how do abstract classes like NumberFormat call the methods from their subclasses?

[Prasad Kharkar]

according to Shaad's question about the private constructors, I had tried a small program that makes us access the object of a class that has all the constructors private
this is it

 
/** A program to demonstrate private constructor
 *  Developer: Prasad Kharkar
 *  Date	 : 03-06-2010
 */

class PrivateConstructor{
	
	private PrivateConstructor(){
	}

	private PrivateConstructor(String s){
		System.out.println(s);
	}
	// the method returns the PrivateConstructor object 
	static PrivateConstructor method(){
		System.out.println("static method and the object rerurned from here");
		return new PrivateConstructor();
	}
}

class TestPrivateConstructor {
	public static void main(String [] args){
		// using new does not work as we are have written the private constructor in the class
		//PrivateConstructor pc = new PrivateConstructor();
		PrivateConstructor p = PrivateConstructor.method();
		//done here
	}
}

   
    

but even if the class constructors are private we could create the object of the class and could access it from outside the class
that also using the static method of the class like this

  PrivateConstructor p = PrivateConstructor.method();  

but in case of abstract classes those are in the java docs I mean classes like NumberFormat, DateFormat etc
we cannot instantiate them even in they are in the same class i.e. because they are abstract
then how can we use the instantiation method like above to get the object of the class?
like this

DateFormat df = DateFormat.getInstance(style, locale);

how is it done in the inbuilt classes ?
please help me

[Ankit Garg]

Try out this program

import java.text.*;
class DateFormatExp {
	public static void main(String[] args) {
		DateFormat formatter = DateFormat.getInstance();
		System.out.println(formatter.getClass());
	}
}

[Prasad Kharkar]

does that mean that we are getting the instance of the class that extends the abstract class ?
and with the use of polymorphism?
means
DateFormat is referring to the subclass?
I will try to write a program for that definityly

[Prasad Kharkar]

I was checking for that
and found out something
like we instantiate DateFormat class as follows

   
DateFormat df = DateFormat.getDateInstance();
DateFormat df1 = DateFormat.getDateInstance(style);
 

as the DateFormat class is abstract and the object is of the subclass that extends the DateFormat class
I tried the program to check whether we can do it manually or not
when I run the program given by Ankit sir
that gives the name of the class is SimpleDateFormat

I tried that method with my code and got some similar results for my classes
here is it

  
/** A program to use get the instance of abstract class
 *  Developer: Prasad Kharkar
 *  Date	 : 06-06-2010
 */

abstract class AbstractClass{ // acts like abstract DateFormat class
	
	AbstractClass(){
		System.out.println("Abstract class constructor");	
	}

	AbstractClass(int i){
		System.out.println("int AbstractClass constructor");
	}

	static AbstractClass getInstance(){
		return new ConcreteClass();
	 }

	static AbstractClass getInstance(int x){
		return new AnotherConcreteClass("Prasad");
	}
}

class ConcreteClass extends AbstractClass { // acts like the SimpleDateFormat that extends DateFormat
	ConcreteClass(){
		System.out.println("in the ConcreteClass constructor");
	}	
	public static void main(String [] args){
		AbstractClass ac = AbstractClass.getInstance();
		AbstractClass ac1 = AbstractClass.getInstance(10);
		System.out.println(ac.getClass());
		System.out.println(ac1.getClass());
	
        }
}

class AnotherConcreteClass extends AbstractClass{

	AnotherConcreteClass(){
		System.out.println("in the AnotherConcreteClass constructor with no args");
	}

	AnotherConcreteClass(String s){
		System.out.println("In the AnotherConcreteClass constructor with String argument");
	}
}


  

this prints out

F:\Java\Javaranch problems>java ConcreteClass
Abstract class constructor
in the ConcreteClass constructor
Abstract class constructor
In the AnotherConcreteClass constructor with String argument
class ConcreteClass
class AnotherConcreteClass

My question here is
is this the way DateFormat and NumberFormat classes are created while creating java?
or is this the way that can resemble to that way?
Expecting help from Ankit sir and the great Henry sir

[Sandy Ghosh]

Prasad Kharkar wrote:according to Shaad's question about the private constructors, I had tried a small program that makes us access the object of a class that has all the constructors private
this is it

 
/** A program to demonstrate private constructor
 *  Developer: Prasad Kharkar
 *  Date	 : 03-06-2010
 */

class PrivateConstructor{
	
	private PrivateConstructor(){
	}

	private PrivateConstructor(String s){
		System.out.println(s);
	}
	// the method returns the PrivateConstructor object 
	static PrivateConstructor method(){
		System.out.println("static method and the object rerurned from here");
		return new PrivateConstructor();
	}
}

class TestPrivateConstructor {
	public static void main(String [] args){
		// using new does not work as we are have written the private constructor in the class
		//PrivateConstructor pc = new PrivateConstructor();
		PrivateConstructor p = PrivateConstructor.method();
		//done here
	}
}

   
    

but even if the class constructors are private we could create the object of the class and could access it from outside the class
that also using the static method of the class like this

  PrivateConstructor p = PrivateConstructor.method();  

but in case of abstract classes those are in the java docs I mean classes like NumberFormat, DateFormat etc
we cannot instantiate them even in they are in the same class i.e. because they are abstract
then how can we use the instantiation method like above to get the object of the class?
like this

DateFormat df = DateFormat.getInstance(style, locale);

how is it done in the inbuilt classes ?
please help me

At first when you calling this static method, The private constructor is getting invoked inside the class, so it is legal, and then it is returning the object to the caller..
the static method() is analogous to getInstance() method...