will be a specialization of another: the new class will have all the methods of the old, plus some extras, just as in this bank account example. To describe this situation we use the metaphor of a family of object classes. The original class is the parent and the specialized version is the child class. We say that the child inherits the methods of the parent. (The names subclass for child and superclass for parent are also sometimes used.)


Here's how we create a subclass of the account class:

(define-class (checking-account init-balance)
    (parent (account init-balance))
    (method (write-check amount)
        (ask self 'withdraw (+ amount 0.10)) ))

This example introduces the parent clause in define-class. In this case, the parent is the account class. Note that because the account class needs one instantiation variable, we need to provide that argument as well (hence the (account init-balance)).

Whenever we send a message to a checking-account object, where does the corresponding method come from? If a method of that name is defined in the checking-account class, it is used; otherwise, the OOP system looks for a method in the parent account class. If the parent doesn't have that method, we will look at the parent's parent, and so on.

Test Your Understanding

These questions follow our class definitions for account and checking-account above.
(define sam (instantiate checking-account 500))
Which one of these will return an error?

The 'self' Keyword

What should write-check do? It should reduce the account's balance by the specified amount and additional fee. We already know how to reduce our balance, it's just the withdraw method! To call a method that we already defined from the body of another method, we use the self, hence the (ask self 'withdraw (+ amount 0.10)). Each object has a local state variable self whose value is the object itself.


Methods defined in a certain class only have access to the local state variables de fined in the same class. For example, a method de fined in the checking-account class can't refer to the balance variable de fined in the account class; likewise, a method in the account class can't refer to the init-balance variable.

This rule corresponds to the usual Scheme rule about scope of variables: each variable is only available within the block in which it's defi ned. (Not every OOP implementation works like this, by the way.)

Test Your Understanding

Classes are wonderful! They keep objects organized. Inheritance is wonderful! They keep classes organized. Be aware about the states that a child has and which ones are updated.
>(define nick (instantiate checking-account 500))
>(ask nick 'init-balance)
>(ask nick 'balance)
>(ask nick 'deposit 50)
What does the following expression return?
(ask nick 'balance)

What does the following expression return?
(ask nick 'init-balance)

Suppose we now have the following snippet of code:
(define-class (checking-account init-balance)
    (parent (account init-balance)) 
    (method (write-check amount)
        (ask self 'withdraw (+ amount 0.10)) )
    (method (show-balance) balance)  )

(define jeffrey (instantiate checking-account 500))
We added a new method, show-balance to the class. What will (ask jeffrey 'show-balance) return?


Several takeaways from this subsection:

  • Some classes will be a more 'specialized' or 'specific' version of another class. In these cases, we want to make the specific class a 'child' of the 'parent' class.
  • A child class inherits all methods of the parent class.
  • Keep track of what variable is actually in scope in your class.

What's Next?

We are going to learn what kinds of variables a class can have.