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Why constuctor can't be marked as final?

 
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Thanks in Advance.
 
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final just means it can't be overridden. A constructor can't be overridden, so what's the point of labelling it final? With most of the examples where a particular modifier isn't allowed in a particular place in Java, the explanation is simply that it wouldn't make much sense there.
 
saloni jhanwar
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Matthew Brown wrote:final just means ..



Thanks Matthew

I got another link also Constructor final
 
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but i make a constructor as final by not using final keyword instead I use private ;)
 
saloni jhanwar
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Seetharaman Venkatasamy wrote:but i make a constructor as final by not using final keyword instead I use private ;)



If final is useless here then private also should be, what is use of private here as we can't inherit constructor.
 
Seetharaman Venkatasamy
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saloni jhanwar wrote: what is use of private here as we can't inherit constructor.


you cant subclass a class[or you cant instantiate a class using that constructor]
 
saloni jhanwar
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Seetharaman Venkatasamy wrote:

saloni jhanwar wrote: what is use of private here as we can't inherit constructor.


you cant subclass a class[or you cant instantiate a class using that constructor]



I want to ask that as we can't inherit constructor then why to use access modifiers with it ,like private etc because they will be useless for constructor.
 
Seetharaman Venkatasamy
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Constructor is a *special* kind of method. and how do you create an instance[by using new operator] of a class with out seeing the constructor ?
 
saloni jhanwar
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Seetharaman Venkatasamy wrote:Constructor is a *special* kind of method. and how do you create an instance[by using new operator] of a class with out seeing the constructor ?



Thanks but my question was something different, yes i know constructor is needed for class, but is there any sense to use any access modifiers with it ?
 
Matthew Brown
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Making a constructor private will (if it's the only constructor) prevent you from instantiating the class from outside the class. Which means it can only be instantiated from inside - e.g. in a static method. See, for example, the Singleton pattern.
 
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saloni jhanwar wrote:but is there any sense to use any access modifiers with it ?


Well, the answer lies in Seetharaman Venkatasamy's post (previous to your last post). What if I want to 'stop' someone(external to my class) from creating objects? What if I want methods of 'my class' only should be able to create the objects?

I hope you get the point.

Edit : I was typing this when Matthew Brown replied, so its almost duplicate
 
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A common example I learn from the industry:



This is how people usually do to create a singleton assuming no two threads accessing getA() method concurrently.
 
Matthew Brown
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That wouldn't work - you're trying to access A's private constructor from B. You'd have to move the static method to A (and fix the other problems with it )
 
Helen Ma
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Sorry,
I type it wrong, the revised code should be:

 
Matthew Brown
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Now there's just the "other things wrong with it" . The most important being, that will actually create a new A every time you call getA().
 
Helen Ma
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The code should be this:

Helen Ma wrote:

 
Anayonkar Shivalkar
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Nops... it still have a problem

Consider below scenario:
1) There are two threads which are calling getA method.
2) Thread 1 reaches line 10. a is null, so it reaches at line 11. Line 11 has not yet executed.
3) Thread 2 reaches line 10. a is null, so it reaches at line 11. Line 11 has not yet executed.
4) Thread 1 creates an object, assigns to 'a' and returns the object.
5) Thread 2 creates another object, assigns to 'a' and returns the object.

So, there are two possibilities to avoid this:

Either


Or



Personally, I prefer the first one for below reasons:
1) Instance is initialized during class loading itself.
2) There's no 'if' condition overhead during getInstance method.
3) We don't have to explicitly make it thread-safe.

I hope this helps.
 
Helen Ma
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Hi, thanks for your thread safe example.
By the way, this quoted code:


is equivalent to


So, if "this" Singleton instance is not created yet, will I get a null pointer exception?
 
Anayonkar Shivalkar
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No. The equivalent code which you have posted will give compile time error (for the exact same reason - you cannot use 'this' inside a static context/method).

The code which I posted contained a static synchronized method which is equivalent to:


Besides, my original code had a problem - getter method was void!
I've edited that post.
 
Matthew Brown
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You want the easiest way to make a singleton in Java? Guaranteed thread safe as well?

(Using singletons is often dubious, but I'm not going to get into that now)
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