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学习java的经典书籍

presumably optimized out of existence.) The difference is shown in the output
from one run:
</FONT><P></DIV>

<font color="#990000"><PRE>fd1: i4 = 15, i5 = 9
Creating <font color="#0000ff">new</font> FinalData
fd1: i4 = 15, i5 = 9
fd2: i4 = 10, i5 = 9 </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that the values of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i4</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>fd1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>fd2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are unique, but the value for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i5</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is not changed by creating the second 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>FinalData</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object. That&#8217;s because it&#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">
and is initialized once upon loading and not each time a new object is created.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
variables 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>v1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
through 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>v4</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
demonstrate the meaning of a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle. As you can see in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
just because 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>v2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
doesn&#8217;t mean that you can&#8217;t change its value. However, you cannot
re-bind 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>v2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to a new object, precisely because it&#8217;s 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
That&#8217;s what 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
means for a handle. You can also see the same meaning holds true for an array,
which is just another kind of handle. (There is know way that I know of to make
the array handles themselves 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.)
Making handles 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
seems less useful than making primitives 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV>
<A NAME="Heading198"></A><H4 ALIGN=LEFT>
Blank
finals
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Java
1.1<A NAME="Index520"></A>
allows the creation of <A NAME="Index521"></A><A NAME="Index522"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>blank
finals
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which are fields that are declared as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
but are not given an initialization value. In all cases, the blank final 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>must</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
be initialized before it is used, and the compiler ensures this. However, blank
finals provide much more flexibility in the use of the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword since, for example, a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
field inside a class can now be different for each object and yet it retains
its immutable quality. Here&#8217;s an example:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: BlankFinal.java</font>
<font color="#009900">// "Blank" final data members</font>

<font color="#0000ff">class</font> Poppet { }

<font color="#0000ff">class</font> BlankFinal {
  <font color="#0000ff">final</font> <font color="#0000ff">int</font> i = 0; <font color="#009900">// Initialized final</font>
  <font color="#0000ff">final</font> <font color="#0000ff">int</font> j; <font color="#009900">// Blank final</font>
  <font color="#0000ff">final</font> Poppet p; <font color="#009900">// Blank final handle</font>
  <font color="#009900">// Blank finals MUST be initialized</font>
  <font color="#009900">// in the constructor:</font>
  BlankFinal() {
    j = 1; <font color="#009900">// Initialize blank final</font>
    p = <font color="#0000ff">new</font> Poppet();
  }
  BlankFinal(<font color="#0000ff">int</font> x) {
    j = x; <font color="#009900">// Initialize blank final</font>
    p = <font color="#0000ff">new</font> Poppet();
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    BlankFinal bf = <font color="#0000ff">new</font> BlankFinal();
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You&#8217;re
forced to perform assignments to finals either with an expression at the point
of definition of the field or in every constructor. This way it&#8217;s
guaranteed that the final field is always initialized before use.
</FONT><P></DIV>
<A NAME="Heading199"></A><H4 ALIGN=LEFT>
Final
arguments
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Java
1.1<A NAME="Index523"></A>
allows you to make <A NAME="Index524"></A><A NAME="Index525"></A>arguments
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
by declaring them as such in the argument list. This means that inside the
method you cannot change what the argument handle points to:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: FinalArguments.java</font>
<font color="#009900">// Using "final" with method arguments</font>

<font color="#0000ff">class</font> Gizmo {
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> spin() {}
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> FinalArguments {
  <font color="#0000ff">void</font> with(<font color="#0000ff">final</font> Gizmo g) {
    <font color="#009900">//! g = new Gizmo(); // Illegal -- g is final</font>
    g.spin();
  }
  <font color="#0000ff">void</font> without(Gizmo g) {
    g = <font color="#0000ff">new</font> Gizmo(); <font color="#009900">// OK -- g not final</font>
    g.spin();
  }
  <font color="#009900">// void f(final int i) { i++; } // Can't change</font>
  <font color="#009900">// You can only read from a final primitive:</font>
  <font color="#0000ff">int</font> g(<font color="#0000ff">final</font> <font color="#0000ff">int</font> i) { <font color="#0000ff">return</font> i + 1; }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    FinalArguments bf = <font color="#0000ff">new</font> FinalArguments();
    bf.without(<font color="#0000ff">null</font>);
    bf.with(<font color="#0000ff">null</font>);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that you can still assign a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>null</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle to an argument that&#8217;s final without the compiler catching it, just
like you can with a non-final argument.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
methods 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>f(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>g(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
show what happens when primitive arguments are 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
you can only read the argument, but you can't change it.
</FONT><a name="_Toc375545319"></a><a name="_Toc408018522"></a><P></DIV>
<A NAME="Heading200"></A><H3 ALIGN=LEFT>
Final
methods
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">There
are two reasons for <A NAME="Index526"></A><A NAME="Index527"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods. The first is to put a &#8220;lock&#8221; on the method to prevent any
inheriting class from changing its meaning. This is done for design reasons
when you want to make sure that a method&#8217;s behavior is retained during
inheritance and cannot be overridden.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
second reason for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods is efficiency. If you make a method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you are allowing the compiler to turn any calls to that method into <A NAME="Index528"></A><A NAME="Index529"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>inline</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
calls. When the compiler sees a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method call it can (at its discretion) skip the normal approach of inserting