Java clone() method

Question

I read Effective Java by J.Bloch and here was wrote:

If you design a class for inheritance, be aware that if you choose not to provide a well-behaved protected clone method, it will be impossible for subclasses to implement Cloneable.

I have Class A:

public class A{}

And class B which extends class A:

public class B extends A implements Cloneable {

}

So in this case I can't to override clone() method? If yes, than please explain why.

Answer 1

In your case yes you can override clone():

public class A {
}

public class B extends A implements Cloneable {

    @Override
    public B clone() throws CloneNotSupportedException {
        return (B) super.clone();
    }
}

and still have an effective clone mechanism - you are therefore telling the truth when you state implements Cloneable.

However, all that is needed to break that promise is to give A a private variable.

public class A {
    private int a;
}

and now your promise is broken - unless A implements clone, in which case you can use super.clone():

public class A {

    private int a;

    @Override
    public A clone() throws CloneNotSupportedException {
        A newA = (A) super.clone();
        newA.a = a;
        return newA;
    }
}

public class B extends A implements Cloneable {

    private int b;

    @Override
    public B clone() throws CloneNotSupportedException {
        B newB = (B) super.clone();
        newB.b = b;
        return newB;
    }
}

Essentially - as Joshua Bloch states - if you don't implement clone your sub-classes also cannot (generally).

Answer 2

Cloneable and clone() are pretty much deprecated, so it doesn't matter anyway. However, since clone() is always supposed to call super.clone(), if a superclass implements it wrong, the subclass won't be able to fix it either.

It has nothing to do with not being able to override clone(), it just won't result in a correct implementation.

Answer 3

Of course you can implement B.clone().

The problem which J. Bloch refers to is (IIRC), that without a good A.clone() method, the private fields within A can not be cloned properly in B. Simple example:

public class A {
  private String myText;
  public A (String myText) {
    this.myText = myText;
  }
  public String getExtendedText() {
    return "Hello something " + myText;
  }
}

public class B extends A implements Cloneable {
    private int someValue;
    // getter/setter

    public clone() {
        // how can you now provide a good clone with A.myText being cloned?
    }    
}