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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<H2 ALIGN=LEFT>
An
object has an interface
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Aristotle
was probably the first to begin a careful study of the concept of type. He was
known to speak of “the class of fishes and the class of birds.” The
concept that all objects, while being unique, are also part of a set of objects
that have characteristics and behaviors in common was directly used in the
first object-oriented language, Simula-67, with its fundamental keyword
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that introduces a new type into a program (thus
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>class</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>type</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are often used synonymously
</FONT><A NAME="fnB3" HREF="#fn3">[3]</A><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">).</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Simula,
as its name implies, was created for developing simulations such as the classic
“bank teller problem.” In this, you have a bunch of tellers,
customers, accounts, transactions, etc. The members (elements) of each class
share some commonality: every account has a balance, every teller can accept a
deposit, etc. At the same time, each member has its own state; each account has
a different balance, each teller has a name. Thus the tellers, customers,
accounts, transactions, etc. can each be represented with a unique entity in
the computer program. This entity is the object, and each object belongs to a
particular class that defines its characteristics and behaviors.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">So,
although what we really do in object-oriented programming is create new data
types, virtually all object-oriented programming languages use the
“class” keyword. When you see the word “type” think
“class” and vice versa.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Once
a type is established, you can make as many objects of that type as you like,
and then manipulate those objects as the elements that exist in the problem you
are trying to solve. Indeed, one of the challenges of object-oriented
programming is to create a one-to-one mapping between the elements in the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>problem
space
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(the place where the problem actually exists) and the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>solution
space
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(the place where you’re modeling that problem, such as a computer).
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">But
how do you get an object to do useful work for you? There must be a way to make
a request of that object so it will do something, such as complete a
transaction, draw something on the screen or turn on a switch. And each object
can satisfy only certain requests. The requests you can make of an object are
defined by its
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>interface</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and the type is what determines the interface. The idea of type being
equivalent to interface is fundamental in object-oriented programming.
</FONT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><IMG HEIGHT=171 WIDTH=114 SRC="tij0004.gif"></FONT><P></DIV><DIV ALIGN=RIGHT><P></DIV><DIV ALIGN=RIGHT><P></DIV><DIV ALIGN=RIGHT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
simple example might be a representation of a light bulb:
</FONT><P></DIV><DIV ALIGN=LEFT><P></DIV>
<font color="#990000"><PRE>Light lt = <font color="#0000ff">new</font> Light();
lt.on();</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
name of the type/class is
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Light</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and the requests that you can make of a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Light</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object are to turn it on, turn it off, make it brighter or make it dimmer. You
create a “handle” for a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Light
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">simply
by declaring a name (
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>lt</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
for that identifier, and you make an object of type
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Light</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
with the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword, assigning it to the handle with the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>=</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
sign. To send a message to the object, you state the handle name and connect it
to the message name with a period (dot). From the standpoint of the user of a
pre-defined class, that’s pretty much all there is to programming with
objects.
</FONT><a name="_Toc375545190"></a><a name="_Toc408018387"></a><P></DIV>
<HR><DIV ALIGN=LEFT><A NAME="fn3" HREF="#fnB3">[3]</A><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
Some people make a distinction, stating that type determines the interface
while class is a particular implementation of that interface.
</FONT><P></DIV>
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