why System class instance cannot be instatiated

[Amirtharaj Chinnaraj]

hi guys

when i go through the API java.lang.System

i found that System class cannot be instantiated

my questios is why the System class cannot be instantiated.

even though it is not having the constructor jvm supplies

a default no-Arg-Constructor

amir

[Stan James]

Your "why" question could have two answers ...

Why the designers decided not to let you make an instance ... They put all the functionality you need in static methods and variables so no instance is necessary.

Why you can't make one against their wishes ... They made the constructor private. This is a common way to build classes that are meant to be all static or that want to control instantiation, for example the Singleton pattern.

You can download the source for the library and look up things like this:

/** Don't let anyone instantiate this class */
private System() {
}

[Amirtharaj Chinnaraj]

thanks indeed James

amir

[Sudhakar Reddy Kurakula]

The reason why System class is not instantiated is ,System class has a private constructor().


Cheers
Sudhakar
[ December 31, 2006: Message edited by: Sudhakar Reddy Kurakula ]

[marc weber]

Originally posted by Sudhakar Reddy Kurakula:
The reason why System class is not instantiated is ,System is a final class...

No, final prevents subclassing.

[isha krishnan]

Hi all,

The reason why System class is not instantiated is ,System class has a private constructor().

System class has no constructor that's why it can not be instantiated not even a private constructor. As we see in string api,there is no constructor defined. Please let me know if its the correct reason.

Thanks,

[Manuel Petermann]

I am sorry but its not the reason.
If a class doesn't have a constructor a default one is implicitly applied.

class Foo{
    void bar(){
        System.out.println(this.getClass());
    }
}
...
    new Foo().bar(); //valid code
...

[Jesper de Jong]

isha krishnan wrote:System class has no constructor that's why it can not be instantiated not even a private constructor.

That is not correct. The System class does have a private constructor, which is not mentioned in the Javadoc documentation. The private constructor is what prevents you from instantiating the class. If a class has no constructor, a public constructor which does not take arguments is automatically added by the Java compiler. So if a class has no constructor, it can still be instantiated.

You can lookup the source code for class System in the file src.zip which is in your JDK installation directory. There you'll see that it has a private constructor, as Stan James already showed above.

isha krishnan wrote:As we see in string api,there is no constructor defined.

Class String has several constructors.

[isha krishnan]

The System class does have a private constructor, which is not mentioned in the Javadoc documentation

So why is it not mentioned in JavaDoc?It should be there i guess so
Yes that's perfectly correct that if no constructor is given,its default constructor is called.I was wrong here.

Also in quote

As we see in string api,there is no constructor defined.

I did mistake in writing, i wanted to write "System" instead of String.

Thanks

[Campbell Ritchie]

It is normal for private members and constructors not to appear in the API documentation. The javadoc tool usually does not produce visible output for private members. Anybody reading about uninstantiable classes would think of this (or similar).

[isha krishnan]

Thanks Campbell

I got all information