如果您引用它的表達式的編譯時類型是您自己的類或子類,則只能訪問不同包中類型的受保護成員. (其中“你的”類是包含代碼的類.)你自己的類必須是原來聲明方法的類的子類.

這是一個例子假設Base與所有其他類在不同的包中：

package first;

public class Base

{

protected void Foo() {}

}

// Yes, each class is really in its own file normally - but treat

// all the classes below as being in package "second"

package second;

public class Child extends Base

{

public void OtherMethod(Object x)

{

((Base) x).Foo(); // Invalid: Base is not Child or subclass

((Child) x).Foo(); // Valid: Child is Child

((GrandChild) x).Foo(); // Valid: GrandChild is subclass of Child

((OtherChild) x).Foo(); // Invalid: OtherChild is not Child or subclass

}

}

public class GrandChild extends Child {}

public class OtherChild extends Base {}

換句話說,它允許您訪問“像你一樣的對象”的受保護成員.

A protected member or constructor of

an object may be accessed from outside

the package in which it is declared

only by code that is responsible for

the implementation of that object.

6.6.2.1 Access to a protected Member

Let C be the class in which a

protected member m is declared. Access

is permitted only within the body of a

subclass S of C. In addition, if Id

denotes an instance field or instance

method, then: If the access is by a

qualified name Q.Id, where Q is an

ExpressionName, then the access is

permitted if and only if the type of

the expression Q is S or a subclass of

S. If the access is by a field access

expression E.Id, where E is a Primary

expression, or by a method invocation

expression E.Id(. . .), where E is a

Primary expression, then the access is

permitted if and only if the type of E

is S or a subclass of S.

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