Question

I’m preparing to SCJP and looks like I don’t understand class cast principles.

class Dog extends Animal

First way of creating Dog() instance- make instance of Animal and cast it to Dog(upcast):

Animal a = new Dog();
Dog d1 = (Dog)a;

VS

Second way of creating Dog() instance – make it directly:

Dog d2 = new Dog();

what is the difference between d1 and d2 objects this case?

Answer 1

Animal a = new Dog();// a dog is created but it will be referred by reference of animal as dog is an animal
Dog d1 = (Dog)a;//now we know that this animal is Dog so we are casting it to dog.

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Dog d2 = new Dog();// we are creating instance of dog which is referred by reference of Dog

what is the difference between d1 and d2 objects this case?

d1 and d2 are just reference to dog, both will eventually refer to an instance of Dog .
there is no difference

Also See

• Polymorphism