ch37.htm

Java_by_Example,初级经典例子哦,珍藏版本

<H2><A NAME="Chapter27"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 27</FONT></A></H2>

<OL>

<LI>Java can load only GIF or JPEG files.

<LI>The two parameters required by many of the image and sound

methods are the base URL and the relative location of the file

to load.

<LI>Java recognizes only AU audio files.

<LI>To display an image after it's loaded, you call the <TT>Graphics</TT>

object's <TT>drawImage()</TT> method.

<LI>No. You can store images and sounds in any directory relative

to the base URL.

<LI>To scale an image, simply supply the <TT>drawImage()</TT>

method with the width and height with which you want the image

displayed.

<LI>To determine the normal width and height of an image, call

the <TT>Image</TT> object's <TT>getWidth()</TT> and <TT>getHeight()</TT>

methods.

<LI>The document base URL is the location of the HTML document,

whereas the code base URL is the location of the applet's CLASS

file.

<LI>You have more control over sounds with an <TT>AudioClip</TT>

object because the <TT>AudioClip</TT> class provides the <TT>play()</TT>,

<TT>stop()</TT>, and <TT>loop()</TT> methods, whereas the applet

can only play an audio file from beginning to end.

</OL>

<H2><A NAME="Chapter28"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 28</FONT></A></H2>

<OL>

<LI>The single argument is a string containing the URL from which

the <TT>URL</TT> object should be constructed.

<LI>An <TT>AppletContext</TT> object represents the application

containing the applet. This application is usually a Web browser.

<LI>To obtain an <TT>AppletContext</TT> object, call the applet's

<TT>getAppletContext()</TT> method.

<LI>To ensure that you have a valid <TT>URL</TT> object, you handle

the exception that may be generated by the <TT>URL</TT> class.

<LI>To create an exception handler, you must create <TT>try</TT>

and <TT>catch</TT> program blocks.

<LI>To connect to an URL, call the <TT>AppletContext</TT> object's

<TT>showDocument()</TT> method.

<LI>The <TT>URL</TT> class throws a MalformedURLException exception

if the URL's string is syntactically incorrect.

</OL>

<H2><A NAME="Chapter29"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 29</FONT></A></H2>

<OL>

<LI>A package is a group of related classes and interfaces.

<LI>To tell Java that a class uses a particular package, you use

the <TT>import</TT> keyword followed by the full name of the package.

<LI>To add a class or interface to a package, place the <TT>package</TT>

keyword at the top of the class's or interface's source code followed

by the name of the package to which the class or interface will

be added.

<LI>To tell Java that a class implements a particular interface,

add the <TT>implements</TT> keyword to the class's declaration

line, followed by the name of the interface.

<LI>The biggest difference between an interface and a class is

that an interface declares, but never implements, its methods.

An interface's methods must be defined in any class that implements

the interface.

<LI>Interfaces and classes are similar in that they are declared

in almost exactly the same way.

<LI>The complete name of a package mirrors the folder hierarchy

on your hard disk into which the package files must be stored.

</OL>

<H2><A NAME="Chapter30"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 30</FONT></A></H2>

<OL>

<LI>You use a <TT>try</TT> block to hold the program statements

that may generate the exceptions you want to handle.

<LI>You use a <TT>catch</TT> block to hold the program statements

that should be executed when a particular exception is thrown.

<LI>You don't have to catch all types of exceptions in your applets,

because Java has default handlers for many of them.

<LI>If you call a method that's declared with a <TT>throw</TT>

phrase, you must handle the exception in your program.

<LI>You can have as many <TT>catch</TT> blocks as you need in

order to respond to all appropriate exceptions.

<LI>To pass an exception on to a calling method, you declare the

called method with the <TT>throws</TT> phrase. Java will then

pass the exception to the calling function, where it must be handled

or thrown again.

<LI>Java throws exceptions that must be handled in your program,

as well as throws system, runtime exceptions that you may or may

not decide to handle in your program.

</OL>

<H2><A NAME="Chapter31"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 31</FONT></A></H2>

<OL>

<LI>Threads are like small programs within the main program. Just

as a multitasking system can run two or more applications simultaneously,

so can an applet or application run more than one thread simultaneously.

<LI>All Java threads must implement the <TT>Runnable</TT> interface,

which contains the <TT>run()</TT> method needed to start a thread.

<LI>To start a thread, you call the thread object's <TT>start()</TT>

method.

<LI>When you start a thread, Java calls the thread object's <TT>run()</TT>

method.

<LI>You'll usually stop your applet's threads in either the <TT>stop()</TT>

or <TT>destroy()</TT> method.

<LI>When you suspend a thread, you put it to sleep so that it

remains ready to run when you next need it. When you stop a thread,

you kill the thread, meaning that you'll have to create a new

thread object when you want to run the thread again.

<LI>To ensure that all threads get a chance to run, you should

call, in any thread that takes a while to run, the <TT>sleep()</TT>

or <TT>yield()</TT> method.

<LI>When retaining a thread's state is not an issue, you can both

create and start the thread in the applet's <TT>start()</TT> method.

<LI>When you want to retain a thread's state while the user switches

to and from the Web page containing your applet, you should create

the threads in <TT>init()</TT>, start or resume the threads in

<TT>start()</TT>, suspend the threads in <TT>stop()</TT>, and

stop the threads in <TT>destroy()</TT>.

<LI>Use the <TT>synchronized</TT> keyword to mark a method or

a block of code as a potential area of conflict for threads trying

to access the same resources.

<LI>When a thread enters a synchronized area of code, Java gives

the thread the class's monitor object. Until the thread releases

the monitor object, no other thread can enter the synchronized

area. In this way, a monitor object is much like a key that unlocks

a synchronized method or block of code.

</OL>

<H2><A NAME="Chapter32"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 32</FONT></A></H2>

<OL>

<LI>The one method that you'll find in every application, but

not in an applet, is <TT>main()</TT>, which is where a Java application's

execution begins.

<LI>To run a Java application, you compile it using javac, and

then run the byte-code .CLASS file with the Java interpreter,

java.

<LI>The single parameter received by <TT>main()</TT> is the application's

command-line parameters.

<LI>The parameters come into the <TT>main()</TT> method as an

array of strings. Each parameter is in one element of the array,

so you can access the elements by indexing the array.

<LI>To convert an applet to an application, you must add the <TT>main()</TT>

method.

<LI>You must instantiate an object from a class before you can

call its methods because a class is just a template for an object,

much the same way a blueprint is a template for a manufactured

object.

<LI>To create and display an application's window, you call the

<TT>Frame</TT> class's constructor, and then call the <TT>Frame</TT>

object's <TT>resize()</TT> and <TT>show()</TT> methods.

</OL>

<H2><A NAME="Chapter33"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 33</FONT></A></H2>

<OL>

<LI>To run more than one applet at a time with Appletviewer, create

an HTML document that loads the applets you want to see. Appletviewer

will load and run each applet in its own window.

<LI>Although HotJava is a good example of the type of programs

you can create with Java, it hasn't been kept up to date with

the Java programming language. For this reason, HotJava cannot

run applets created with the latest version of Java. Netscape

Navigator 2.0, on the other hand, can load and run these newer

applets.

<LI>A doc-comment block, which is placed at the top of a class

or right before a method, provides the information javadoc needs

to create useful HTML documentation files.

<LI>To start a program that's been loaded into the Java debugger,

type run.

<LI>To document a method's return value and parameters in a doc-comment,

you use the <TT>@return</TT> and <TT>@param</TT> doc tags.

<LI>To start a debugging session with appletviewer, type <TT>appletviewer

-debug applet.html</TT>, where <TT>applet.html</TT> is the HTML

document that loads the applet.

<LI>To create hyperlinks in javadoc's HTML documents, add <TT>@see</TT>

doc tags where appropriate in your doc-comment blocks.

<LI>To start a debugging session with jdb, type <TT>jdb app.class</TT>,

where <TT>app.class</TT> is the .CLASS file of the application

you want to debug.

<LI>The javap tool is a disassembler that converts byte-code .CLASS

files into a description of the source code.

<LI>Native methods are methods written in a language other than

Java.

<LI>The javah tool helps you implement native methods by creating

the header files you need to gain access to data fields in Java

classes from your C source code.

<LI>To set a breakpoint with the debugger, type <TT>stop at class:line</TT>,

where <TT>class:line</TT> is the class's name and breakpoint line

number separated by a colon.

</OL>

<H2><A NAME="Chapter34"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 34</FONT></A></H2>

<OL>

<LI>You must use the compiler to convert your source-code files

into byte-code (.CLASS) files that Java's interpreter can read

and run.

<LI>If you fail to include the .java extension when specifying

a source-code file in the javac command line, the compiler will

not compile the file and will instead generate an illegal argument

error.

<LI>The options for the javac command line go between the <TT>javac</TT>

command and the name of the source-code file.

<LI>Yes, you can specify multiple options in one command line.

Just place them one after the other between the <TT>javac</TT>

command and the name of the source-code file.

<LI>To set a target directory for the compiler's output files,

use the <TT>-d</TT> option followed by the name of the directory.

<LI>The <TT>-g</TT> command-line option instructs the compiler

to add debugging information to the byte-code files it generates.

<LI>The <TT>-nowarn</TT> command-line option instructs the compiler

to suppress warning messages as it compiles a file. The compiler

will still generate error messages.

<LI>To instruct the compiler to generate status information as

it works, specify the <TT>-verbose</TT> command-line option.

<LI>Byte-code files are the same format on every computer system.

Each type of computer has a Java interpreter specially written

for it. The interpreter reads and executes the byte-code files.

</OL>

<H2><A NAME="Chapter35"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 35</FONT></A></H2>

<OL>

<LI>Everything will run fine if you leave off the file extension.

In fact, the interpreter won't accept the file extension and will

generate an error if you include it.

<LI>Byte-code files have the .CLASS file extension. These are

the types of files that the interpreter can load and execute.

<LI>You can specify verbose output from the interpreter with the

<TT>-verbose</TT> or <TT>-v</TT> command options. The second is

just a short version of the first.

<LI>When writing a Java application, you must compile the source

code before the interpreter can load and execute the program.

<LI>To get a list of options supported by the interpreter, type

the command <TT>javac -help</TT>.

<LI>The interpreter is the only tool that can run a Java application.

This is because the compiler produces Java byte-code files rather

than regular executable files like most other compilers.

<LI>The <TT>-checksource</TT> (or <TT>-cs</TT>) command-line option

instructs the interpreter to recompile any source-code files that

have been changed since the last compilation.

</OL>

<H2><A NAME="Chapter36"><FONT SIZE=5 COLOR=#Ff0000>

Chapter 36</FONT></A></H2>

<OL>

<LI>The six main Java packages are <TT>lang</TT>, <TT>awt</TT>,

<TT>applet</TT>, <TT>io</TT>, <TT>util</TT>, and <TT>net</TT>.

<LI>The classes needed to write applications and applets that

run in a windowed environment are found in the <TT>awt</TT> package.

<LI>You would use the <TT>Math</TT> class whenever you need to

perform sophisticated mathematical calculations that require functions

like sin, cosine, tangent, and so on.

<LI>By creating an object of the <TT>String</TT> class, you can

use the class's many methods to manipulate the string.

<LI>To join (or concatenate) two strings, call the <TT>String</TT>

class's <TT>concat()</TT> method.

<LI>No, you do not instantiate an object from the <TT>Math</TT>

class. Because the class's methods are all static, you can call

them like this: <TT>Math.Method()</TT>, where <TT>Method</TT>

is the name of the method you want to call.

<LI>The data-type wrapper classes provide methods for manipulating

Java's primitive data types, such as <TT>int</TT>, <TT>float</TT>,

and <TT>boolean</TT>. To use the classes, you can either call

static methods through the class's name (such as, <TT>Integer.parseInt()</TT>)

or create an object of the class and call the methods through

that object.

<LI>The <TT>System</TT> class provides two methods-<TT>getProperty()</TT>

and <TT>getProperties()</TT>- that enable you to obtain information

about the system.

<LI>The <TT>io</TT> package features many classes that you can

use to perform various types of I/O operations using input and

output streams. Usually, you create an object of the appropriate

class and then manipulate the stream through the object's methods.

</OL>

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