Java类型转换和作用域

Question

I have two interfaces: MinServer and MaxServer. MaxServer extends MinServer. So, is the following code normal and right?

MinServer server=foo.getUndefinedServer();
...code according to MinServer
if (isThisServerMax){
  server=(MaxServer)server;
...code according to MaxServer
}
...code according to MinServer

Answer 1

No, it is not correct.

At this point:

server=(MaxServer)server
...code according to MaxServer

You are casting a MaxServer to a MinServer (which is OK) but then you assign the result of the casting to a MinServer ... so ehhm, you are in the same point where you started.

If you change it like this:

MaxServer server2 = (MaxServer)server
...code according to MaxServer using server2

then it will be correct.

Answer 2

No.

If the isThisServerMax is a boolean variable that correctly indicates whether server 's class implements MaxServer , then everything actually shown will run without error. However, nothing you present establishes a context wherein methods can be invoked on server that do not belong to interface MinServer , as I suppose is the intended meaning of "code according to MaxServer".

Answer 3

There is nothing wrong with the downcast you are doing. It will compile fine. Just know that if it can't be downcasted it will throw a runtime exception that you should handle. If it is not actually a MaxServer then it will throw an error...