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Re: Are you sure, Mr. Gosling?

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On 10.06.2016 21:20, Lew wrote:
> On Wednesday, June 8, 2016 at 11:55:13 AM UTC-7, Robert Klemme wrote:
>> On 08.06.2016 15:31, Eric Douglas wrote:

>>> Defaults from clone() to consider (if we don't want these, we either
>>> need to specify the alternative in the copy constructor, or replace in
>>> the clone method on the new instance returned from super.clone()):
>>> Immutable objects get copied by value, bucket objects get copied by
>>> reference.
>>
>> This is wrong.  Objects ("reference types") are copied by reference by
>> Object.clone() - regardless of whether they are mutable or not.  Only
>> primitive types are copied by value.
>
> That isn't correct. References are copied, by default, but they are
> copied by value. When 'clone' is overridden appropriately, those
> references are dereferenced, and the values to which they point are
> copied. However you look at it, it is values that are copied - either
> pointer values or primitive values.

Yes, references (pointers) are copied by value, i.e. there is a new 
object that has a pointer to the same object.  That is the default 
behavior of clone().  This is different from what Eric had written (see 
above) and which prompted my response.

Now, of course, if you override it typically you would retain some 
references (e.g. to immutable objects) and provide new values for others 
(e.g. to collections), typically by creating a new object one way or 
another (i.e. invoking clone(), a constructor, a factory method or 
something else) or using a default value (e.g. null).

> Copy-by-reference means that the original reference will point to the
> new object, which is not what we want nor what 'clone' does.

I am not sure.  Now that we have talked about it so much I am not sure 
any more that "copy by reference" is actually properly defined.  It 
seems a parallel construction to "call by reference" but that describes 
another situation, namely a non-copy of a value.  You could describe 
this as copying an immutable pointer (in C++ you cannot change what a 
reference points to, as opposed to a pointer).

> This is similar to the logic that proves that method arguments in
> Java are always passed by value. Java doesn't actually have
> references, as C++ programmers and others understand them. It has
> pointers.

Well, underneath all are pointers (to memory addresses).  In C++ the 
difference is that you can change a pointer (i.e. where it points to) 
but not a reference.  In that sense references in Java are really pointers.

Kind regards

	robert


-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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Re: Are you sure, Mr. Gosling? Arne Vajhøj <arne@vajhoej.dk> - 2016-06-04 15:27 -0400
  Re: Are you sure, Mr. Gosling? Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-04 16:43 -0400
    Re: Are you sure, Mr. Gosling? Arne Vajhøj <arne@vajhoej.dk> - 2016-06-04 18:44 -0400
      Re: Are you sure, Mr. Gosling? Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-05 08:05 -0400
        Re: Are you sure, Mr. Gosling? Arne Vajhøj <arne@vajhoej.dk> - 2016-06-05 08:31 -0400
        Re: Are you sure, Mr. Gosling? Eric Douglas <e.d.programmer@gmail.com> - 2016-06-06 05:22 -0700
          Re: Are you sure, Mr. Gosling? Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-06 09:00 -0400
            Re: Are you sure, Mr. Gosling? v_borchert@despammed.com (Volker Borchert) - 2016-06-06 16:06 +0000
              Re: Are you sure, Mr. Gosling? Robert Klemme <shortcutter@googlemail.com> - 2016-06-06 22:56 +0200
                Re: Are you sure, Mr. Gosling? Eric Douglas <e.d.programmer@gmail.com> - 2016-06-07 05:08 -0700
                Re: Are you sure, Mr. Gosling? Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-07 08:41 -0400
                Re: Are you sure, Mr. Gosling? Martin Gregorie <martin@address-in-sig.invalid> - 2016-06-07 13:14 +0000
                Re: Are you sure, Mr. Gosling? Eric Douglas <e.d.programmer@gmail.com> - 2016-06-07 06:30 -0700
                Re: Are you sure, Mr. Gosling? Robert Klemme <shortcutter@googlemail.com> - 2016-06-07 19:10 +0200
                Re: Are you sure, Mr. Gosling? Eric Douglas <e.d.programmer@gmail.com> - 2016-06-07 10:22 -0700
                Re: Are you sure, Mr. Gosling? Mike Amling <mamling@chaff.us> - 2016-06-07 15:31 -0500
                Re: Are you sure, Mr. Gosling? Eric Douglas <e.d.programmer@gmail.com> - 2016-06-08 06:31 -0700
                Re: Are you sure, Mr. Gosling? Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-08 10:41 -0400
                Re: Are you sure, Mr. Gosling? Eric Douglas <e.d.programmer@gmail.com> - 2016-06-08 10:21 -0700
                Re: Are you sure, Mr. Gosling? Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-08 14:10 -0400
                Re: Are you sure, Mr. Gosling? Robert Klemme <shortcutter@googlemail.com> - 2016-06-08 20:54 +0200
                Re: Are you sure, Mr. Gosling? Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-08 16:36 -0400
                Re: Are you sure, Mr. Gosling? Robert Klemme <shortcutter@googlemail.com> - 2016-06-08 23:11 +0200
                Re: Are you sure, Mr. Gosling? Lew <lewbloch@gmail.com> - 2016-06-10 12:20 -0700
                Re: Are you sure, Mr. Gosling? Robert Klemme <shortcutter@googlemail.com> - 2016-06-14 19:10 +0200
                Re: Are you sure, Mr. Gosling? Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-07 17:12 -0400
                Re: Are you sure, Mr. Gosling? Robert Klemme <shortcutter@googlemail.com> - 2016-06-07 23:25 +0200
                Re: Are you sure, Mr. Gosling? Arne Vajhøj <arne@vajhoej.dk> - 2016-06-07 17:33 -0400
          Re: Are you sure, Mr. Gosling? Arne Vajhøj <arne@vajhoej.dk> - 2016-06-06 21:18 -0400

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