Definitions of Terms
Many terms in Smalltalk are also used in other languages
such as C++, Eiffel, and Simula. While there may be a surface similarity,
the terms often mean quite different things. This chapter defines many common
terms in the context of Smalltalk.
Definitions of Terms: Objects
What is an object?
In Smalltalk, an object is a collection
of data and code. The data is hidden within the object and cannot be accessed
except through code in the object.
What is a method?
A method is a Smalltalk subroutine, no more and no less.
It always takes at least one parameter (self) and always returns
one value.
What is a class?
A class is the definition of an object.
The class of an object holds a list of the data that an object will have,
and it holds the code for the object. Each different kind of object has
its own class.
Smalltalk programs are created by writing classes. Each
class defines data and a set of methods, the code, which operates on that
data. Figure 1 illustrates a class and two instances.
What is an instance?
An instance of a class is simply an
object. It holds data and a pointer to the class which holds the code.
What is a subclass?
A subclass is a class that is defined
using another class as a basis. The other class is called the superclass
(or parent class). The subclass adds to the implementation (or shares the
implementation) of the superclass.
Subclassing in Smalltalk is implementation sharing not
subtyping.
What are encapsulation and information hiding?
Encapsulation and information hiding refer to the act
of hiding data behind a wall of code. Encapsulation is a basic characteristic
of all OOP languages and of some non-OOP languages.
What is a hierarchy?
A hierarchy is an outline or tree of class definitions.
In Smalltalk, the top (or root) of the hierarchy is a very general object
named Object. All other objects are subclasses of Object
or of some other class (which must be a subclass of object or another class).
There are some weird exceptions; see xxxxxxx.
What is polymorphism?
Polymorphism means multiple meanings. In Smalltalk,
polymorphism means that there are multiple methods with the same name and
that any one of them might be invoked by a single message send.
Example
A group of classes define various kinds of employees.
There is one class for retired employees, one for active employees, one
for executives, one for part time employees, and so on.
Each class might have a computePaycheck
method to compute the amount of an employee's paycheck, but the methods
would be different because of the differing requirements of each kind of
employee.
At the point where a paycheck amount needs to be computed,
the computePaycheck message is sent to an instance of one of
the employee classes:
anEmployee computePaycheck
Definitions of Terms: Messages
What is a message?
A message is an operation that can be requested of an
object. It consists of the name of the message, which is the name of a method,
and values for any associated parameters. All messages are considered to
have at least one parameter, the value that will become self.
If an object has a method named zotFarb:
then these three parts are a message:
- The name
zotFarb:
- The value that will be
self
- A value to pass for its one parameter.
What is a message send?
A message send is the process of invoking a method in
an object using a message. It has two steps: finding the method, and invoking
the method.
Finding the method
- The name of the message is compared with methods in the target object.
If not found, then each of the parents are examined successively until one
is found. (If none is found, an error condition is raised.) In general,
the object to which the message is sent is not known until runtime and this
lookup process must logically occur each time a message is sent.
Invoking the method
- Once the appropriate method is found, then the method is invoked just
as if it were a procedure in a procedural language. Parameters are bound
to their local names, the stack is appropriately massaged, and execution
begins.
Upon termination, control returns to the method that
sent the message to a point just after the message send. A value is always
returned although the sender has the option of ignoring it.
How does a message send differ from a procedure call?
A message send is identical to a procedure call once
a method is found to invoke.
Definitions of Terms: Classes
Are classes objects?
Yes; classes in Smalltalk are objects and are instances
of some other object.
What is an abstract class?
An abstract class is just a class; Smalltalk makes no
distinction between abstract classes and any other kind. However, programmers
find it useful to define classes that are not complete, and that will never
have instances. Such classes define some general concept and some protocol
(a set of methods) that all its subclasses will inherit.
The base class library contains a number of abstract
classes, or abstract superclasses, as they are sometime called. These include:
Class Abstracts
Object The basic idea of an object: existence, size, ...
Magnitude Values that compare uniquely with < and >
Number Values that can be added, subtracted, ...
Stream Sequences of values
Collection Groups of values
Last Modified: 02:26pm PST, January 06, 1996