Classes provide a means to gather functions and data together to provide a blueprint for objects.

Classes define an objects behavior and state. Behavior is defined by methods which live in the class. Every object of the same class supports the same methods. State is defined in fields, whose values are stored in each instance.

Defining a class

Classes are created using the class keyword:

class Mocha { }

This creates a class named Mocha with no methods or attributes.

Methods

To let our mocha do stuff, we need to give it methods:

class Mocha
{
    brew()
    {
        print("Brewing a rich and delightful mocha, just for you!")
    }
}

This defines a brew method that takes no arguments. To add parameters, put their names inside the parentheses:

class Mocha
{
    brew(size, flavor)
    {
        print("Brewing a ${size} ${flavor} mocha!")
    }
}

Constructors

We’ve seen how to define kinds of objects and define methods on them. Our mochas can brew any kind of coffee beans, but we don’t actually have any mochas to do it. To create instances of a class, we need a constructor. You define one like so:

class Mocha
{
    init(size, flavor)
    {
        print("Creating a ${size} ${flavor} mocha!")
    }
}

The init name says we’re defining a constructor.

To make a new mocha now, we just call the class itself:

var myMocha = Mocha("large", "vanilla")

A constructor returns the instance of the class being created, even if you don’t explicitly use return. It is valid to use return inside of a constructor, but it is an error to have an expression after the return. That rule applies to return this as well, return handles that implicitly inside a constructor, so just return is enough.

return          // valid, returns 'this'

return variable // invalid
return nil     // invalid
return this     // also invalid

Inheritance

A class can inherit from a “parent” or superclass. When you invoke a method on an object of some class, if it can’t be found, it walks up the chain of superclasses looking for it there.

By default, any new class inherits from Object, which is the superclass from which all other classes ultimately descend. You can specify a different parent class using : when you declare the class:

class IcedMocha is Mocha {}

This declares a new class Iced Mocha that inherits from Mocha.