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学习java的经典书籍

it means that data or method is not tied to any particular object instance of
that class. So even if you’ve never created an object of that class you
can call a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method or access a piece of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
data. With ordinary, non-
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
data and methods you must create an object and use that object to access the
data or method, since non-
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
data and methods must know the particular object they are working with. Of
course, since 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods don&#8217;t need any objects to be created before they are used, they
cannot 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>directly
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">access
non-
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
members or methods by simply calling those other members without referring to a
named object (since non-
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
members and methods must be tied to a particular object).
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Some
object-oriented languages use the terms 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>class
data
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>class
methods
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
meaning that the data and methods exist only for the class as a whole, and not
for any particular objects of the class. Sometimes the Java literature uses
these terms too.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">To
make a data member or method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you simply place the keyword before the definition. For example, this produces a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
data member and initializes it:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#0000ff">class</font> StaticTest {
    <font color="#0000ff">static</font> <font color="#0000ff">int</font> i = 47;
}</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Now
even if you make two 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StaticTest</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects, there will still be only one piece of storage for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StaticTest.i.</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
Both objects will share the same 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i.
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Consider:</FONT><P></DIV>

<font color="#990000"><PRE>StaticTest st1 = <font color="#0000ff">new</font> StaticTest();
StaticTest st2 = <font color="#0000ff">new</font> StaticTest(); </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">At
this point, both 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>st1.i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>st2.i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
have the same value of 47 since they refer to the same piece of memory.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">There
are two ways to refer to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
variable. As indicated above, you can name it via an object, by saying, for
example, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>st2.i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
You can also refer to it directly through its class name, something you cannot
do with a non-static member. (This is the preferred way to refer to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
variable since it emphasizes that variable&#8217;s 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
nature.)
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">StaticTest.i++;</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>++</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
operator increments the variable. At this point, both 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>st1.i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>st2.i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
will have the value 48.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Similar
logic applies to static methods. You can refer to a static method either
through an object as you can with any method, or with the special additional
syntax 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>classname.method(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
You define a static method in a similar way:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#0000ff">class</font> StaticFun {
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> incr() { StaticTest.i++; }
}</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StaticFun</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>incr(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
increments the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
data 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
You can call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>incr(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in the typical way, through an object:
</FONT><P></DIV>

<font color="#990000"><PRE>StaticFun sf = <font color="#0000ff">new</font> StaticFun();
sf.incr();</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Or,
because 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>incr(&#160;)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">is
a static method, you can call it directly through its class:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">StaticFun.incr();</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">While
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
when applied to a data member, definitely changes the way the data is created
(one for each class vs. the non-
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">one
for each object), when applied to a method it&#8217;s not so dramatic. An
important use of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for methods is to allow you to call that method without creating an object.
This is essential, as we will see, in defining the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method that is the entry point for running an application.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Like
any method, a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method can create or use named objects of its type, so a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is often used as a &#8220;shepherd&#8221; for a flock of instances of
its own type.
</FONT><a name="_Toc375545233"></a><a name="_Toc408018434"></a><P></DIV>

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