What is a cloneable interface and how many methods does it contain?

[Rd Dari]

Hi,

If it is not any method then why we use these?

Please explain it.

[Henry Wong]

See java.lang.Cloneable in the Javadoc. And if there is still confusion, please elaborate the question.

Henry

[Rd Dari]

Hi Henry sir,

I am little confuse about it I read that link.

Please give me simple example to understand easily.

Thank you!

[Henry Wong]

Basically, if a class wants to be clonable, it just needs to implement the Cloneable interface -- which has one method, the clone() method. The clone() method is called when the application needs to make a copy of a cloneable instance.

Henry

[Rd Dari]

ok I have understand now

Thanks a lot for giving me early response.

I can understand that www.coderanch.com why is so popular.

Thanks once again.

[Rob Spoor]

Henry Wong wrote:the Cloneable interface -- which has one method, the clone() method.

No it doesn't. The clone() method is declared in Object as protected. Cloneable does not have it, because that would mean that the clone() method of any class that implements Cloneable would need to become public. For some reason Sun decided it should be possible to keep it protected.

[Henry Wong]

Rob Spoor wrote:No it doesn't. The clone() method is declared in Object as protected. Cloneable does not have it, because that would mean that the clone() method of any class that implements Cloneable would need to become public. For some reason Sun decided it should be possible to keep it protected.

Yeah, very good point. Maybe "has" is too strong a word. The Cloneable interface and clone() method should be used together -- but the interface doesn't specify the method. Sorry for the confusion.

Henry

[Arun Singh Raaj]

So @henry, you mean to say that cloneable interface has a clone() method but implementation has been done in Object class?

[Campbell Ritchie]

Arun Singh Raaj wrote:So @henry, you mean to say that cloneable interface has a clone() method but implementation has been done in Object class?

No.

There is a rudimentary implementation in the Object class, but you cannot simply clone an Object because its clone() method has protected access and is therefore inaccessible outside the java.lang package.

Object ob1 = new Object(); // runs all right
Object ob2 = ob1.clone(); // fails to compile because of access

So what you do is to override the clone() method with public access and also implement the Cloneable interface, which, as you can see from this link, is a tagging interface with no methods. You must do both.
If you find a copy of Effective Java by Joshua Bloch you can find more about the clone() method (page 54) and you will find a different opinion about marker/tagging interfaces (page 179). You will see that Bloch warns that the clone() method is difficult to use.

[Dave Tolls]

Not actually the case.

public class Test {
    public static void main(String[] args) {
        Test t = new Test();
        try {
            Object clone = t.clone();
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }
    }
}

The above will happily compile since clone() is protected, so accessible by any class (since they all extend Object).
However, it will throw the exception as Test does not implement Cloneable.

[Campbell Ritchie]

That is different from what I showed; I showed plain simple Object whereas you are inside the Test class and the inherited clone() method is now inside the same package so it is accessible. You still have to handle the Exception which appears to be checked.

[ganesh chowdhary]

Campbell Ritchie wrote:

Arun Singh Raaj wrote:So @henry, you mean to say that cloneable interface has a clone() method but implementation has been done in Object class?

No.

There is a rudimentary implementation in the Object class, but you cannot simply clone an Object because its clone() method has protected access and is therefore inaccessible outside the java.lang package.

Object ob1 = new Object(); // runs all right
Object ob2 = ob1.clone(); // fails to compile because of access

So what you do is to override the clone() method with public access and also implement the Cloneable interface, which, as you can see from this link, is a tagging interface with no methods. You must do both.
If you find a copy of Effective Java by Joshua Bloch you can find more about the clone() method (page 54) and you will find a different opinion about marker/tagging interfaces (page 179). You will see that Bloch warns that the clone() method is difficult to use.

[Campbell Ritchie]

Welcome to the Ranch

Have you got a question?

[ganesh chowdhary]

Campbell Ritchie wrote:Welcome to the Ranch

Have you got a question?

[ganesh chowdhary]

Campbell Ritchie wrote:Welcome to the Ranch

Have you got a question?

I have a question for u.Since every class extends Object class,why can't it access the protected clone method

[Campbell Ritchie]

Don't use abbreviations like “u”; they are difficult for non‑English‑speakers to understand.

You want to be able to call myObject.clone() from outside the class. That won't work because the protected modifier permits access within the same package or in code responsible for the implementation, which may be in a subclass.Try this:-

class CloneDemo
{
    private final Object o = new Object();
    private final Object p = o.clone(); // fails to compile
}

The only way you can get such a method to execute is to override it with public access.

[ganesh chowdhary]

Campbell Ritchie wrote:Don't use abbreviations like “u”; they are difficult for non‑English‑speakers to understand.

You want to be able to call myObject.clone() from outside the class. That won't work because the protected modifier permits access within the same package or in code responsible for the implementation, which may be in a subclass.Try this:-

class CloneDemo
{
    private final Object o = new Object();
    private final Object p = o.clone(); // fails to compile
}

The only way you can get such a method to execute is to override it with public access.

thanks a lot.
Cloneable should be implemented and there should be a provision of public access clone method for access in outside the package and outside implemented class.Superb Campbell.

[Campbell Ritchie]

Please don't quote the whole of the preceding post; that simply makes the thread longer and longer.

And . . . “That's a pleasure ” Find the Joshua Bloch references I showed you earlier.