Why assign a subclass object to a superclass reference? [duplicate]

问题

Animal is a superclass

Feline is a subclass of Animal

Cat is a subclass of Feline

Canine is a sublcass of Animal

Dog is a subclass of Canine

All of the classes have their own eat() method that outputs:

"(class) is eating"

I've already tried creating an array of Animals, looping through them, and calling the eat() method, which outputs the proper output for each given animal.

My question is, what does one gain by doing this:

Cat j = new Animal();

wouldn't Cat j = new Cat() do anything you need do regarding methods with Animal types, since it's already an animal through inheritance?

Edit: I'm sorry, I meant it the other way around--what does one gain by declaring: Animal j = new Cat();

sorry!

回答1:

Why assign a subclass object to a superclass reference?

Polymorphism
Mock Object while doing testing
Flexibility if you want to replace it with another implementation in future
Most likely used in Inheritance and abstract classes

回答2:

by doing the same we achieve runtime polymorphism.

based on the type of object the overridden method will be called

now coming toy your question

just a single line answer to that will be

A cat is always an animal

Animal a = new Cat();//ok

All animals are not cat

Cat c = new Animal();//not ok

here is a good example to learn and understand this

来源：https://stackoverflow.com/questions/23254070/why-assign-a-subclass-object-to-a-superclass-reference

标签

java

inheritance

polymorphism