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Java - Dynamic Method Dispatch
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  Dynamic Method Dispatch - Example Code

Abstract Class :

package base;

/*
Abstract classes cannot be instantiated, but they can be subclassed
*/
public abstract class ClsVirusScanner {

    //With One Abstract method
    public abstract void fnStartScan();
    
    protected void fnCheckForUpdateVersion(){
        System.out.println("Perform Virus Scanner Version Check");
    }
    
    protected void fnBootTimeScan(){
        System.out.println("Perform BootTime Scan");
    }
    protected void fnInternetSecutiry(){
        System.out.println("Scan for Internet Security");
    }
    
    protected void fnRealTimeScan(){
        System.out.println("Perform RealTime Scan");
    }
    
    protected void fnVirusMalwareScan(){
        System.out.println("Detect Virus & Malware");
    }    
}

Overriding Abstract Method in Child Class :

import base.ClsVirusScanner;

//All the 3 child classes inherits the base class ClsVirusScanner
//Child Class 1
class ClsPaidVersion extends ClsVirusScanner{
    @Override
    public void fnStartScan() {
        super.fnCheckForUpdateVersion();
        super.fnBootTimeScan();
        super.fnInternetSecutiry();
        super.fnRealTimeScan();
        super.fnVirusMalwareScan();
    }
};    //ClsPaidVersion IS-A ClsVirusScanner
//Child Class 2

class ClsTrialVersion extends ClsVirusScanner{
    @Override
    public void fnStartScan() {
        super.fnInternetSecutiry();
        super.fnVirusMalwareScan();
    }
};    //ClsTrialVersion IS-A ClsVirusScanner

//Child Class 3
class ClsFreeVersion extends ClsVirusScanner{
    @Override
    public void fnStartScan() {
        super.fnVirusMalwareScan();
    }
};     //ClsTrialVersion IS-A ClsVirusScanner

Dynamic/Late Binding leads to Dynamic method dispatch :

//Calling Class
public class ClsRunTheApplication {

    public static void main(String[] args) {

        final String VIRUS_SCANNER_VERSION = "TRIAL_VERSION";

        //Parent Refers Null
        ClsVirusScanner objVS=null; 

        //String Cases Supported from Java SE 7
        switch (VIRUS_SCANNER_VERSION){
        case "FREE_VERSION":

            //Parent Refers Child Object 3
            //ClsFreeVersion IS-A ClsVirusScanner
            objVS = new ClsFreeVersion(); //Dynamic or Runtime Binding
            break;
        case "PAID_VERSION":

            //Parent Refers Child Object 1
            //ClsPaidVersion IS-A ClsVirusScanner
            objVS = new ClsPaidVersion(); //Dynamic or Runtime Binding
            break;
        case "TRIAL_VERSION":

            //Parent Refers Child Object 2
            objVS = new ClsTrialVersion(); //Dynamic or Runtime Binding
            break;
        }

        //Method fnStartScan() is the Version of ClsTrialVersion()
        objVS.fnStartScan();

    }
}

Result :

Scan for Internet Security  
Detect Virus & Malware

Upcasting :

objVS = new ClsFreeVersion();
objVS = new ClsPaidVersion();
objVS = new ClsTrialVersion()

Remarks

  • Dynamic Binding = Late binding
  • Abstract classes cannot be instantiated, but they can be sub-classed (Base for a child class)
  • An abstract method is a method that is declared without an implementation
  • Abstract class may contain a mix of methods declared with or without an implementation
  • When an abstract class is sub-classed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract
  • Dynamic method dispatch is a mechanism by which a call to an overridden method is resolved at runtime. This is how java implements runtime polymorphism.
  • Upcasting : Casting a subtype to a supertype, upward to the inheritance tree.
  • Runtime Polymorphism = Dynamic Polymorphism
 

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https://develop.horje.com/learn/1434/reference

 

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