Java中的Class類有兩個重要的方法：cast()和asSubclass()。這兩個方法都是用于Class類進行運行時(run-time)階段的類型轉換的。下面來看看他們的定義和使用：

第一個方法asSubclass()

public?Class?extends?U>?asSubclass(Class?clazz)

Casts?thisClassobject?to?represent?a?subclass?of?the?class?represented?by?the?specified?class?object.?Checks?that?that?the?cast?is?valid,?and?throws?aClassCastExceptionif?it?is?not.?If?this?method?succeeds,?it?always?returns?a?reference?to?this?class?object.

This?method?is?useful?when?a?client?needs?to?"narrow"?the?type?of?aClassobject?to?pass?it?to?an?API?that?restricts?theClassobjects?that?it?is?willing?to?accept.?A?cast?would?generate?a?compile-time?warning,?as?the?correctness?of?the?cast?could?not?be?checked?at?runtime?(because?generic?types?are?implemented?by?erasure).

Returns:

thisClassobject,?cast?to?represent?a?subclass?of?the?specified?class?object.

Throws:

ClassCastException-?if?thisClassobject?does?not?represent?a?subclass?of?the?specified?class?(here?"subclass"?includes?the?class?itself).

Since:

1.5

使用實例：

Class>?c?=?Class.forName(“MyClass”);

Class?child?=?c.asSubClass(Father.class);??//?“Child”?extends?“Father”

第二個方法cast()

publicTcast(Objectobj)

Casts?an?object?to?the?class?or?interface?represented?by?thisClassobject.

Parameters:

obj-?the?object?to?be?cast

Returns:

the?object?after?casting,?or?null?if?obj?is?null

Throws:

ClassCastException-?if?the?object?is?not?null?and?is?not?assignable?to?the?type?T.

Since:

1.5

使用實例：

這個函數可以簡單的理解為將一個Object類型的對象轉換成Class表示的類型的對象(其實在java中Class類就是所有類型字節碼的一種抽象，或者簡單理解為在java中就是用于來說明類型的類型，因為java中一切都是類，所以“類”(類型)也是是類，而這個類就是Class)。

Class?c?=?String.class;

Object?o?=?“hello?world!”;

String?s?=?c.cast(o);??????//?將Object類型轉成這里的Class表示的類型，即String類型

Java中的Class類可以表示java中的所有類型，主要通過Class中的T來指定具體表示的是哪種類型。

總結

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