在J2SE?5.0版本中，java語言引入了很多新的特性，本文將主要介紹static?import。static?import主要解決的問題是方便開發(fā)人員創(chuàng)建和使用全局的常量以及靜態(tài)的方法。要使用這一新的特性您應(yīng)該首先從java.sun.com下載最新的j2sdk?5.0。?
????在《effective?java》中作者曾經(jīng)談到在接口中定義常量是很糟糕的一種使用方法，我們應(yīng)該始終使用接口來定義類型。但是在實(shí)際開發(fā)工作中還是有很多人這樣使用接口，他們這樣做的原因是這樣定義常量使用起來很方便。例如如下定義方式：
???public?interface?BadIrrationalConstants?{
?????public?static?final?double?SQRT_TWO?=?1.414;
?????public?static?final?double?SQRT_THREE?=?1.732;
???}
?public?interface?BadTranscendentalConstants?{
?????public?static?final?double?PI?=?3.14159;
?????public?static?final?double?E?=?2.71828;}
???
????如果你想在自己的類中使用這些接口中定義的常量的時候，那么你必須要實(shí)現(xiàn)這些接口。比如
?????public?class?BadUseOfConstants?implements
?????BadTranscendentalConstants,?BadIrrationalConstants?{
????public?static?double?sinPiOverFour()?{
???????return?SQRT_TWO?/?2;
?????}
?????public?static?void?main(String[]?args)?{
???????System.out.println("Pi?is?approximately?"?+?PI);
???????System.out.println("The?sin?of?Pi/4?is?about?"?+
?????????sinPiOverFour());
?????}
???}
????這樣這些常量就變成你的類的一部分了，如果這個類不是final的話，其它的類繼承了這個類又使得它繼承了這些常量，但是這也許不是用戶需要的結(jié)果。
????針對這樣的情況，我們可以這樣做，那就是在一個類中定義這些常量，使用的時候可以通過ClassName.variableName來訪問他們。例如
package?staticEx;
???public?class?IrrationalConstants?{
?????public?static?final?double?SQRT_TWO?=?1.414;
?????public?static?final?double?SQRT_THREE?=?1.732;
???}
package?staticEx;
???public?class?TranscendentalConstants?{
?????public?static?final?double?PI?=?3.14159;
?????public?static?final?double?E?=?2.71828;
???}
????現(xiàn)在J2SE?5.0提供了靜態(tài)導(dǎo)入的功能，你只需要在import關(guān)鍵字后面寫一個static關(guān)鍵字就可以直接使用類中定義的常量了，例如
???import?static?staticEx.IrrationalConstants.SQRT_TWO;
???import?static?staticEx.IrrationalConstants.SQRT_THREE;
???import?static?staticEx.TranscendentalConstants.PI;
????當(dāng)然你也可以使用.*的格式，例如
import?static?staticEx.IrrationalConstants.*；的格式??
package?staticEx;
???import?static?staticEx.IrrationalConstants.SQRT_TWO;
???import?static?staticEx.IrrationalConstants.SQRT_THREE;
???import?static?staticEx.TranscendentalConstants.PI;
???public?class?ConstantsWithStaticImport?{
?????public?static?double?sinPiOverFour()?{
???????return?SQRT_TWO?/?2;
?????}
?????public?static?void?main(String[]?args)?{
???????System.out.println("Pi?is?approximately?"?+?PI);
???????System.out.println("The?sin?of?Pi/4?is?about?"?+
?????????sinPiOverFour());
?????}
???}
????運(yùn)行該程序會得到
???Pi?is?approximately?3.14159
???The?sin?of?Pi/4?is?about?0.707
????在這里提醒大家一下，如果你過多的使用.*的樣式，那么可能會給程序的可讀性帶來負(fù)面的影響。因?yàn)槟愫茈y看出這個變量在哪里定義的。
????對常量定義是如此，對靜態(tài)方法的使用也適用。例如下面的例子
??package?staticEx;
???public?class?IrrationalConstants2?{
?????public?static?final?double?SQRT_TWO?=?1.414;
?????public?static?final?double?SQRT_THREE?=?1.732;
?????public?static?double?sinPiOverFour()?{
???????return?SQRT_TWO?/?2;
?????}
???}
package?staticEx;
???import?static?staticEx.IrrationalConstants2.*;
???import?static?staticEx.TranscendentalConstants.*;
???public?class?ConstantsWithStaticImport2?{
?????public?static?void?main(String[]?args)?{
???????System.out.println("Pi?is?approximately?"?+?PI);
???????System.out.println("The?sin?of?Pi/4?is?about?"?+
?????????sinPiOverFour());
?????}
???}
????在ConstantsWithStaticImport2類中靜態(tài)導(dǎo)入IrrationalConstants2后就可以直接使用它的靜態(tài)方法sinPiOverFour()了。
????以上簡單的介紹了如何使用static?import，您可以參考一下連接獲得更多的介紹：
http://java.sun.com/developer/JDCTechTips/2004/tt1005.htmlhttp://java.sun.com/j2se/1.5.0/docs/guide/language/static-import.html凡是有該標(biāo)志的文章，都是該blog博主Caoer（草兒）原創(chuàng)，凡是索引、收藏
、轉(zhuǎn)載請注明來處和原文作者。非常感謝。