Question

class Animal {

}

class Horse extends Animal {

}

public class Test {
    public static void main(){
        Animal a = new Horse();//mark1
    }
}


Animal ah = new Horse();// reference type is Animal and object is Horse

I can’t fully understand this below:
Animal ah = new Horse();// reference type is Animal and object is Horse
I always keep the reference type the same with the object
I want to know the reason of using the not-same reference type.Please tell me some condition that it’s more appropriate to use it

Answer 1

this works for several occasions, a simple example would be if you needed an array of animals:

Animal array[] = new Animal[3];
array[0] = new Horse();
array[1] = new Cow();
array[2] = new Animal();

You could have some defined generic methods or have them specified in for each class that extends.

public class Cow extends Animal {
    public void sound() {
        System.out.println("MOO");
    }
}

public class Horse extends Animal {
    public void sound() {
        System.out.println("HEEEHREHE");
    }
}

While in Animal you could not know the thing to do, so you could use somthing like

public class Animal {
    public void sound() {
        System.out.println(".");
    }
}

The thing is that if you go throug every animal in your array it would use the method defined for it.

for (int i = 0; i < 3; i++) {
    array[i].sound();
}

would output:

HEEEHREHE
MOO
.