CODEWITHSUNDEEP
javadesign-patterns
strauberry
strauberry

Reputation: 4199

Type mismatch in object factory

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!

Upvotes: 2

Views: 425

Answers (5)

Yurii Hohan
Yurii Hohan

Reputation: 4171

Usually you want to treat the objects of the same factory similarly, that is why Superclass is created to have all the common behaviour in. If you really want to use them class specific, then, yes, you should do the cast. But I feel like it kills the purpose of abstract factory.

if you want a specific product or group of products when subclass your factory and receive it from the child factory

Upvotes: 0

Stephan
Stephan

Reputation: 7388

You could explicitely cast the return value to Subclass which will fail as soon as the factory returns another Subclass of Superclass and will throw an exception during runtime. Therefore, don't do it.

The more appropriate way would be to declare myObj as type Superclass and have the interface contain the methods you need. If you need to know the exact subtype that is returned, using a factory pattern is a bit pointless.

Upvotes: 1

Jayendra
Jayendra

Reputation: 52789

As the method returns Superclass you cant directly assign it to a Subclass.

You need to use this or type cast.

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

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

Upvotes: 0

Tobias
Tobias

Reputation: 9380

It's the other way around. If you get a SubClass you can assign it to a SuperClass variable (if we assume SubClass extends SuperClass).

Upvotes: 0

aioobe
aioobe

Reputation: 421040

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");

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");

Upvotes: 2

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