Many times people confuse inheritance with “contains” relationship, which is not correct at all. Inheritance is “
A type of” relationship not “
contains” relationship. So a child is a superset of Parent’s abilities.
Example: Each fox is an Animal. But the other way is not always true (Not every Animal is Fox). Hence, Fox is a type of Animal.
class Animal{
1. Ability of walking
}
class Fox extends Animal{
1. Inherited abilities: Ability of Animal (walking)
2. It’s own abilities (like hunting)
}
Now let's try to digest few concepts here.
When I write this : Animal a = new Fox(). What does it mean?
It means Fox is also an Animal or every Fox is a type of Animal which makes sense too.
What about : Fox f = new Animal()
It means Animal is a type of Fox or every Animal is a Fox, which is absolutely incorrect.
So "Fox is an Animal" is fine but the reverse doesn't make sense. For example, Apple is a Fruit, Car is a Vehicle etc. Remember inheritance is uni-directional. For example, House is a Building. But Building is not a House.
Let's formulate our finding in terms of coding: A reference to child class object can't refer to the parent class object. Whereas a reference to parent class object can refer to child class object.
Example 1:
Animal a = new Fox(); //True
Fox f = new Animal(); //False (Compilation error)
Example 2:
Animal a = new Animal();
Fox f = new Fox();
a = f; //True
f = a; //False (Compilation error)
So we learned few things here:
A child class object is inherently a parent class object. In simple terms, objects/reference of parent class can hold objects of child class but the reverse is not true.
Example 3:
public class TestMain {
public static void main(String[] args) {
Animal a = new Fox();
a.walk();
((Fox)a).hunt(); //need to typecast because reference "a" doesn't hold "hunt" method
}
}
class Animal{
void walk(){
System.out.println("Animal can walk");
}
}
class Fox extends Animal{
void hunt(){
System.out.println("Fox can hunt");
}
}
Output:
Animal can walk
Fox can hunt
Few points to mention here:
1. A subclass can't inherit private members of Superclass obviously.
2. Try to keep all variables private or protected and use public accessor(set/get) methods to access them.
3. On a light note: Initially don't try to learn this concept with "child" and "parent" example as you might get confused. Playing with a dog or a cat is safe and easier. :)
