Query on constructor

[abin joy]

Can a constructor of a class be marked as private?

[Ulf Dittmer]

What happened when you tried it?

[Santhi Bharath]

up to my knowledge you can mark as private.but,you can't instantiate that class.this is mainly used in singleton creation

[venkatesh badrinathan]

you have two cases here.
1)you can instantiate the class with it.
2)but, you either cannot extend the class nor instantiate it outside the class

 
public class Mickle
{
private Mickle()
{}
 
    public static void main(String argv[])
    {
    new Mickle(); //no problem here
}
 
}
class base extends Mickle //error
{
void m()
{
new Mickle(); //error
}
}
   

i think this helps you..??

[venkatesh badrinathan]

sorry, i should have told like,
you can instantiate it within the same class(ie. class with constructor marked as private)

[Janardan Kelkar]

Since a constructor (although private) has been defined in the superclass, the base class cannot access it, hence even though the base class has a default constructor, it cannot make an implicit call to the constructor in the superclass (since it is private). Any constructor must have a call to a constructor in the supercalss as its first statement.
So effectively what we are doing is trying to access the private member of the superclass which is illegal in the first place.
The class by itself can be instantiated with a private constructor, there are no problems.

[Campbell Ritchie]

Yes, you can have a private constructor. It is used for two main reasons:

To prevent the class being instantiated at all.

To restrict the class from being instantiated outside a factory method. Do a search for singleton pattern, which uses that trick.

If you have a class which only has private constructors, it is impossible to inherit from that class, so you ought to mark the class "final" as well.