Classes and Objects

A MzScheme class specifies

• a collection of instance variables,
• initial value expressions for the instance variables, and
• initialization variables that are bound to initialization arguments.

An object is a collection of bindings for instance variables that are instantiated according to a class description. There is no distinction between ``methods'' and ``instance variables''; a method is simply an instance variable with a procedural value.

The core feature of the object system is the ability to define a new class (a derived class) in terms of an existing class (the superclass) using inheritance and overriding:

• inheritance: An object of a derived class instantiates variables declared by the derived class's superclass as well as variables declared in the derived class expression.
• overriding: A variable declared in a superclass can be redeclared in the derived class. References to the overridden variable in the superclass use the binding of the derived class's declaration.

An interface is a collection of variable names that could be declared by a class. A class implements an interface when it

• declares (or inherits) a public instance variable for each variable in the interface; and
• specifically declares its intention to implement the interface.

A single class can implement any number of interfaces. A derived class automatically implements any interface that its superclass implements.

A new interface can extend one or more interfaces with additional variables; classes that implement the extended interface also implement the original interfaces.

Classes, objects, and interfaces are all first-class Scheme values. However, a MzScheme class or interface is not a MzScheme object (i.e., there are no ``meta-classes'' or ``meta-interfaces'').

• Object Example
• Creating Interfaces
• Creating Classes
  • Initialization Variables
  • Instance Variables
  • Initial Values
• Creating Objects
• Instance Variable Access
  • Generic Procedures
• Object Utilities