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 my program is not well conceived?
Is there a better way I missed?

Thanks a lot! nbarraille

How to downcast a Java object?

4 Answers