Inheritance - Creating object with super class and subclass

[Mahesh Kumar ...]

Hi

There is a animal class

public class Animal {
    public void eat() { System.out.println('Eating from super class');  }
}

public class Dog extends Animal {
   public void eat() { System.out.println('I am dog. I am eating now');  }
   public void showLove() {System.out.println('I am very lovable');}

}

public cass Tiger extends Animal {
      public void eat() { System.out.println('I am Tiger. I am eating now');  }
}

We can create objects like this.

Animal animal = new Animal();
Animal animal = new Dog();
Dog dog = new Dog();

My question is, why should we create new Dog() with Animal reference? and what is the use?
In what conditions i should create

Animal animal = new Dog();
Animal animal = new Tiger();

and in what conditions i should use

Dog dog = new Dog();

Please kindly explain.
Thanks

[Knute Snortum]

Welcome to the Ranch, Mahesh Kumar!

Think about a program that just deals with what animals eat, not the specific animal.  You might have a method like this:

public void whatDoIEat(Animal animal) {
    animal.eat();
}

How would you be able to do this without an interface or a supertype?

[Ryan McGuire]

Also, sometimes you don't know ahead of time what kind of Animal you need.

Animal pet;
System.out.println("What kind of pet would you like?");
Scanner sc = new Scanner(System.in);
String answer = sc.nextLine();
if (answer.equals("dog")) {
    pet = new Dog();
}
else if (answer.Equals("cat")) {
    pet = new Cat();
}

[Mahesh Kumar ...]

Thank you. I understand now.

[Campbell Ritchie]

Ryan McGuire wrote:. . .if (answer.equals("dog")) {    pet = new Dog(); } else if (answer.Equals("cat")) {    pet = new Cat(); }

...but, if there is any possibility of its being a cat, it cannot show love.

Because the showLove() method isn't declared in Animal, it cannot be invoked on the pet, not even if it is a dog.

[Mahesh Kumar ...]

Animal animal = new Dog();

The animal class has some default methods for all the animals.
But Dog class has one specific method "showLove"


Now

Animal animal = new Dog();
animal.showLove();

Is not working. Animal class does not know anything about Dog class.
How do we handle this situation?

[Campbell Ritchie]

Only say, “default method,” when referring to interfaces. You have concrete methods in the Animal class (which class should probably be declared abstract) and you are overriding them in the subclasses. Whenever you are overriding methods, tag them with @Override. Look at the three Java® Language Specification (=JLS) links in that page, and this Java™ Tutorials page for more information..
You can access showLove() like this:-

if (pet instanceof Dog)
{
   ((Dog)pet).showLove();
}

Not only is that code awkward to read, but also I think any use of instanceof is a sign of bad design in your inheritance.

[Eduardo Bueno]

Just to expand further on the response of the first question, I suggest you take a look at https://en.wikipedia.org/wiki/Liskov_substitution_principle