A class cannot be subclassed...

[Thiru Mu]

By making the constructor private, a class cannot be subclassed

1. True
2. False

is this True???

[Jesper de Jong]

Your question talks about "the constructor", but a class can have more than one constructor. If all constructors of a class are private, then you can't subclass it. Try it and see what the compiler says:

class Super {
    private Super() { }
}
 
class Sub extends Super {
}

[ August 31, 2007: Message edited by: Jesper Young ]

[Thiru Mu]

Hi Joseph,
Yes I have tested it.

If I make the default Constructor private, then the Class cannot be subclasses. for instance,

class Super{
private Super(){
}
Super( int a){
}
}
public class Test extends Super{ // LINE
public static void main(String args[]){

}
}

I am getting a compiler error here saying Super() is not visible at //LINE

But if I make the default constructor as public and the other one as private, the class can be subclassed.

So here I can conclude that, if the default constuctor is provided and marked as private the class cannot be subclessed. You agree this??

[Burkhard Hassel]

class Super{ 
private Super(){
}
Super(int a){
}
}
 
public class Test2 extends Super{
Test2() {
super(42);
}
public static void main(String args[]){
}
}

Test2 compiles happily.


Note by the way that you should not mix up the technical terms "default constructor" and "parameterless constructor" or you may confuse yourself and others:

class One {
}
 
class Two {
Two() {
}
}

Class one has a default constructor, class two has a parameterless constructor.

Yours,
Bu.

[raghu nagabandi]

If all the constructors of a class are private then we can not create subclass for that class.
if it contains combinations i.e. both private and others , then we can create subclass.

[shubhra chauhan]

if we are using more than one constructor in superclass for eg
class Super{
Super(){}//1
Super(int a){}//2
}

If 1 is private and when called in main give compilation error and same we'll get if 2 is private.
now as in above eg
class Super{
private Super(){}
Super(int a){}
}

public class Test2 extends Super{
Test2() {
super(42);
}
public static void main(String args[]){}
}


so here we are using constructor of super class which is not private at alland moreover it is called from constructor test2().

its not a solution as we always can't depend on other constructor to call first one and the problem was originally calling a private constructor not any constructor.

[dhwani mathur]

Hi All

The above discussion has confused me a bit
to what is the answer please explain me
that if which type of constructor(default,no-arg,parameterised)
if made private will lead to a


class having that constructor cannot be subclassed?


Please explain!!!

[Burkhard Hassel]

OK, back to the question:

By making the constructor private, a class cannot be subclassed

1. True
2. False

is this True???

Yes, it is true. Plain and simple.

The question states: "By making the constructor private..."

Using the article "the" implies that there is only one constructor.

If a class had more than one constructor, you could not ask about the constructor, because this would be bad english.



Yours,
Bu.