tij0203.html

学习java的经典书籍

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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Using
present systems (such as Pentium 200 pro, Netscape 3, and JDK 1.1.5), these
relative times show the extraordinary cost of new objects and arrays, the heavy
cost of synchronization, and the modest cost of an unsynchronized method call.
References [5] and [6] give the Web address of measurement applets you can run
on your own machine.
</FONT><P></DIV>
<A NAME="Heading644"></A><H4 ALIGN=LEFT>
General
modifications 
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here
are some modifications that you can make to speed up time-critical parts of
your Java program. (Be sure to test the performance before and after you try
them.)
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Replace</FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">With</FONT><P></DIV>
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<TD WIDTH=162 COLSPAN=1 ROWSPAN=1 VALIGN=TOP>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Why</FONT><P></DIV>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Interface</FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Abstract
Class (when only one parent is needed)
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Multiple
inheritance of interfaces prevents some optimizations.
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Non-local
or array loop variable
</FONT><P></DIV>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Local
loop variable
</FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Time
(above) shows an instance integer assignment is 1.2 local integer assignments,
but an array assignment is 2.7 local integer assignments.
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Linked
list (fixed size)
</FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Saving
discarded link items or replacing the list with a circular array (in which
approximate size is known)
</FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Each
new object takes 980 local assignments. See Reusing Objects (below), Van Wyk
[12] p. 87 and Bentley[15] p. 81
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">x/2
(or any power of 2)
</FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">X
&gt;&gt; 2 
</FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">(or
any power of 2)
</FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Uses
faster hardware instructions
</FONT><P></DIV>
</TD>
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<a name="_Toc408018859"></a></TABLE></DIV>
<A NAME="Heading645"></A><H3 ALIGN=LEFT>
Specific
situations
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>The
cost of Strings
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
concatenation operator + looks innocent but involves a lot of work. The
compiler can efficiently concatenate constant strings, but a variable string
requires considerable processing. For example, if 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>s</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>t</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
variables:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">System.out.println("heading"
+ s + "trailer" + t);
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">this
requires a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
appending arguments, and converting the result back to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">with
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This costs both space and time. If you&#8217;re appending more than one 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
consider using a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
directly, especially if you can repeatedly reuse it in a loop. Preventing the
creation of a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringBuffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
on each iteration saves the object creation time of 980 seen earlier. Using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>substring(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and the other 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods is usually an improvement. When feasible, character arrays are even
faster. Also notice that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>StringTokenizer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is costly because of synchronization.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Synchronization</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
In the JDK interpreter, calling a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is typically 10 times slower than calling an unsynchronized method. With
JIT compilers, this performance gap has increased to 50 to 100 times (notice
the timing above shows it to be 97 times slower). Avoid 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods if you can &#8211; if you can&#8217;t, synchronizing on methods rather
than on code blocks is slightly faster.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Reusing
objects
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
It takes a long time to create an object (the timing above shows 980 assignment
times for a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and 3100 assignment times for a small new array), so it&#8217;s often worth
saving and updating the fields of an old object instead of creating a new
object. For example, rather than creating a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Font</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object in your 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method, you can declare it an instance object, initialize it once, and then
just update it when necessary in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
See also Bentley, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>Programming
Pearls
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
p. 81 [15].
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Exceptions</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
You should only throw exceptions in abnormal situations, which are usually
cases in which performance is not an issue since the program has run into a
problem that it doesn&#8217;t normally expect. When optimizing, combine small 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>try</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">-</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>catch</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
blocks, which thwart compiler optimization by breaking the code into small
independent sections. On the other hand, be careful of sacrificing the
robustness of your code by over-zealous removal of exception handling.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Hashing:
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
standard 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Hashtable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class in Java 1.0 and 1.1 requires casting and costly synchronization (570
assignment times). Furthermore, the early JDK library doesn&#8217;t
deliberately choose prime number table sizes. Finally, a hashing function
should be designed for the particular characteristics of the keys actually
used. For all these reasons, the generic 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Hashtable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
can be improved by designing a hash class that fits a particular application
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>.</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
Note that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>HashMap</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in the Java 1.2 collections library has much greater flexibility and
isn&#8217;t automatically 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Method
inlining:
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
Java compilers can inline a method only if it is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
or 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and in some cases it must have no local variables. If your code spends a lot of
time calling a method that has none of these modifiers, consider writing a
version that is 
</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><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>I/O</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
Use buffers wherever possible, otherwise you can end up doing I/O a single byte
at a time. Note that the JDK 1.0 I/O classes use a lot of synchronization, so
you might get better performance by using a single &#8220;bulk&#8221; call such
as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>readFully(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and then interpreting the data yourself. Also notice that the Java 1.1
&#8220;reader&#8221; and &#8220;writer&#8221; classes were designed for
improved performance.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Casts
and instanceof
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Casts
take from 2 to 200 assignment times. The more costly ones require travel up the
inheritance hierarchy. Other costly operations lose and restore capabilities of
the lower level constructs.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Graphics:</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
Use clipping to reduce the amount of work done in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>repaint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
double buffering to improve perceived speed, and image strips or compression to
speed downloading times. 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>Animation
in Java Applets
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
from JavaWorld and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>Performing
Animation
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
from Sun are two good tutorials. Remember to use high-level primitives;
it&#8217;s much faster to call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>drawPolygon(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
on a bunch of points than looping with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>drawLine(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If you must draw a one-pixel-wide line, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>drawLine(x,y,x,y)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is faster than 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>fillRect(x,y,1,1)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Using
API classes
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
Use classes from the Java API when they offer native machine performance that
you can&#8217;t match using Java. For example, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>arrayCopy(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is much faster than using a loop to copy an array of any significant size. 
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Replacing
API classes
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
Sometimes API classes do more than you need, with a corresponding increase in
execution time. For these you can write specialized versions that do less but
run faster. For example, one application that needed a container to store many
arrays was speeded by replacing the original 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
with a faster dynamic array of objects.
</FONT><P></DIV>
<A NAME="Heading646"></A><H4 ALIGN=LEFT>
Other
suggestions
</H4>
<UL>
<LI><FONT FACE="Symbol" SIZE=3 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Move
repeated constant calculations out of a critical loop, for example, computing 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>buffer.length</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for a constant-size 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>buffer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><LI><FONT FACE="Symbol" SIZE=3 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static
final 
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">constants
can help the compiler optimize the program. 
</FONT><LI><FONT FACE="Symbol" SIZE=3 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Unroll
fixed length loops.
</FONT><LI><FONT FACE="Symbol" SIZE=3 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Use
javac&#8217;s optimization option, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>-O</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which optimizes compiled code by inlining 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>final</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods. Note that your classes may get larger in size (JDK 1.1 or later only
&#8211; earlier versions might not perform byte verification). Newer
just-in-time (JIT) compilers will dramatically speed the code.
</FONT><LI><FONT FACE="Symbol" SIZE=3 COLOR="Black">	</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Count
down to zero whenever possible &#8211; this uses a special JVM byte code.
</FONT><a name="_Toc408018860"></a></UL>
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