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Q) Explain program on abstract modifier usage?




A)
//AbstractExample.java

import java.util.Scanner;
/*1*/ abstract class vehicle5
{
       private int wheel;
       vehicle5()
       {
              System.out.println("Some vehicles instanciated");
              wheel=4;
       }
       void start()
       {
              System.out.println("Every vehicle starts like this");
       }
/*2*/  abstract void move();
}
class car6 extends vehicle5
{
/*3*/  void move()
       {
              System.out.println("Exellent transportation");
       }
}
class zeep extends vehicle5
{
/*3*/  void move()
       {
              System.out.println("Good transportation");
       }
}
class AbstractExample {

       public static void main(String[] args) {
/*4*/         vehicle5 v;
              Scanner s = new Scanner(System.in);
              System.out.println("What vehicle you want car/zeep");
              String choice=s.next();
              if(choice.equals("car"))
/*5*/                v=new car6();
/*6*/         /*if(choice.equals("zeep"))
                     v=new zeep();*/ //error
              else
/*5*/                v=new zeep();
              v.move();
       }
}

Output:

Example1:
What vehicle you want car/zeep
car
Some vehicles instanciated
Exellent transportation

Example2:
What vehicle you want car/zeep
zeep
Some vehicles instanciated
Good transportation

Explanation of the above program:
1)      The abstract class is defined it contain its constructor. That is called by child class constructor only.
2)      The abstract class method is declared(i.e. move())
3)      The both car and zeep classes the move() method is implemented as overridden method.
4)      The vehicle class reference is created but its object is not created. Because abstract class should not created its object. But it holds its children class object addresses.
5)      Vehicle class/ abstract class reference is holding the its children object addresses.
6)      Error is raised, because if we check both with “if” conditions the compiler raised error when enter other than 2 class object so that’s why it raised.

->Even though the parent class reference is holding the object of children class, but that vehicle reference should not call own method of child class. Only the overridden methods only called by the parent class reference.

Ex:

abstract class vehicle {
       {
              void move();
       }
class car extends vehicle
{
       void move()
       {
              System.out.println("");
       }
       void fly()
       {
             
       }
}
class example
{
       public static void main(String[] args) {
              vehicle v;
              v=new car();
              v.move();//ok. The overridden methods should call by the parent reference.
              v.fly();//error. Because super class reference doesn't call the child's own methods.
       }
}

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