21-01-25
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Q)Example Program on default constructor.
class Account{
      int accno;
      String name;
      float balance;//instance variables
      Account(){
          accno=100001;
	      name="Rama";
	      balance=770000;
      }//default constructor
      void displayAccountData(){
          System.out.println("ACCNO:"+accno);
          System.out.println("NAME:"+name);
          System.out.println("BALANCE:"+balance);
      }
}//Account data type created
class DefaultConstructorApplication{
   public static void main(String args[]){
        Account acc1=new Account();
	    System.out.println("First account details..........");
	    acc1.displayAccountData();
	    Account  acc2=new Account();
	    System.out.println("Second account details..........");
        acc2.displayAccountData();
	}
}
Q)Example program on parameterised constructor.
class Account{
      int accno;
      String name;
      float balance;//instance variables
       Account(int ano,String nm,float bal){
          accno=ano;
	      name=nm;
	      balance=bal;
      }//parameterised constructor
      void displayAccountData(){
          System.out.println("ACCNO:"+accno);
          System.out.println("NAME:"+name);
          System.out.println("BALANCE:"+balance);
      }
}//Account data type created
class ParameterisedConstructor{
   public static void main(String args[]){
        Account acc1=new Account(101,"Rama",40000);
		acc1.displayAccountData();
		Account acc2=new Account(102,"Rahim",50000);
	    acc2.displayAccountData();
	}
}
Q)How is a constructor different from an instance method?
=>A constructor is different from an instance method in the following areas.
1)Name*
2)Return type
3)Purpose
4)Type of calling
5)Time of calling
6)frequency of calling
7)who calls it
8)Is it the behaviour of the object?

constructor                                  instance method
*************************************************
1)class name                               generally not
and constructor name
should be the same.

2)should not have                       should have return type.
return type.

3)mostly constructor                    mostly, data processing
is meant for object                    code is written here.
initialization

4)It is called implicitly                It should be called explicitly
(automatically)

5)object creation time             After the object is bound to the reference

6)called only                          can be called any number of times.
once in the lifetime 
of an object.

7)JVM calls it.                         other method can call it.

8)It is not at                             It is.
all the behaviour of the
object.
*****************************************
Q)Can an object of a Java class be created without explicitly writing a constructor in that class?
=>Yes.
=>In a Java class if we don't write a constructor explicitly, Java compiler provides by default,
a zero argument constructor for the class. Therefore, zero argument constructor
has got name called "default constructor".

class A{
   A(){
   }//explict default constructor
}
class B{
}//It has implicit default constructor