栈类应当受益于指定元素类型的能力。有了泛型以后，这个将成为可能。
The Stack class would clearly benefit from the ability to specify its element type. With generics, that becomes possible.
要使操作成功完成，调用栈中的每个类必须有必需的许可权。
Each class in the calling stack must have the required permissions for the operations to complete successfully.
调用栈的检查在遇到PrivilegedAction实例时停止，有效地将执行指定操作所必需的许可权授予所有的后继类调用。
The check of the calling stack stops when it encounters a PrivilegedAction instance, effectively granting all subsequent class calls the required permission to perform the specified operation.
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