Instanceof with a javax.jms.Topic

Question

I have an instance of Topic called "testTopic". Can somebody explain to me why both of these statements return true?
testTopic instanceof javax.jms.Topic
testTopic instanceof javax.jms.Queue
How can something be both a Topic and a Queue. The second one should return false.
Try it and see for yourself.

Answer 1

Translating bbritta's code into plain text (nice example!): many JMS vendors actually use the same implementation class for both Queues and Topics, which is why you got the results you did.
Now, there are a couple of ways to tell them apart. You'll have to experiment to see which works "best for you." And some of these may be vendor-specific handling, so not all results will apply on different servers.
1) Invoke getTopicName() after casting your Destination to Topic (or Queue); check the result or see if you get an exception.
2) If you have control (or input) over the server environment, make the names of the Destinations "BlahQueue" and "BlechTopic" and use the name to determine which one.
3) Try to send a message and catch it when it barfs - if it does, that is.
4) Pass a boolean or int to your methods using Destination, designating which one it is.
My opinion: don't use generic Destinations - always code explicitly to a Queue or Topic JMS method. This helps you keep track of how your messages are handled and avoids confusion later on.