design patterns - Long list of if statements in Java

Question

Sorry I can't find a question answering this, I'm almost certain someone else has raised it before.

My problem is that I'm writing some system libraries to run embedded devices. I have commands which can be sent to these devices over radio broadcasts. This can only be done by text. inside the system libraries I have a thread which handles the commands which looks like this

if (value.equals("A")) { doCommandA() }
else if (value.equals("B")) { doCommandB() } 
else if etc. 

The problem is that there are a lot of commands to it will quickly spiral to something out of control. Horrible to look out, painful to debug and mind boggling to understand in a few months time.

Answer 1

using Command pattern:

public interface Command {
     void exec();
}

public class CommandA() implements Command {

     void exec() {
          // ... 
     }
}

// etc etc

then build a Map<String,Command> object and populate it with Command instances:

commandMap.put("A", new CommandA());
commandMap.put("B", new CommandB());

then you can replace your if/else if chain with:

commandMap.get(value).exec();

EDIT

you can also add special commands such as UnknownCommand or NullCommand, but you need a CommandMap that handles these corner cases in order to minimize client's checks.