窝窝爱蛋 2008-5-12 21:39
如何使用Static的变量和方法
[color=#000000]如何使用Static的变量和方法
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]有时你希望定义一个类成员，使它的使用完全独立于该类的任何对象。通常情况下，类成员必须通过它的类的对象访问，但是可以创建这样一个成员，它能够被它自己使用，而不必引用特定的实例。在成员的声明前面加上关键字static(静态的)就能创建这样的成员。如果一个成员被声明为static，它就能够在它的类的任何对象创建之前被访问，而不必引用任何对象。你可以将方法和变量都声明为static。[/color][color=#0000ff]static[/color][color=#000000] 成员的最常见的例子是main( ) 。因为在程序开始执行时必须调用main() ，所以它被声明为static。
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]声明为static的变量实质上就是全局变量。当声明一个对象时，并不产生static变量的拷贝，而是该类所有的实例变量共用同一个static变量。声明为static的方法有以下几条限制：
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img][/color][color=#000000]1[/color][color=#000000].它们仅能调用其他的static 方法。
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img][/color][color=#000000]2[/color][color=#000000].它们只能访问static数据。
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]它们不能以任何方式引用this 或super（关键字super 与继承有关）。
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]如果你需要通过计算来初始化你的static变量，你可以声明一个static块，Static 块仅在该类被加载时执行一次。下面的例子显示的类有一个static方法，一些static变量，以及一个static 初始化块：
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img][/color][color=#008000]//[/color][color=#008000] Demonstrate static variables，methods，and blocks. [/color][color=#008000]
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif[/img][img]http://www.blogjava.net/Images/OutliningIndicators/ContractedBlock.gif[/img][/color][color=#0000ff]class[/color][color=#000000] UseStatic [/color][img]http://www.blogjava.net/Images/dot.gif[/img][color=#000000]{
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img][/color][color=#0000ff]static[/color]
[color=#0000ff]int[/color][color=#000000] a [/color][color=#000000]=[/color]
[color=#000000]3[/color][color=#000000];
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img][/color][color=#0000ff]static[/color]
[color=#0000ff]int[/color][color=#000000] b;
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img][img]http://www.blogjava.net/Images/OutliningIndicators/ContractedSubBlock.gif[/img][/color][color=#0000ff]static[/color]
[color=#0000ff]void[/color][color=#000000] meth([/color][color=#0000ff]int[/color][color=#000000] x) [/color][img]http://www.blogjava.net/Images/dot.gif[/img][color=#000000]{
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img]System.out.println([/color][color=#000000]"[/color][color=#000000]x = [/color][color=#000000]"[/color]
[color=#000000]+[/color][color=#000000] x);
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img]System.out.println([/color][color=#000000]"[/color][color=#000000]a = [/color][color=#000000]"[/color]
[color=#000000]+[/color][color=#000000] a);
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img]System.out.println([/color][color=#000000]"[/color][color=#000000]b = [/color][color=#000000]"[/color]
[color=#000000]+[/color][color=#000000] b);
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]}[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img][img]http://www.blogjava.net/Images/OutliningIndicators/ContractedSubBlock.gif[/img][/color][color=#0000ff]static[/color]
[img]http://www.blogjava.net/Images/dot.gif[/img][color=#000000]{
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img]System.out.println([/color][color=#000000]"[/color][color=#000000]Static block initialized.[/color][color=#000000]"[/color][color=#000000]);
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img]b [/color][color=#000000]=[/color][color=#000000] a [/color][color=#000000]*[/color]
[color=#000000]4[/color][color=#000000];
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]}[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img][img]http://www.blogjava.net/Images/OutliningIndicators/ContractedSubBlock.gif[/img][/color][color=#0000ff]public[/color]
[color=#0000ff]static[/color]
[color=#0000ff]void[/color][color=#000000] main(String args[]) [/color][img]http://www.blogjava.net/Images/dot.gif[/img][color=#000000]{
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img]meth([/color][color=#000000]42[/color][color=#000000]);
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]}[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockEnd.gif[/img]}[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]一旦UseStatic 类被装载，所有的static语句被运行。首先，a被设置为3，接着static 块执行(打印一条消息)，最后，b被初始化为a[/color][color=#000000]*[/color][color=#000000]4[/color][color=#000000] 或12。然后调用main()，main() 调用meth() ，把值42传递给x。3个println ( ) 语句引用两个static变量a和b，以及局部变量x 。
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]注意：在一个static 方法中引用任何实例变量都是非法的。
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]下面是该程序的输出：
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]Static block initialized.
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]x [/color][color=#000000]=[/color]
[color=#000000]42[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]a [/color][color=#000000]=[/color]
[color=#000000]3[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]b [/color][color=#000000]=[/color]
[color=#000000]12[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]在定义它们的类的外面，[/color][color=#0000ff]static[/color][color=#000000] 方法和变量能独立于任何对象而被使用。这样，你只要在类的名字后面加点号运算符即可。例如，如果你希望从类外面调用一个static方法，你可以使用下面通用的格式：
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]classname.method( )
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]这里，classname 是类的名字，在该类中定义static方法。可以看到，这种格式与通过对象引用变量调用非static方法的格式类似。一个static变量可以以同样的格式来访问——类名加点号运算符。这就是Java 如何实现全局功能和全局变量的一个控制版本。
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]下面是一个例子。在main() 中，static方法callme() 和static 变量b在它们的类之外被访问。
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif[/img][img]http://www.blogjava.net/Images/OutliningIndicators/ContractedBlock.gif[/img][/color][color=#0000ff]class[/color][color=#000000] StaticDemo [/color][img]http://www.blogjava.net/Images/dot.gif[/img][color=#000000]{
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img][/color][color=#0000ff]static[/color]
[color=#0000ff]int[/color][color=#000000] a [/color][color=#000000]=[/color]
[color=#000000]42[/color][color=#000000];
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img][/color][color=#0000ff]static[/color]
[color=#0000ff]int[/color][color=#000000] b [/color][color=#000000]=[/color]
[color=#000000]99[/color][color=#000000];
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img][img]http://www.blogjava.net/Images/OutliningIndicators/ContractedSubBlock.gif[/img][/color][color=#0000ff]static[/color]
[color=#0000ff]void[/color][color=#000000] callme() [/color][img]http://www.blogjava.net/Images/dot.gif[/img][color=#000000]{
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img]System.out.println([/color][color=#000000]"[/color][color=#000000]a = [/color][color=#000000]"[/color]
[color=#000000]+[/color][color=#000000] a);
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]}[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockEnd.gif[/img]}[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockStart.gif[/img][img]http://www.blogjava.net/Images/OutliningIndicators/ContractedBlock.gif[/img][/color][color=#0000ff]class[/color][color=#000000] StaticByName [/color][img]http://www.blogjava.net/Images/dot.gif[/img][color=#000000]{
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img][img]http://www.blogjava.net/Images/OutliningIndicators/ContractedSubBlock.gif[/img][/color][color=#0000ff]public[/color]
[color=#0000ff]static[/color]
[color=#0000ff]void[/color][color=#000000] main(String args[]) [/color][img]http://www.blogjava.net/Images/dot.gif[/img][color=#000000]{
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img]StaticDemo.callme();
[img]http://www.blogjava.net/Images/OutliningIndicators/InBlock.gif[/img]System.out.println([/color][color=#000000]"[/color][color=#000000]b = [/color][color=#000000]"[/color]
[color=#000000]+[/color][color=#000000] StaticDemo.b);
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]}[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/ExpandedBlockEnd.gif[/img]}[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]下面是该程序的输出：
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]a [/color][color=#000000]=[/color]
[color=#000000]42[/color][color=#000000]
[img]http://www.blogjava.net/Images/OutliningIndicators/None.gif[/img]b [/color][color=#000000]=[/color]
[color=#000000]99[/color]
