Why clone() method is not directely accessible from any class ?

[joni novhia]

Hi All,
I wonder on why any user defined class's object not able to invoke clone() method. I think as Object is the superclass of all the claasses that we define, so Object's clone() method should
be directely accessible from any subclass . But until we implement Cloneable its not getting possible. Can anyone explain me why this is so ?

Thanks in advance.

class StringWrap
{
	public static void main(String[] args) 
	{
		Nil n=new Nil();
		Object obj=((Object)n).clone();
	
	}
}

class Nil
{
	
}

Output :

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
The method clone() from the type Object is not visible

at StringWrap.main(StringWrap.java:6)

[Jesper de Jong]

You don't always want your objects to be cloneable. Sometimes you have classes that represent something that should not be cloneable.

Unfortunately, the designers of the Java language used an odd design when they added this feature to class Object. There's a protected clone() method which throws a CloneNotSupportedException, unless the class implements interface Cloneable. Interface Cloneable is a marker interface (an interface without any methods - its only purpose is to indicate that the class that implements it is allowed to be cloned). If you want, you can override the clone() method in your class and make it coderanch.

[Sumit Patil]

A nice article here

[joni novhia]

Jesper de Jong wrote:You don't always want your objects to be cloneable. Sometimes you have classes that represent something that should not be cloneable.

Unfortunately, the designers of the Java language used an odd design when they added this feature to class Object. There's a protected clone() method which throws a CloneNotSupportedException, unless the class implements interface Cloneable. Interface Cloneable is a marker interface (an interface without any methods - its only purpose is to indicate that the class that implements it is allowed to be cloned). If you want, you can override the clone() method in your class and make it coderanch.

Hi Jesper, Thanks for the reply.

actually i doubt on the thing that, why this clone() method is not accessible directly from any user defined class's object. As we know, the protected methods can be accessed by classes which inherit the superclass (In our case class Nil inherit class Object).
So , with this rule in mind, if i do this :

Nil n=new Nil();
n.clone(); <----- here's the compiler error that the clone() is not found for n.

I agree that it throws cloneNotSupportedException if i won't implement cloneable. but my point is why this method is not visible for any user defined class ?


Hope you got this now .....

[Jesper de Jong]

When a method is protected, it can only be accessed by the class itself, subclasses of the class, or classes in the same package as the class.

Your class StringWrap cannot access the clone() method of class Nil, because class StringWrap is not a subclass of class Nil. StringWrap and Nil are in the same package, but the clone() method is defined in class Object, which is in a different package (java.lang) - so the clone method of class Nil is not accessible from class StringWrap.

If you want to make the clone() method of class Nil publicly visible, override it and make it public:

class Nil implements Cloneable {
    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

Also, make class Nil implement Cloneable, otherwise you'll get a CloneNotSupportedException when you run the code.

One more thing: Casting to Object is not necessary, because everything is already an Object:

// Too complicated!
Object obj=((Object)n).clone();

// Just write this:
Object obj = n.clone();

[joni novhia]

Jesper de Jong wrote:When a method is protected, it can only be accessed by the class itself, subclasses of the class, or classes in the same package as the class.

Your class StringWrap cannot access the clone() method of class Nil, because class StringWrap is not a subclass of class Nil. StringWrap and Nil are in the same package, but the clone() method is defined in class Object, which is in a different package (java.lang) - so the clone method of class Nil is not accessible from class StringWrap.

If you want to make the clone() method of class Nil publicly visible, override it and make it public:

class Nil implements Cloneable {
    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

Also, make class Nil implement Cloneable, otherwise you'll get a CloneNotSupportedException when you run the code.

One more thing: Casting to Object is not necessary, because everything is already an Object:

// Too complicated!
Object obj=((Object)n).clone();

// Just write this:
Object obj = n.clone();

Ohk ......
got the funda protected things ....
feels like heaven now ..... Thanks a lot Jesper