Cloning objects of inner classes

Question

I have written a class A such as following

class A <E> {  // class A has objects of E

    int x;

    private B<T> next // a object of inner class

    class B<T> { 
        // an inner class having objects of type T

        T[] elements = (T[]) Object[x];
        // has other stuff including many objects  
    }

 public A<E> func ( B<E> val ){
  Here I want to clone the value of val so that I can do different operations on other   
 }

The problem comes that I wish to write B<E> Temp = B<E>Value.clone() where Value is defined in the code.

but it says that clone is not visible.

What should I do to make it so....

Thanks a lot...

Can you provide a code sample which includes your `clone()` routine? You might also want to implement the `Cloneable` interface. — Johan Sjöberg, Feb 16 '11 at 17:51

score 1 · Answer 1 · answered Feb 16 '11 at 17:54

1

clone() is protected so you just need to redefine in B to do whatever you need.

answered Feb 16 '11 at 17:54

Peter Hull

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What do you mean by redefine ??? Could you give an example ?? as in I need to implement Cloneable in the nested class or what ?? – Feb 16 '11 at 18:00
1

Just overwrite it and increase its visibility: make it public. Inside, you will call super.clone() and then -- if needed -- you'll make a deep copy. – Costi Ciudatu Feb 16 '11 at 18:31
again could you give an example ... I wrote super.clone() in the inner class and eclipse does'nt let me run this. Thanks a lot... – Feb 16 '11 at 18:35

Andreas Dolk · Answer 2 · 2011-02-17T08:07:42.980

0

It's better to not use clone() but to implement a copy constructor on B:

 class B extends A { 
    // an inner class having objects of type 
    List<String> strings = ArrayList<String>();

    public B(B that) {
       super(that); // call the superclass's copy constructor
       this.strings = new ArrayList<String>();
       this.strings.add(that.strings);
    }

    // ...
}

then call

public C func ( B val ){
   B clone = new B(val);
}

(removed the generics stuff to limit demonstration on the copy constructor itself)

edited Feb 17 '11 at 08:07

answered Feb 16 '11 at 17:59

Andreas Dolk

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clone is broken, there are a lot of questions and answers on this topic (Example: http://stackoverflow.com/questions/2427883/clone-vs-copy-constructor-which-is-recommended-in-java) – Andreas Dolk Feb 16 '11 at 19:33
On "It's better to not use clone()", what if you're cloning a `B` type but are not sure of its runtime type (because it could be a subclass of `B`)? – Steve Kuo Feb 17 '11 at 07:01
@Steve Kuo - I don't understand that comment. A `B` is a `B` at runtime, a subclass of `B` is not a `B`. Subclassing was not part of the question, but anyway, I edited the question. Copy Constructor is compatible with inheritance. (and `clone` is still broken and we should simply avoid it). – Andreas Dolk Feb 17 '11 at 08:10
How is a subclass of `B` not a `B` (it will pass instanceof)? Are you saying `LinkedHashMap` is not a `HashMap`? – Steve Kuo Feb 17 '11 at 21:19
@Steve - A `LinkedHashMap` is a `LinkedHashMap` and it extends `HashMap`. You *create* it by calling the `LinkedHashMap` constructor. Calling `getClass()` on an instance of `LinkedHashMap` returns `"java.util.LinkedHashMap"`. *is-a* is ambigious in this case, I was not talking about inheritance, I was talking about the type of an instance. – Andreas Dolk Feb 18 '11 at 07:00
@Andreas_D: Your copy constructor answer is incomplete and incorrect if B can have subclasses - which it can, as its non-final. `public C func ( B val ){ B clone = new B(val); }` This code will always create a B instance, even if the val passed is actually a subclass of B. This is where clone is needed, but I agree it's tricky to use and somewhat broken in places. – davidfrancis May 06 '11 at 11:06
@Zebsy - subclasses is a different story - if you want to clone some `E extends B`, then call the copy constructor of `E`. If you use `new B(B source)` with an `E` instance, you'll *not* create a clone of `E`, but that was never the intention. – Andreas Dolk May 08 '11 at 20:32

score 0 · Accepted Answer · answered Feb 16 '11 at 19:00

Here an example to the answers from Peter (not tested, may need some syntax checks):

class A <E> {  // class A has objects of E

    int x;

    private B<T> next // a object of inner class

    class B<T> implements Cloneable {
        public B<T> clone() {
           try {
              B<T> klon = (B<T>) super.clone();
              // TODO: maybe clone the array too?
              return klon;
           }
           catch(CloneNotSupportedException) {
              // should not occur
              throw new Error("programmer is stupid");
           }
        }
        // an inner class having objects of type T

        T[] elements = (T[]) Object[x];
        // has other stuff including many objects  
    }

 public A<E> func ( B<E> val ){
     B<E> clone = val.clone();
     // more code
 }

Cloning objects of inner classes

3 Answers3