what is an object.mht

什么是对象

  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> because they contain the state for a =
particular=20
  bicycle object, and in object-oriented terminology, a particular =
object is=20
  called an <EM>instance</EM>.=20
  <P>The following figure illustrates a bicycle modeled as a software =
object.=20
  <P>
  <CENTER><IMG height=3D133 alt=3D""=20
  src=3D"http://java.sun.com/docs/books/tutorial/figures/java/con18.gif" =
width=3D230=20
  align=3Dbottom NATURALSIZEFLAG=3D"3"></CENTER>
  <P></P>In addition to its variables, the software bicycle would also =
have=20
  methods to brake, change the pedal cadence, and change gears. (The =
bike would=20
  not have a method for changing the speed of the bicycle, as the bike's =
speed=20
  is just a side effect of what gear it's in, how fast the rider is =
pedaling,=20
  whether the brakes are on, and how steep the hill is.) These methods =
are=20
  formally known as <A=20
  onmouseover=3D"self.status=3D'Look up instance methods in glossary'; =
return true;"=20
  onmouseout=3D"self.status=3D''; return true;"=20
  =
href=3D"javascript:var%20meth=3DopenWin;%20meth('instance%20method');"><F=
ONT=20
  color=3D#00bb00><EM>instance methods</EM></FONT></A><A=20
  onmouseover=3D"self.status=3D'Look up instance methods in glossary'; =
return true;"=20
  onmouseout=3D"self.status=3D''; return true;"=20
  =
href=3D"javascript:var%20meth=3DopenWin;%20meth('instance%20method');"><I=
MG=20
  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> because they inspect or change the =
state of a=20
  particular bicycle instance.=20
  <P>The object diagrams show that the object's variables make up the =
center, or=20
  nucleus, of the object. Methods surround and hide the object's nucleus =
from=20
  other objects in the program. Packaging an object's variables within =
the=20
  protective custody of its methods is called <A=20
  onmouseover=3D"self.status=3D'Look up encapsulation in glossary'; =
return true;"=20
  onmouseout=3D"self.status=3D''; return true;"=20
  =
href=3D"javascript:var%20meth=3DopenWin;%20meth('encapsulation');"><FONT =

  color=3D#00bb00><EM>encapsulation</EM></FONT></A><A=20
  onmouseover=3D"self.status=3D'Look up encapsulation in glossary'; =
return true;"=20
  onmouseout=3D"self.status=3D''; return true;"=20
  =
href=3D"javascript:var%20meth=3DopenWin;%20meth('encapsulation');"><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> This conceptual picture of an =
object-a nucleus=20
  of variables packaged within a protective membrane of methods-is an =
ideal=20
  representation of an object and is the ideal that designers of =
object-oriented=20
  systems strive for. However, it's not the whole story. Often, for =
practical=20
  reasons, an object may wish to expose some of its variables or hide =
some of=20
  its methods. In the Java programming language, an object can specify =
one of=20
  four access levels for each of its variables and methods. The access =
level=20
  determines which other objects and classes can access that variable or =
method.=20
  Variable and method access in Java is covered in <A=20
  =
href=3D"http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol=
.html"=20
  target=3D_top>Controlling Access to Members of a Class</A><A=20
  =
href=3D"http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol=
.html"=20
  target=3D_top><IMG height=3D11 alt=3D"(in the Learning the Java =
Language trail)"=20
  =
src=3D"http://java.sun.com/docs/books/tutorial/images/tutorialIcon.gif" =
width=3D11=20
  align=3DabsMiddle border=3D0></A>. Encapsulating related variables and =
methods=20
  into a neat software bundle is a simple yet powerful idea that =
provides two=20
  primary benefits to software developers:=20
  <UL>
    <LI><B>Modularity:</B> The source code for an object can be written =
and=20
    maintained independently of the source code for other objects. Also, =
an=20
    object can be easily passed around in the system. You can give your =
bicycle=20
    to someone else, and it will still work.=20
    <LI><B>Information hiding:</B> An object has a public interface that =
other=20
    objects can use to communicate with it. The object can maintain =
private=20
    information and methods that can be changed at any time without =
affecting=20
    the other objects that depend on it. You don't need to understand =
the gear=20
    mechanism on your bike to use it. </LI></UL></BLOCKQUOTE><IMG =
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