Overriding

[sandhi mridul]

Hi,

   
      class Base{
        int value = 0;
        
        Base(){
        addValue();
        }
 
         void addValue(){
         value += 10;
         System.out.println(value);
        }
       int getValue(){
         return value;
       }
     }
    class Derived extends Base{
      
      Derived(){
       addValue();
      }
 
     void addValue(){
     value +=  20;
     System.out.println(value);
      }
    }
    public class Test {
      public static void main(String[] args){
          Base b = new Derived();
          System.out.println(b.getValue());
      }
    }
 

Output= 20
40
40

What is going on in this method?
Plaese help me.

Thanks

[Daniel Chemko]

As you can see from this simple stack dump, both constructors are being called (even if you don't explicitly call super(), its still being called).

Since the Base had its addValue() overwritten, with derived's addValue(), the total is then incremented by 20 each time.

class Base
{
   int value = 0;
   
   Base()
   {
      addValue();
   }
   
   void addValue()
   {
      value += 10;
      System.out.println(Thread.currentThread().getStackTrace()[2]);
      System.out.println(value);
   }
   
   int getValue()
   {
      return value;
   }
   
   public static void main(String[] args)
   {
      Base b = new Derived();
      System.out.println("Final: " + b.getValue());
   }
   
   protected static class Derived extends Base
   {
      
      Derived()
      {
         addValue();
      }
      
      void addValue()
      {
         value += 20;
         System.out.println(Thread.currentThread().getStackTrace()[2]);
         System.out.println(value);
      }   
   }
}

[Shahid Ahmed]

please can anyone explain me the control flow..in the first program..i.e in the actual question.
I dont understand,how the control flows...because,here we have :
Base b=new Derived() so,is it that first the Derived() constructor is called? if so,then Derived() constructor has the super() as its first statement, then it should go to Base() constructor..there,we get the value printed '10' but here in the answer,the first one is '20' ...how.???
how is the control flowing..?? :roll:

[Bednarski Adam]

Hi,
I've changed output methods, look at code:

    
      class Base{
        int value = 0;
        
        Base(){
        addValue();
        }
 
         void addValue(){
         value += 10;
         System.out.println("B:addValue -> " + value);
        }
       int getValue(){
         return value;
       }
     }
    class Derived extends Base{
      
      Derived(){
       addValue();
      }
 
     void addValue(){
     value +=  20;
     System.out.println("D:addValue -> " + value);
      }
    }
    public class Test {
      public static void main(String[] args){
          Base b = new Derived();
          System.out.println("Test:main -> " + b.getValue());
      }
    }
 

as results you will get:

D:addValue -> 20
D:addValue -> 40
Test:main -> 40

So only explanation is that because you are using object "Derived" (...new Derived()...) then in Base() constructror you are using overriden method from "Derived".

Please correct me if I'm wrong !

[Mlody]

/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/

package javaapplication4;

/**
*
* @author michal
*/
import java.io.IOException;
import java.util.*;



class Base{
int value = 0; // local variable is unique

Base(){
addValue();
}

void addValue(){
value += 10;
System.out.println(value);
}
int getValue(){
return value;
}
}
class Derived extends Base{
int value = 3; // local variable is unique

Derived(){
addValue();
}

void addValue(){ // <<<<<======== US Method
value += 20;
System.out.println(value);
}
}
public class Main {
public static void main(String[] args){
Base b = new Derived(); // Firstly Basic constructor (extends Base), there addValue. But this method is overriden.
// Secondly Derived constructor, and addValue +20
System.out.println(b.getValue()); // +20 + 20 = 40
}
}

[sandhi mridul]

Thanks .... It helped.

[Dean Jones]

The code given by Mlody is realy interesting, gives "0" on invocation of getValue() method.