ch34.htm

Java_by_Example,初级经典例子哦,珍藏版本

whose default value is the folder that contains Java's classes.

Java will also look in the active folder (the one you're in when

you type the <TT>javac</TT> command line). However, you can change

the setting of CLASSPATH temporarily for the current compilation

by using the <TT>-classpath</TT> option, like this:

<BLOCKQUOTE>

<PRE>

javac -classpath path FileName.java

</PRE>

</BLOCKQUOTE>

<P>

In the above line, <TT>path</TT> is the path you want to include,

each separated by a semicolon. For example, assuming that you

installed Java in a folder called C:\JAVA and that your own classes

are in the C:\CLASSES folder, the following line compiles your

program using the same settings the compiler would use by default:

<BLOCKQUOTE>

<PRE>

javac -classpath c:\java\lib\classes.zip;c:\classes FileName.java

</PRE>

</BLOCKQUOTE>

<P>

Notice that Java's classes are in a file called CLASSES.ZIP. You

must include this file name in the path in order for the compiler

to find the classes it needs to successfully compile your applet.

<H3><A NAME="SpecifyingtheTargetDirectory">

Specifying the Target Directory</A></H3>

<P>

When you run javac by typing the <TT>javac Applet.java</TT> command

line, the compiler reads the source-code file (or files), converts

it to byte-code form, and stores the resultant .CLASS file in

the directory from which the compiler was run. You can control

this target directory by specifying the <TT>-d</TT> command option,

like this:

<BLOCKQUOTE>

<PRE>

-d directory

</PRE>

</BLOCKQUOTE>

<P>

In this command, <TT>directory</TT> is the directory in which

you want the output files (.CLASS files) stored.

<H3><A NAME="ExampleSettingtheTargetDirectory">

Example: Setting the Target Directory</A></H3>

<P>

Suppose you have your Java source code files (the ones with the

.java extension) in a folder called C:\CLASSES, as you have for

the applets you've created in this book. Now you want to have

the .CLASS files that are created by the Java compiler placed

in a subdirectory of CLASSES called COMPILED. You'd first create

the subdirectory with the command <TT>md compiled</TT>. Then you'd

issue the following command:

<BLOCKQUOTE>

<PRE>

javac -d c:\classes\compiled applet.java

</PRE>

</BLOCKQUOTE>

<P>

Following the <TT>javac</TT> command are the <TT>-d</TT> option,

the name of the directory in which to store the output files,

and the Java source-code file to compile.

<P>

The directory name is either a full path or a path relative to

your current directory. For example, if you're current directory

is c:\CLASSES, when you type the above command, you can shorten

the directory name, like this:

<BLOCKQUOTE>

<PRE>

javac -d compiled applet.java

</PRE>

</BLOCKQUOTE>

<H3><A NAME="CreatingDebuggingTables">

Creating Debugging Tables</A></H3>

<P>

To get the most out of a debugger, your programs need to be compiled

in a special way, so that debugging information is included in

the compiled byte-code files. The compiler switch that turns this

option on is <TT>-g</TT>, and you use it like this:

<BLOCKQUOTE>

<PRE>

javac -g applet.java

</PRE>

</BLOCKQUOTE>

<P>

As you can see, the only thing extra here is the <TT>-g</TT> option

itself, which requires no additional arguments.

<P>

You may decide that it would be cool to use the <TT>-g</TT> option

all the time, so that your programs are always loaded with debugging

information. Don't do it. This wouldn't be a good idea because

programs with debugging information are not only a bit larger

than programs without the debugging information, but also tend

to run slower. The larger the program is, the more debugging information

the <TT>-g</TT> option adds to the file.

<H3><A NAME="ExampleAddingDebuggingTablestoanApplet">

Example: Adding Debugging Tables to an Applet</A></H3>

<P>

To test the <TT>-g</TT> option, copy the ShapeApplet.java file

from the CHAP34 folder on this book's CD-ROM to your CLASSES directory.

Then, compile the file with the following command:

<BLOCKQUOTE>

<PRE>

javac ShapeApplet.java

</PRE>

</BLOCKQUOTE>

<P>

Now, check the size of the ShapeApplet.class file. It should be

1,334 bytes.

<P>

Next, compile the applet again, this time using the <TT>-g</TT>

option, like this:

<BLOCKQUOTE>

<PRE>

javac -g ShapeApplet.java

</PRE>

</BLOCKQUOTE>

<P>

When you check the file size this time, you'll find it's 1,612

bytes, over 20% larger. The extra size is caused by the additional

debugging information the compiler has stored in the .CLASS file.

<H3><A NAME="SuppressingWarnings">

Suppressing Warnings</A></H3>

<P>

Sometimes, when the Java compiler finds something questionable

in your code, it issues a warning. Warnings represent the kind

of errors that don't prevent a program from compiling properly,

but that may generate a runtime error or just be bad programming

practice. Because warnings are not critical to the compilation

process, Java enables you to turn them off. You might do this,

for example, when you already know about the problems that are

creating the warnings. To turn off the warnings, you use the <TT>-nowarn</TT>

option, like this:

<BLOCKQUOTE>

<PRE>

javac -nowarn applet.java

</PRE>

</BLOCKQUOTE>

<P>

Like the <TT>-g</TT> option, <TT>-nowarn</TT> requires no additional

arguments.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD><B>TIP</B></TD></TR>

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

You can use more than one command-line option at a time. For example, you can both turn on debugging information and set the target directory with a command like this: javac -g -d c:\classes applet.java.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="OptimizingaProgram">

Optimizing a Program</A></H3>

<P>

When a compiler runs, it reads in source code and converts that

source code to some other format, in Java's case, a byte-code

file. As a programmer, though, you know that there are many ways

to accomplish the same task in a program. A compiler doesn't normally

take this sort of thing under consideration when it's working,

though. It generates its output the same way for every source-code

file.

<P>

However, the javac compiler knows how to perform certain types

of optimization on your programs, but it only does so when asked.

(And don't forget to say pretty please). To tell the Java compiler

to optimize your program, you use the <TT>-O</TT> option, like

this:

<BLOCKQUOTE>

<PRE>

javac -O applet.java

</PRE>

</BLOCKQUOTE>

<P>

The <TT>-O</TT> option requires no arguments.

<P>

Notice that the letter after the hyphen is an uppercase O. A lowercase

o will not work. Also, be aware that compiling with the optimizing

option may make the resulting .CLASS file incompatible with some

other Java tools. For this reason, optimizing should be done only

when compiling the program for the final time.

<H3><A NAME="SwitchingOnVerboseOutput">

Switching On Verbose Output</A></H3>

<P>

When you run the Java compiler with no command-line option, the

compiler runs and performs its task without displaying any sort

of information on the screen (unless the program contains errors).

With a large program that takes a while to compile, you may want

to know what's going on behind your back, if for no other reason

than to reassure yourself that everything is going okay. You can

make the compiler report to you as it works by using the <TT>-verbose</TT>

option, like this:

<BLOCKQUOTE>

<PRE>

javac -verbose applet.java

</PRE>

</BLOCKQUOTE>

<P>

When you add this command-line option, the compiler will tell

you which files it's loading and compiling. It'll even tell you

how long each step took to complete (Figure 34.4).

<P>

<A HREF="f34-4.gif"><B> Figure 34.4 : </B><I>When you use the -verbose option, the compiler reports to you every step of the way.



</I></A><P>

<H2><A NAME="Summary"><FONT SIZE=5 COLOR=#Ff0000>

Summary</FONT></A></H2>

<P>

The compiler is one of the most important of the Java tools, because

without it you'd be unable to convert your source-code files into

byte-code files, which are the only kind of files that Java's

interpreter understands. Ordinarily, you can run the compiler

simply by typing the command <TT>javac</TT> followed by the name

of the file you want to compile. However, javac also recognizes

a number of options that you can add to the command line. These

include options to set directories, to optimize the program, to

add debugging information, and to display status information as

the compiler works. In the next chapter, you learn about the compiler's

counter-part, the interpreter, which reads the files created by

the compiler in order to run the program.

<H2><A NAME="ReviewQuestions"><FONT SIZE=5 COLOR=#Ff0000>

Review Questions</FONT></A></H2>

<OL>

<LI>Why do you need to use a compiler?

<LI>What happens if you fail to include the .java extension when

specifying your source-code file to the compiler?

<LI>When you use options with the javac command, where do you

place them in the command line?

<LI>Can you specify more than one command-line option at a time?

<LI>How do you set a different target directory for the compiler's

output files?

<LI>What does the -g command-line option do?

<LI>What does the -nowarn command-line option do?

<LI>How do you get the compiler to show you what it's doing as

it works?

<LI>How do byte-code files enable Java to run the same programs

on different types of computers?

</OL>

<H2><A NAME="ReviewExercises"><FONT SIZE=5 COLOR=#Ff0000>

Review Exercises</FONT></A></H2>

<OL>

<LI>Compile an applet, instructing the compiler to include debugging

information in the byte-code file.

<LI>Compile an applet with the verbose setting, and study the

information the compiler displays on the screen.

<LI>Compile an applet, specifying no warnings, optimization, and

an output directory of C:\CLASSES\MYCLASSES.

</OL>

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