Replace conditional with polymorphism - nice in theory but not practical

Question

"Replace conditional with polymorphism" is elegant only when type of object you're doing switch/if statement for is already selected for you. As an example, I have a web application which reads a query string parameter called "action". Action can have "view", "edit", "sort", and etc. values. So how do I implement this with polymorphism? Well, I can create an abstract class called BaseAction, and derive ViewAction, EditAction, and SortAction from it. But don't I need a conditional to decided which flavor of type BaseAction to instantiate? I don't see how you can entirely replace conditionals with polymorphism. If anything, the conditionals are just getting pushed up to the top of the chain.

EDIT:

public abstract class BaseAction {     public abstract void doSomething(); }  public class ViewAction : BaseAction {     public override void doSomething() { // perform a view action here... } }  public class EditAction : BaseAction {     public override void doSomething() { // perform an edit action here... } }  public class SortAction : BaseAction {     public override void doSomething() { // perform a sort action here... } }   string action = "view";  // suppose user can pass either "view", "edit", or "sort" strings to you. BaseAction theAction = null;  switch (action) {     case "view":         theAction = new ViewAction();         break;      case "edit":         theAction = new EditAction();         break;      case "sort":         theAction = new SortAction();         break; }  theAction.doSomething();    // So I don't need conditionals here, but I still need it to decide which BaseAction type to instantiate first. There's no way to completely get rid of the conditionals. 

Answer 1

You're right - "the conditionals are getting pushed up to the top of the chain" - but there's no "just" about it. It's very powerful. As @thkala says, you just make the choice once; from there on out, the object knows how to go about its business. The approach you describe - BaseAction, ViewAction, and the rest - is a good way to go about it. Try it out and see how much cleaner your code becomes.

When you've got one factory method that takes a string like "View" and returns an Action, and you call that, you have isolated your conditionality. That's great. And you can't properly appreciate the power 'til you've tried it - so give it a shot!