Whats is the use of making a Constructor Protected

[Haricharan Ramachandra]

Hi

Can anyone please let me know what is the use of making a Constructor Protected. I know that if we make a Constructor private you can't instanciate that class. But if i make it Public or Protected i can still instanciate it. So what is the difference between maing a Constructor Protected instead of Public

thanks for your help in advance

thanks
Hari

[Ernest Friedman-Hill]

If a constructor is protected, then (outside of the package where it is defined) it can only be used by a subclass -- i.e., in a super() call.

[sven studde]

I know that if we make a Constructor private you can't instanciate that class.

Sure you can. A public method in a class can access private members--including private methods.

class Apple
{
private String color;

        //private constructor:
private Apple(String c)
{
color = c;
}
 
String getColor()
{
return color;
}
 
public static Apple createApple(String color)
{
Apple a = new Apple(color);
return a;
}
}
 
class AATest1
{
public static void main(String[] args) 
{
 
Apple a = Apple.createApple("red");
System.out.println(a.getColor());

 
}
}

But if i make it Public or Protected i can still instanciate it. So what is the difference between maing a Constructor Protected instead of Public

A class that is not in the same package as a class that has a protected constructor cannot create objects of that class (that is assuming there is no helper method like in the above example). Or, in other words, if a class has a protected constructor, then only classes in the same package can create objects of that class.

On the other hand, if a class has a public constructor, then any class can create objects of that class.
[ October 27, 2006: Message edited by: sven studde ]

[sven studde]

I have a question that came to mind when thinking about the op's question: is there any effect in declaring a method protected if the method is not a constructor or a static method? It seems to me that if the method isn't a constructor or a static method, then it will only be called with an object of the class, and therefore protected has no meaning for such a method.

Ok, I just realized there are inheritance implications: a protected method will be inherited by a subclass unlike private methods, which aren't inherited. So, that means you can override the method in the subclass and therefore polymorphism will work.
[ October 27, 2006: Message edited by: sven studde ]