Dynamic Binding

When the type of object is determined at runtime.

Happens when object of child is assigned to reference of parent.

Example:

Human h = new Student()
//Student IS-A Human

foo(Human h) //method definition

//method call
foo(Student s)

Reference of Human type ————> Object of Student type.

(Reference Variable) ----------------> (Object)

Argument -- Human h = new Student() is not dynamic binding

#interview-question

Someone could very easily make an argument that Human h = new Student() is not Dynamic Binding

class Human{
    foo(){}
}

class Student extends Human{
    foo(){}
    xyz(){}
}

main(){
    Human h = new Student();
    
    h.xyz(); //This line will give error. 
    h.foo(); 
}

Why it looks like its not runtime polymorphism

The line h.xyz() will immediately show an error.

• h.xyz() shows error
• ⇒ Program doesn't know h is an object of class Student
• ⇒ h is an object of Human type
• ⇒ Its not runtime polymorphism

What actually happens

Compiler doesn't know at compile time that 'h' is an object of Student type.

• Object type determined at runtime
• ⇒ Compiler still thinks 'h' is an object of Human type
• ⇒ COMPILE TIME Error.

Even though we can't call Student-exclusive methods from this reference without casting, the type of object or the version of method to be called is still determined at runtime. Which still makes it a case of dynamic binding and Runtime Polymorphism.

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Backlinks

• Runtime Polymorphism (cs)