Selecting a class dynamically at runtime

Question

Im trying to come up with a solution where the class that processes a "message" is selected at runtime depending on the message type. I know that i can use something like this

if msg_type = "A"
  MsgAProcessor.execute(message);
else if msg_type = "B"
  MsgBProcessoror = execute(message);
....
....
....

I dont want to use the above approach as i dont want the code to know anything about the message types that i could be processing. I want to be able to in the future add a new message processor for a new message type. The solution i am thinking of at the moment is as follows

There are 3 message processors at the moment

MsgAProcessor
MsgBProcessor
MsgBProcessor

All three classes have a method called execute which will process the message in its own way. I have created an interface called MsgProcessor and added the execute() method in the interface.

Now i am having difficulty in knowing which message processor the caller should call without having to check the message type. For example, i cant do this

MsgProcessor proc = new MsgAprocessor() proc.execute()

The above will stil be required to be in an if statement as it needs to be called just after finding out the message type. I would also like to avoid instantiating using the implementation class type.

Is there a better way of achieving the same?

I want to be able to just call MsgProcessor.execute from the interface and have the runtime environment know which implementation class to call based on the message type.

Answer 1

1. Have an interface Processor that has a method execute(). All your three MsgProcessors implement this.
2. Have a separate class ProcessorControl that has a submit(String message) method. Whenever a new message comes, you simply do ProcessorControl.submit(message)
3. Now ProcessorControl has a method addProcessor(Proccesor proc, String type) which adds the processors to a hashtable with type as the key. Hence each processor is now assigned with a type.
4. In the submit method, just get hashtable.get(type).execute(proc)

This is a simple command pattern.