prog.html

java 基础的 一点东西,,可以看看

<CODE>static println</CODE> method in the <CODE>System</CODE> class, without 
creating an instance of the <CODE>System</CODE> class.

<P>
However, a program must create an instance of a class to access its non-static
fields and methods. Accessing static and non-static fields and methods is
discussed further with several  examples in the next section. 

<A NAME="fields"></A>
<H3>Fields and Methods</H3>

The <A HREF="./Code/LessonTwoA.java">LessonTwoA.java</A> program 
alters the simple example to store the text string in 
a static field called <CODE>text</CODE>. The <CODE>text</CODE> field
is static so its data can be accessed directly without creating an 
<EM>instance</EM> of the <CODE>LessonTwoA</CODE> class. 

</FONT>

<PRE>
class LessonTwoA {
   static String text = &quot;I'm a Simple Program&quot;;
   public static void main(String[] args){
        System.out.println(text);
   }
}
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

The <A HREF="./Code/LessonTwoB.java">LessonTwoB.java</A> and
<A HREF="./Code/LessonTwoC.java">LessonTwoC.java</A> programs
add a <CODE>getText</CODE> method to the program to 
retrieve and print the text. 

<P>
The <A HREF="./Code/LessonTwoB.java">LessonTwoB.java</A> program
accesses the non-static <CODE>text</CODE> field with the non-static
<CODE>getText</CODE> method. Non-static methods and fields are called 
instance methods and fields. This approach requires that an instance 
of the <CODE>LessonTwoB</CODE> class be created in the 
<CODE>main</CODE> method. To keep things interesting, this example 
includes a static text field and a non-static instance method 
(<CODE>getStaticText</CODE>) to retrieve it.

<BLOCKQUOTE>
<HR>
<STRONG>Note:</STRONG> The field and method return values 
are all type <CODE>String</CODE>.
<HR>
</BLOCKQUOTE>


</FONT>

<PRE>
class LessonTwoB {

   String text = &quot;I'm a Simple Program&quot;;
   static String text2 = &quot;I'm static text&quot;;

   String getText(){
        return text;
   }

   String getStaticText(){
        return text2; 
   }

   public static void main(String[] args){
        LessonTwoB progInstance = new LessonTwoB();
        String retrievedText = progInstance.getText();
        String retrievedStaticText = 
                 progInstance.getStaticText(); 
        System.out.println(retrievedText);
        System.out.println(retrievedStaticText);
   }
}
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

The <A HREF="./Code/LessonTwoC.java">LessonTwoC.java</A> program
accesses the static <CODE>text</CODE> field with the 
static <CODE>getText</CODE> method.  Static methods and fields are called 
class methods and fields. This approach allows the program to
call the static <CODE>getText</CODE> method directly without creating
an instance of the <CODE>LessonTwoC</CODE> class. 

</FONT>

<PRE>
class LessonTwoC {

   static String text = &quot;I'm a Simple Program&quot;;

//Accessor method
   static String getText(){
        return text;
   }

   public static void main(String[] args){
        String retrievedText = getText();
        System.out.println(retrievedText);
   }
}
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

So, class methods can operate only on class fields, and instance
methods can operate on class and instance fields. 

<P>
You might wonder what the difference means. In short, there is only
one copy of the data stored or set in a class field but each instance has
its own copy of the data stored or set in an instance field. 

<P>
<IMG SRC="./Art/diff.gif" WIDTH="436" HEIGHT="158" ALT="">
<P>

The figure above shows three class instances with one static field and one instance
field. At runtime, there is one copy of the value for static Field A and
each instance points to the one copy. When setFieldA(50) is called on the
first instance, the value of the one copy changes from 36 to 50 and all
three instances point to the new value. But, when setFieldB(25) is called
on the first instance, the value for Field B changes from 0 to 25
for the first instance only because each instance has its own copy of
Field B.

<P>
See
<A HREF="http://java.sun.com/docs/books/tutorial/java/javaOO/classvars.html">
Understanding Instance and Class Members</A> lesson in 
<A HREF="http://java.sun.com/docs/books/tutorial/index.html">The Java tutorial</A> for a 
thorough discussion of this topic. 

<A NAME="const"></A>
<H3>Constructors</H3>

Classes have a special method called a <EM>constructor</EM> that is called 
when a class instance is created. The class constructor always has the
same name as the class and no return type.
The <A HREF="./Code/LessonTwoD.java">LessonTwoD</A> program converts the 
<CODE>LessonTwoB</CODE> program to use a constructor to initialize the 
text string. 

<BLOCKQUOTE>
<HR>
<STRONG>Note:</STRONG> If you do not write your own constructor, 
the compiler adds an empty constructor, which calls the no-arguments constructor of its parent class. The empty constructor
is called the default constructor. The default constructor
initializes all non-initialized fields and variables to zero.

<HR>
</BLOCKQUOTE>

</FONT>

<PRE>
class LessonTwoD {

   String text;

//Constructor
   LessonTwoD(){
     text = &quot;I'm a Simple Program&quot;;
   }

//Accessor method
   String getText(){
        return text;
   }

   public static void main(String[] args){
        LessonTwoD progInst = new LessonTwoD();
        String retrievedText = progInst.getText();
        System.out.println(retrievedText);
   }
}
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<A NAME="summ"></A>
<H3>To Summarize</H3>

A simple program that prints a short text string to the console
would probably do everything in the <CODE>main</CODE> method
and do away with the constructor, <CODE>text</CODE> field, and
<CODE>getText</CODE> method. But, this lesson used a very simple
program to show you the structure and elements in a basic
Java program.

<A NAME="more"></A>
<H3>More Information</H3>

See
<A HREF="http://java.sun.com/docs/books/tutorial/java/javaOO/classvars.html">Understanding 
Instance and Class Members</A> lesson in
<A HREF="http://java.sun.com/docs/books/tutorial/index.html">The Java
tutorial</A> for a thorough discussion of this topic.

<P>
_______<BR>
<A NAME="TJVM"><SUP>1</SUP></A> As used on this web site, 
the terms &quot;Java virtual 
machine&quot; or &quot;JVM&quot; mean a virtual machine 
for the Java platform.

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