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一個 Generic問題?
來自 http://blog.csdn.net/zhangxiaoxiang/archive/2006/11/07/1372076.aspx?Pending=true

下面程序的main方法中的第二行代碼和注釋中的兩行代碼表達的意思完全相同，注釋中的兩行代碼不能通過編譯（這很容易理解），而第二行（采用方法調用鏈）卻可以順利通過編譯（這就很難理解了）。

public ? class ?Test
{
???? public ? void ?func()
???? {
?????????System.out.println( " func " );
???}
?
??? public ? static ? void ?main(String?args[])? throws ?Exception
??? {
???????????Object?obj? = ? new ?Test();

??????????? // 下面這行可以成功編譯??
???????????((Test)obj).getClass().newInstance().func();

??????????? // 下面這兩行無法通過編譯
??????????? /**/ /* Class?c?=?((Test)obj).getClass();
???????????c.newInstance().func();? */
???
?}
} ?

public?class?Test
{
????public?void?func()
????{
?????????System.out.println("func");
???}
?
???public?static?void?main(String?args[])?throws?Exception
???{
???????????Object?obj?=?new?Test();

???????????//下面這行可以成功編譯??
???????????((Test)obj).getClass().newInstance().func();

???????????//下面這兩行無法通過編譯
???????????
???????????
???????????//下面這兩行無法通過編譯
???????????/**//*Class?c?=?((Test)obj).getClass();
???????????c.newInstance().func();?*/

???????????
?????????Class?<???extends?Test>?c?=?((Test)obj).getClass();
?????????//因為Generic,?編譯器可以在編譯期獲得類型信息所以可以編譯這類代碼。你將下面那兩行改成?

???????????c.newInstance().func();?
???
?}
}?

下面的代碼
?? Object obj = new Test();

???? //下面這行可以成功編譯??
??? ((Test)obj).getClass().newInstance().func();

javap之后

public static void main(java.lang.String[])?? throws java.lang.Exception;
? Code:
?? 0:?? new???? #5; //class Test
?? 3:?? dup
?? 4:?? invokespecial?? #6; //Method "<init>":()V
?? 7:?? astore_1
?? 8:?? aload_1
?? 9:?? checkcast?????? #5; //class Test
?? 12:? invokevirtual?? #7; //Method java/lang/Object.getClass:()Ljava/lang/Class;
?? 15:? invokevirtual?? #8; //Method java/lang/Class.newInstance:()Ljava/lang/Object;
?? 18:? checkcast?????? #5; //class Test
?? 21:? invokevirtual?? #9; //Method func:()V
?? 24:? return

}
???

??? ((Test)c.newInstance()).func();

?? 52:? invokevirtual?? #16; //Method java/lang/Class.newInstance:()Ljava/lang/Object;
?? 55:? checkcast?????? #5; //class Test



virtualf?發表于2006-11-08 14:19:00??IP: 222.209.220.*

//的確如此。




???? Object obj = new Test();
???? Class < ? extends Test> c = ((Test)obj).getClass();
?????? c.newInstance().func();
??
??
?? 9:?? checkcast?????? #5; //class Test
?? 12:? invokevirtual?? #7; //Method java/lang/Object.getClass:()Ljava/lang/Class;
?? 15:? astore_2
?? 16:? aload_2
?? 17:? invokevirtual?? #8; //Method java/lang/Class.newInstance:()Ljava/lang/Object;
?? 20:? checkcast?????? #5; //class Test
?? 23:? invokevirtual?? #9; //Method func:()V
?? 26:? return

}

? Class < ? extends Test> c = ((Test)obj).getClass();
? System.out.println();?
? Test? ff=? c.newInstance();//.func();

就無需轉換?.直接拿到對象了.

?

getClass

public final Class<? extends Object> getClass()






當系統認為  class 變量 < 來自那個類的class > 
 通過 class 變量.newInstance; 被賦予了 那個類的類型.