Polymorphism is one of the OOPs feature that allows us to perform a single action in different ways. For example, lets say we have a class Animal that has a method sound(). Since this is a generic class so we can’t give it a implementation like: Roar, Meow, Oink etc. We had to give a generic message.

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

Now lets say we two subclasses of Animal class: Horse and Cat that extends (see Inheritance) Animal class. We can provide the implementation to the same method like this:

public class Horse extends Animal{
...
    @Override
    public void sound(){
        System.out.println("Neigh");
    }
}
and
public class Cat extends Animal{
...
    @Override
    public void sound(){
        System.out.println("Meow");
    }
}

As you can see that although we had the common action for all subclasses sound() but there were different ways to do the same action. 

What is polymorphism in programming?

Polymorphism is the capability of a method to do different things based on the object that it is acting upon. In other words, polymorphism allows you define one interface and have multiple implementations. As we have seen in the above example that we have defined the method sound() and have the multiple implementations of it in the different-2 sub classes.

Which sound() method will be called is determined at runtime so the example we gave above is a runtime polymorphism example.

Types of polymorphism and method overloading & overriding are covered in the separate tutorials. You can refer them here:

1. Method Overloading in Java – This is an example of compile time (or static polymorphism)

2. Method Overriding in Java – This is an example of runtime time (or dynamic polymorphism)

Static and dynamic binding in java

Association of method call to the method body is known as binding. There are two types of binding: Static Binding that happens at compile time and Dynamic Binding that happens at runtime. Before I explain static and dynamic binding in java,

What is reference and object?

class Human{
....
}
class Boy extends Human{
   public static void main( String args[]) {
       /*This statement simply creates an object of class
        *Boy and assigns a reference of Boy to it*/  
       Boy obj1 = new Boy();

       /* Since Boy extends Human class. The object creation
        * can be done in this way. Parent class reference 
        * can have child class reference assigned to it
        */
       Human obj2 = new Boy();
   }
}

Static and Dynamic Binding in Java

As mentioned above, association of method definition to the method call is known as binding. There are two types of binding: Static binding and dynamic binding. Lets discuss them.

Static Binding or Early Binding

The binding which can be resolved at compile time by compiler is known as static or early binding. The binding of static, private and final methods is compile-time. 

Static binding example

Here we have two classes Human and Boy. Both the classes have same method walk() but the method is static, which means it cannot be overriden so even though I have used the object of Boy class while creating object obj, the parent class method is called by it. Because the reference is of Human type (parent class). So whenever a binding of static, private and final methods happen, type of the class is determined by the compiler at compile time and the binding happens then and there.

class Human{
   public static void walk()
   {
       System.out.println("Human walks");
   }
}
class Boy extends Human{
   public static void walk(){
       System.out.println("Boy walks");
   }
   public static void main( String args[]) {
       /* Reference is of Human type and object is
        * Boy type
        */
       Human obj = new Boy();
       /* Reference is of HUman type and object is
        * of Human type.
        */
       Human obj2 = new Human();
       obj.walk();
       obj2.walk();
   }
}

Dynamic Binding or Late Binding

When compiler is not able to resolve the call/binding at compile time, such binding is known as Dynamic or late Binding. Method Overriding is a perfect example of dynamic binding as in overriding both parent and child classes have same method and in this case the type of the object determines which method is to be executed. The type of object is determined at the run time so this is known as dynamic binding.