Replacing if else statement with pattern

Question

I have a if else statement which might grow in the near future.

    public void decide(String someCondition){

        if(someCondition.equals("conditionOne")){
            //
            someMethod("someParameter");

        }else if(someCondition.equals("conditionTwo")){

           //
           someMethod("anotherParameter");

        }
        .
        .
        else{

            someMethod("elseParameter");

        }
}

Since, this is already looking messy, I think it would be better if I can apply any design patterns here. I looked into Strategy pattern but I am not sure if that will reduce if else condition here. Any suggestions?

Answer 1

This is a classic Replace Condition dispatcher with Command in the Refactoring to Patterns book.

Basically you make a Command object for each of the blocks of code in your old if/else group and then make a Map of those commands where the keys are your condition Strings

interface Handler{
    void handle( myObject o);
}


 Map<String, Handler> commandMap = new HashMap<>();
 //feel free to factor these out to their own class or
 //if using Java 8 use the new Lambda syntax
 commandMap.put("conditionOne", new Handler(){
         void handle(MyObject o){
                //get desired parameters from MyObject and do stuff
          }
 });
 ...

Then instead of your if/else code it is instead:

 commandMap.get(someCondition).handle(this);

Now if you need to later add new commands, you just add to the hash.

If you want to handle a default case, you can use the Null Object pattern to handle the case where a condition isn't in the Map.

 Handler defaultHandler = ...

if(commandMap.containsKey(someCondition)){
    commandMap.get(someCondition).handle(this);
}else{
    defaultHandler.handle(this);
}