📄 what is an object.mht
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Subject: What Is an Object?
Date: Fri, 6 Sep 2002 23:26:03 +0800
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<BODY link=3D#000099 bgColor=3D#ffffff><B><FONT size=3D-1>The =
Java</FONT><SUP><FONT=20
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align=3Dbottom NATURALSIZEFLAG=3D"3"> <BR><FONT size=3D-1><B>Trail</B>: =
Learning the=20
Java Language <BR><B>Lesson</B>: Object-Oriented Programming Concepts =
</FONT>
<H2>What Is an Object? </H2>
<BLOCKQUOTE>Objects are key to understanding <A=20
onmouseover=3D"self.status=3D'Look up object-oriented in glossary'; =
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=
href=3D"javascript:var%20meth=3DopenWin;%20meth('object-oriented%20design=
');"><FONT=20
color=3D#00bb00><EM>object-oriented</EM></FONT></A><A=20
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=
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height=3D11 alt=3D"(in the glossary)"=20
=
src=3D"http://java.sun.com/docs/books/tutorial/images/glossaryIcon.gif" =
width=3D11=20
align=3DabsMiddle border=3D0></A> technology. You can look around you =
now and see=20
many examples of real-world objects: your dog, your desk, your =
television set,=20
your bicycle.=20
<P>These real-world objects share two characteristics: They all have=20
<EM>state</EM> and <EM>behavior</EM>. For example, dogs have state =
(name,=20
color, breed, hungry) and behavior (barking, fetching, and wagging =
tail).=20
Bicycles have state (current gear, current pedal cadence, two wheels, =
number=20
of gears) and behavior (braking, accelerating, slowing down, changing =
gears).=20
<P>Software objects are modeled after real-world objects in that they =
too have=20
state and behavior. A software object maintains its state in one or =
more <A=20
onmouseover=3D"self.status=3D'Look up variables in glossary'; return =
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onmouseout=3D"self.status=3D''; return true;"=20
href=3D"javascript:var%20meth=3DopenWin;%20meth('variable');"><FONT=20
color=3D#00bb00><EM>variables</EM></FONT></A><A=20
onmouseover=3D"self.status=3D'Look up variables in glossary'; return =
true;"=20
onmouseout=3D"self.status=3D''; return true;"=20
href=3D"javascript:var%20meth=3DopenWin;%20meth('variable');"><IMG =
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=
src=3D"http://java.sun.com/docs/books/tutorial/images/glossaryIcon.gif" =
width=3D11=20
align=3DabsMiddle border=3D0></A>. A variable is an item of data named =
by an=20
identifier. A software object implements its behavior with <A=20
onmouseover=3D"self.status=3D'Look up methods in glossary'; return =
true;"=20
onmouseout=3D"self.status=3D''; return true;"=20
href=3D"javascript:var%20meth=3DopenWin;%20meth('method');"><FONT=20
color=3D#00bb00><EM>methods</EM></FONT></A><A=20
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true;"=20
onmouseout=3D"self.status=3D''; return true;"=20
href=3D"javascript:var%20meth=3DopenWin;%20meth('method');"><IMG =
height=3D11=20
alt=3D"(in the glossary)"=20
=
src=3D"http://java.sun.com/docs/books/tutorial/images/glossaryIcon.gif" =
width=3D11=20
align=3DabsMiddle border=3D0></A>. A method is a function (subroutine) =
associated=20
with an object.=20
<BLOCKQUOTE>
<HR>
<STRONG>Definition:</STRONG> An object is a software bundle of=20
variables and related methods.=20
<HR>
</BLOCKQUOTE>You can represent real-world objects by using software =
objects.=20
You might want to represent real-world dogs as software objects in an=20
animation program or a real-world bicycle as a software object in the =
program=20
that controls an electronic exercise bike. You can also use software =
objects=20
to model abstract concepts. For example, an <EM>event</EM> is a common =
object=20
used in GUI window systems to represent the action of a user pressing =
a mouse=20
button or a key on the keyboard.=20
<P>The following illustration is a common visual representation of a =
software=20
object:=20
<P>
<CENTER><IMG height=3D133 alt=3D""=20
src=3D"http://java.sun.com/docs/books/tutorial/figures/java/con17.gif" =
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align=3Dbottom NATURALSIZEFLAG=3D"3"></CENTER>
<P></P>Everything that the software object knows (state) and can do =
(behavior)=20
is expressed by the variables and the methods within that object. A =
software=20
object that modeled your real-world bicycle would have variables that=20
indicated the bicycle's current state: its speed is 10 mph, its pedal =
cadence=20
is 90 rpm, and its current gear is the 5th gear. These variables are =
formally=20
known as <A=20
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=
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<FONT=20
color=3D#00bb00><EM>instance variables</EM></FONT></A><A=20
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<IMG=20
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