Java Reference
In-Depth Information
ics called
operations research
. Simula was described as a process-oriented
language. A few years later a new version, Simula 67, was released.
Simula and Simula 67 never became popular programming languages.
Their historical importance is the introduction of the notion of a
class
.In
object-oriented systems, a class is a template that packages together data
and processing routines. The class is a formal part of an OO language, not
a concrete entity. The programmer creates instances of a class as they be-
come necessary. The instances of a class are called
objects
.
The first description of a fully operational, object-oriented program-
ming language was due to Alan Kay. Kay foresaw that desktop computers
would have megabytes or memory and would be equipped with proces-
sors capable of executing millions of instructions per second. Since, in
Kay's vision of the future, desktop machines would be used mostly by
nonprogrammers, it was necessary that these computers be equipped
with a powerful graphical interface.
An information processing system named Dynabook was a first effort
in this direction. Dynabook was based on a virtual desktop on which
some documents were visible and others were partially covered. The user
would select documents, and move items around on the desktop using a
touch-sensitive screen. The interface was similar to our present day win-
dowing environments. The Flex programming language, loosely based on
Simula 67, performed the processing.
These ideas eventually evolved into the Smalltalk programming lan-
guage, still considered one of the most powerful and refined object-ori-
ented systems. The Smalltalk design team was led by Adele Goldberg. The
Smalltalk language has become the standard to which other object-ori-
ented languages are compared. In addition to Smalltalk, other notable ob-
ject-oriented languages are Clos, Ada 95, C++, and Java.
Object-Oriented Fundamentals
Not all object-oriented languages are created equal. Smalltalk, for example,
implements object-orientation in a pure and strict form. In other OO lan-
guages, such as C++, object-orientation is an optional feature that can be
turned on or off by the programmer. Object orientation in Java is not op-
tional. In contrast with C++, all Java code must reside inside a class. Never-
theless, it is possible to write Java programs in which object-orientation is
minimally visible. The programs developed so far in this topic fall into this
group.
