How to downcast a Java object?

Question

I am trying to understand Java's polymorphism, and I have one question about downcasting an object. Let's say for this example I have two subclasses Dog and Cat that inherit from a superclass Animal From what I understood, the only way to downcast an object is if this Object is already of the good type, like this: ``` Animal a = new Dog(); Dog d = (Dog) a; ``` This works right? But what if I want to create a regular animal without knowing what it would be, and then cast it when I know, how can I do that? ``` Animal a = new Animal(); Dog d = (Dog) a; ``` This will throw a ClassCastException at runtime right? The only way I found to do that is to create a new Dog constructor that creates a dog from a regular animal: ``` Animal a = new Animal(); Dog d = new Dog(a); ``` with ``` public Class Dog extends Animal{ public Dog(Animal a){ super(a); } } ``` So my question is, how am I supposed to do this? - Am I doing it the best way? - Am I not supposed to do this at all, if I have to it means…

Accepted Answer

If you want to create an instance of a type that may vary depending upon non-local conditions, use an Abstract Factory (as described in the Design Patterns book).

In it's simplest form:

```
interface AnimalFactory {
    Animal createAnimal();
}

class DogFactory implements AnimalFactory {
    public Dog createAnimal() {
        return new Dog();
    }
}
```

Note also there is a difference between the static type of a reference and the dynamic type of the object. Even though you have an `Animal` reference, if the original object is a `Dog`, it still behaves like a `Dog`.

How to downcast a Java object?

Solution

Problem