instance variables - polymorphism(overriding)

[Harikrishna Gorrepati]

Hi,

Please advice why instance variables do not participate in polymorphism(overriding) but instance methods can participate..

[Harikrishna Gorrepati]

Hi,

Please advice why instance variables do not participate in polymorphism(overriding) but instance methods can participate..

class BAse11_1
{
	int instanceX = 10;
	void instanceMethod()
	{
		System.out.println("BAse11 instanceMethod");
	}
}

class ParentOfBase_1 extends BAse11_1
{
	int instanceX = 20;
	void instanceMethod()
	{
		System.out.println("ParentOfBase instanceMethod");
	}
}

public class P102_Polymorphism_Instance
{
	public static void main(String[] args)
	{
		BAse11_1 b = new ParentOfBase_1();
		b.instanceMethod();		           //ParentOfBase instanceMethod		
		System.out.println(b.instanceX);   // printing 10  :confused: 
	}
}

Why Line 25 is printing 10 instead of 20 ?

[Ram Narayan.M]

Since instance variables are binded up with the type of reference variables, fields are not involved in Polymorphism...

[Arjun Srivastava]

always remember
overriding-depends upon object type(run-time type).
overloading-depends upon reference type(compile type).
which instance variable invoked determines by the reference type(compile type).

[Ram Narayan.M]

Polymorphism does not apply to:
1. static methods
2. private methods.
3. variables.
4. overloaded methods.
5. Generics

Above lists are done in Compile time called Compile time binding or static binding

Dynamic Binding or Runtime binding is done in Method Overriding

[Harikrishna Gorrepati]

Arjun Srivastava wrote:always remember
overriding-depends upon object type(run-time type).
overloading-depends upon reference type(compile type).
which instance variable invoked determines by the reference type(compile type).

Hi Arjun,

I want to get insight why "instance variable invoked determines by the reference type(compile type)." ?? whereas "instance methods invoked determines by the object type(run-time type)."

[Ram Narayan.M]

Please visit this link

https://coderanch.com/t/508371/java-programmer-SCJP/certification/Why-Properties-cannot-Overridden#2297158

[Harikrishna Gorrepati]

Ram Narayan.M wrote:Please visit this link

https://coderanch.com/t/508371/java-programmer-SCJP/certification/Why-Properties-cannot-Overridden#2297158

Thanks Ram..This link resolved my question. Earlier you mentioned that, Polymorphism doesn't apply to point#4 i.e., overloaded methods..The basic purpose of polymorphism is overloading and overriding know ?? That's why for overloading we are using same name different parameters.. Could you please advice on that ?

[Arjun Srivastava]

Harikrishna Gorrepati wrote:
That's why for overloading we are using same name different parameters.. Could you please advice on that ?

when you are overloading a method (by only changing the no of arguments),you are creating a fresh new different method.

[Harikrishna Gorrepati]

Arjun Srivastava wrote:

Harikrishna Gorrepati wrote:
That's why for overloading we are using same name different parameters.. Could you please advice on that ?

when you are overloading a method (by only changing the no of arguments),you are creating a fresh new different method.

So, Overloading doesn't apply to Polymorphism. Only Overriding applies to Polymorphism.. Is that right ?? People have different opinions on this concept.

[Ram Narayan.M]

I have observed some experts saying that Method Overloading does not fall under Polymorphism....

According to Polymorphism, an entity can be of any form...

E.g.,

Car carObj = new FiatCar();

carObj = new VolkswagonCar();

That is, Car can be FiatCar or VolkswagonCar(Any form)... So dynamically carObj can refer to any subclass instance... Such a flexibility given by Polymorphism...

Polymorphism leads to the building of Generic method (not the Generics) which can work for any type...


And in Method Overloading, once the method pattern is binded at compile time... thats all... cannot be changed...So its not Polymorphism...

[Harikrishna Gorrepati]

Excellent Ram. I will stick to Polymorphism is only for overriding..

[Harikrishna Gorrepati]

Hi Ram, You mentioned Polymorphism doesn't apply to Generics. I am not clear on that. Can you please give simple code for that. Thanks

[Arjun Srivastava]

Harikrishna Gorrepati wrote:You mentioned Polymorphism doesn't apply to Generics. I am not clear on that. simple code for that.

polymorshism doesn't apply to generics until and unless you are using wildcards(?) within the parametrized type.

  List<Object>=ArrayList<String>(); //error 

because polymorshism doesn't apply to generics

  List<Object>=ArrayList<object>(); //works fine 

unless you are using wildcards(?) within the parametrized type.

  List<? extends Number>=ArrayList<Integer>(); //works fine as you are using wildcard.