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Re: Are you sure, Mr. Gosling?

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On 6/7/2016 1:22 PM, Eric Douglas wrote:
> On Tuesday, June 7, 2016 at 1:10:36 PM UTC-4, Robert Klemme wrote:
>> That sounds more like a use case for clone() to me as it seems to
>> stress that the original type is preserved.  It really depends on
>> the usage.
>
> Maybe I'm getting confused then.  I thought clone() sounded wrong
> because it should return an instance of that class, so if you extend
> the class and call super.clone it gets an instance of the base class?
> If your extended class needs to return an instance of itself, which
> needs to copy all the same values as the super class plus a few
> more...

You have already gotten several examples.

I will just add a ref to the docs:

https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#clone--

<quote>
Creates and returns a copy of this object. The precise meaning of "copy" 
may depend on the class of the object. The general intent is that, for 
any object x, the expression:

      x.clone() != x

will be true, and that the expression:

      x.clone().getClass() == x.getClass()

will be true, but these are not absolute requirements. While it is 
typically the case that:

      x.clone().equals(x)

will be true, this is not an absolute requirement.

By convention, the returned object should be obtained by calling 
super.clone. If a class and all of its superclasses (except Object) obey 
this convention, it will be the case that x.clone().getClass() == 
x.getClass().

By convention, the object returned by this method should be independent 
of this object (which is being cloned). To achieve this independence, it 
may be necessary to modify one or more fields of the object returned by 
super.clone before returning it. Typically, this means copying any 
mutable objects that comprise the internal "deep structure" of the 
object being cloned and replacing the references to these objects with 
references to the copies. If a class contains only primitive fields or 
references to immutable objects, then it is usually the case that no 
fields in the object returned by super.clone need to be modified.

The method clone for class Object performs a specific cloning operation. 
First, if the class of this object does not implement the interface 
Cloneable, then a CloneNotSupportedException is thrown. Note that all 
arrays are considered to implement the interface Cloneable and that the 
return type of the clone method of an array type T[] is T[] where T is 
any reference or primitive type. Otherwise, this method creates a new 
instance of the class of this object and initializes all its fields with 
exactly the contents of the corresponding fields of this object, as if 
by assignment; the contents of the fields are not themselves cloned. 
Thus, this method performs a "shallow copy" of this object, not a "deep 
copy" operation.

The class Object does not itself implement the interface Cloneable, so 
calling the clone method on an object whose class is Object will result 
in throwing an exception at run time.
</quote>

Which pretty well explains why sub classing is actually a thing that
clone handles very well.

Arne

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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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