当可能在运行时成功进行向下转换时：
Object o = getSomeObject(),String s = (String) o; // this is allowed because o could reference a String
在某些情况下，这不会成功：
Object o = new Object();String s = (String) o; // this will fail at runtime, because o doesn't reference a String
当强制转换（例如最后一个）在运行时失败时，
ClassCastException将抛出a。
在其他情况下，它将起作用：
Object o = "a String";String s = (String) o; // this will work, since o references a String
请注意，某些强制类型转换将在编译时被禁止，因为它们根本不会成功：
Integer i = getSomeInteger();String s = (String) i; // the compiler will not allow this, since i can never reference a String.
