What is dynamic method dispatch in Java?

Dynamic method dispatch is the mechanism in which a call to an overridden method is resolved at run time instead of compile time. This is an important concept because of how Java implements run-time polymorphism.

Java uses the principle of ‘a superclass reference variable can refer to a subclass object’ to resolve calls to overridden methods at run time. When a superclass reference is used to call an overridden method, Java determines which version of the method to execute based on the type of the object being referred to at the time call.

In other words, it is the type of object being referred to that determines which version of an overridden method will be executed.

// Implementing Dynamic Method Dispatch
class Apple 
{ 
    void display() 
    { 
        System.out.println("Inside Apple's display method"); 
    } 
} 
  
class Banana extends Apple 
{ 
    void display()   // overriding display()
    { 
        System.out.println("Inside Banana's display method"); 
    } 
} 
  
class Cherry extends Apple 
{ 
    void display()   // overriding display()
    { 
        System.out.println("Inside Cherry's display method"); 
    } 
} 
  
class Fruits_Dispatch 
{ 
    public static void main(String args[]) 
    {  
        Apple a  = new Apple();   // object of Apple
        Banana b = new Banana();  // object of Banana
        Cherry c = new Cherry();  // object of Cherry 
  
        Apple ref;    // taking a reference of Apple
           
        ref = a;   // r refers to a object in Apple
        ref.display();   // calling Apple's version of display()
        
        ref = b;   // r refers to a object in Banana
        ref.display();   // calling Banana's version of display()
  
        ref = c;  // r refers to a object in Cherry
        ref.display();  // calling Cherry's version of display()
    } 
}

This program creates one superclass (i.e., class Apple) and two subclasses of it (i.e., Banana class and Cherry class). Subclasses Banana and Cherry override the display() method declared in Apple. Inside the main() method in class Fruits_Dispatch, objects of type Apple, Banana, and Cherry are declared. A reference of type Apple, called ref, is declared.

The program then assigns a reference to each type of object to ref and uses the reference to invoke display().

The version of display() executed is determined by the type of the object being referred to at the time of the call.

// Implementing Dynamic Method Dispatch with Multiple Inheritance Levels
// Superclass
class Fruit {
    void display() {
        System.out.println("Inside Fruit's display method");
    }
}
// Subclass 1
class Apple extends Fruit {
    void display() {
        System.out.println("Inside Apple's display method");
    }
}
// Subclass 2
class Banana extends Fruit {
    void display() {
        System.out.println("Inside Banana's display method");
    }
}
// Subclass 3
class Cherry extends Fruit {
    void display() {
        System.out.println("Inside Cherry's display method");
    }
}
// Subclass 4
class Orange extends Fruit {
    void display() {
        System.out.println("Inside Orange's display method");
    }
}
// Driver class
class Main {
    public static void main(String[] args) {
        Fruit ref; // Reference variable of type Fruit
        // Object of type Apple
        ref = new Apple();
        ref.display(); // Calls Apple's display() method
        // Object of type Banana
        ref = new Banana();
        ref.display(); // Calls Banana's display() method
        // Object of type Cherry
        ref = new Cherry();
        ref.display(); // Calls Cherry's display() method
        // Object of type Orange
        ref = new Orange();
        ref.display(); // Calls Orange's display() method
    }
}

License: Creative Commons-Attribution-ShareAlike 4.0 (CC-BY-SA 4.0)

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What is dynamic method dispatch in Java?

Advantages of dynamic method dispatch

Dynamic method: single levels of inheritance.

Dynamic method: multiple levels of inheritance.