Just revisiting and explaining myself Polymorphism concept here through a blog post. The words Polymorphism means multiple forms. In Java ,Polymorphism means multiple forms of an object.
We shall divide this article into 3 sections.
1.Syntax
2.Calling a variable polymorphically.
3.Calling a method polymorphically.

1.Syntax

Now in polymorphism in Java, the thumb key rule to remember is

super = sub

Meaning the variable reference (LHS) must always be a SUPER class reference and the object initialization(RHS) must a SUB class object.

For Example:

class A{

}
class B extends A{
}

class C extends B{
}

class D extends A{
} 

So valid and invalid syntax according to the thumb rule will be

A a =new B();           //VALID
B b=new D();            //NOT VALID 
C c=new A();           //VALID
A a1=new D();           //VALID

2.Calling a variable polymorphically.

If a variable is called from a polymorphic object,we follow the reference i.e. the super class.
And if the variable is not present in the super class ,it results in a COMPILE ERROR.
EG:

class A{
int x=5;
}
class B extends A{
int x=10;
}
class App{
public static void main(String[]args){
A a=new B();
System.out.println(a.x); 
//What do u think is the output class A x value (5)or class B x value(10)?Follow the rule.

}
}
OUTPUT:
5

3.Calling a method polymorphically.

If a method is called from a polymorphic object ,we follow a 2 step procedure:
1.We got to the super class and check whther the method is present or not.

if(present)
 Goto to step 2 
else
 COMPILE ERROR

2.Come to the sub class and check wther the method is overrided or not.

if(overrided)
 Call the sub-class version
else
 Call the super -class version.

Eg:

class A{
void m1(){
System.out.println("A");
}}
class B extends A{
void m1(){
System.out.println("B");
}}
class App{
public static void main(String[]args){
A a=new B();
a.m1();          //Follow the rule
B=new B();
b.m1();          //Normal sub class object method call
}}
OUTPUT:
B
B

So that’s all for polymorphism in java. If any mistake please feel free to correct me in the comments(Grammatically or otherwise).

Happy Learning 🙂