Question

I’m using a very simple factory class to create an initialize some of my objects:

   public class MyFactory {
      public static Superclass createObject(String type) {
         Superclass myObject = null;
         if (type.equals("type1")) {
            myObject = new Subclass();
            myObject.setParam("val1");
         }
         return myObject;
      }
   }

Not that complex 🙂 No generic code etc., only this way. Nevertheless I get a type mismatch error in Eclipse if I use my factory this way:

Subclass myObj = MyFactory.createObject("type1");

The error says that it cannot convert from Superclass to Subclass, but everywhere I look (Heads First Design Patterns etc.) I can see it this way: return as type the superclass of created subclasses… So why do I get this error :-)? Thank you!

Answer 1

The error says that it cannot convert from Superclass to Subclass

That’s right. It’s the same reason you can’t store a Number in an Integer for instance (The Number could actually be a Double!)

If you know that Subclass is returned from ProbeFactory.createProbe("type1") then you can cast it like this:

Subclass myObj = (Subclass) ProbeFactory.createProbe("type1");

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Down-casting should be avoided if possible. Either you design it so that you can do with

Superclass myObj = ProbeFactory.createProbe("type1");

or you could try to create a type-safe version of the factory:

Subclass myObj = ProbeFactory.createSubProbe("type1");