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cloning

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  cloning albert kao <albertkao3@gmail.com> - 2012-01-16 10:27 -0800
    Re: cloning Arne Vajhøj <arne@vajhoej.dk> - 2012-01-16 15:29 -0500
      Re: cloning Lew <noone@lewscanon.com> - 2012-01-16 13:47 -0800
        Re: cloning Arne Vajhøj <arne@vajhoej.dk> - 2012-01-16 16:52 -0500
          Re: cloning Lew <noone@lewscanon.com> - 2012-01-16 14:04 -0800
            Re: cloning Arne Vajhøj <arne@vajhoej.dk> - 2012-01-16 17:14 -0500
    Re: cloning Lew <noone@lewscanon.com> - 2012-01-16 14:03 -0800
    Re: cloning Roedy Green <see_website@mindprod.com.invalid> - 2012-01-17 09:55 -0800
      Re: cloning Arne Vajhøj <arne@vajhoej.dk> - 2012-01-17 17:37 -0500

#11381 — cloning

I want to clone the following class so is the following code ok?
public class TestImpl implements Serializable {

    private int id = 0;
    private java.sql.Timestamp createDateTime;
    private java.sql.Date expiryDate;
    private java.math.BigDecimal amount;
    private java.sql.Timestamp deleteTimestamp;

    // other methods
    // ...



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

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#11389

On 1/16/2012 1:27 PM, albert kao wrote:
> I want to clone the following class so is the following code ok?
> public class TestImpl implements Serializable {
>
>      private int id = 0;
>      private java.sql.Timestamp createDateTime;
>      private java.sql.Date expiryDate;
>      private java.math.BigDecimal amount;
>      private java.sql.Timestamp deleteTimestamp;
>
>      // other methods
>      // ...
>
>
>
>      @Override
>      public Object clone() throws CloneNotSupportedException {
>          return (TestImpl)super.clone();
>      }
> }

http://docs.oracle.com/javase/6/docs/api/java/lang/Object.html#clone%28%29

<quote>
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. 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>

Note what it says about mutable objects.

Arne

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#11390

On 01/16/2012 12:29 PM, Arne Vajhøj wrote:
> On 1/16/2012 1:27 PM, albert kao wrote:
>> I want to clone the following class so is the following code ok?
>> public class TestImpl implements Serializable {
>>
>> private int id = 0;
>> private java.sql.Timestamp createDateTime;
>> private java.sql.Date expiryDate;
>> private java.math.BigDecimal amount;
>> private java.sql.Timestamp deleteTimestamp;
>>
>> // other methods
>> // ...
>>
>>
>>
>> @Override
>> public Object clone() throws CloneNotSupportedException {
>> return (TestImpl)super.clone();
>> }
>> }
>
> http://docs.oracle.com/javase/6/docs/api/java/lang/Object.html#clone%28%29
>
> <quote>
> 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. 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>
>
> Note what it says about mutable objects.

Besides Arne's excellent advice to read the relevant documentation (always the 
first step, really, wouldn't you concur?), and generosity in saving you the 
weighty burden of clicking a link, let me highlight certain points:

>> the returned object should be obtained by calling super.clone.

You can get really weird bugs if you violate that one.

>> if the class of this object does not implement the interface Cloneable,

You didn't implement that interface.

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg

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#11391

On 1/16/2012 4:47 PM, Lew wrote:
> On 01/16/2012 12:29 PM, Arne Vajhøj wrote:
>> On 1/16/2012 1:27 PM, albert kao wrote:
...
>> if the class of this object does not implement the interface Cloneable,
>
> You didn't implement that interface.

The "good" thing about that bug is that it will show up
quickly and consistent.

Arne

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#11393

Arne Vajhøj wrote:
> Lew wrote:
>> Arne Vajhøj wrote:
>>> albert kao wrote:
> ...
>>> if the class of this object does not implement the interface Cloneable,
>>
>> You didn't implement that interface.
>
> The "good" thing about that bug is that it will show up
> quickly and consistent.

At runtime.

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg

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#11394

On 1/16/2012 5:04 PM, Lew wrote:
> Arne Vajhøj wrote:
>> Lew wrote:
>>> Arne Vajhøj wrote:
>>>> albert kao wrote:
>> ...
>>>> if the class of this object does not implement the interface Cloneable,
>>>
>>> You didn't implement that interface.
>>
>> The "good" thing about that bug is that it will show up
>> quickly and consistent.
>
> At runtime.

Hopefully runtime during unit tests.

Arne

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#11392

On 01/16/2012 10:27 AM, albert kao wrote:
> I want to clone the following class so is the following code ok?
> public class TestImpl implements Serializable {

'private static final long serialVersionUID'?

Have you studied up on 'Serializable'?  It's a dangerous tool in the hands of 
the sorcerer's apprentice.
<http://java.sun.com/docs/books/effective/>
Chapter 11, "Serialization"

Yes, he gives it an entire chapter.

>      private int id = 0;

You don't need to initialize 'id' to zero twice.

What's that?  Well, zero is already the default value.  Then you assign zero 
to the variable in a second, explicit initialization.  That's twice.

Having said that, there are sometimes reasons for an explicit initialization 
of a default value, and it's required for 'final' variables.

>      private java.sql.Timestamp createDateTime;

import directives, anyone?

>      private java.sql.Date expiryDate;

I agree that you don't want an import for 'java.sql.Date'.

>      private java.math.BigDecimal amount;

Good choice of type.

>      private java.sql.Timestamp deleteTimestamp;
>
>      // other methods
>      // ...

Really good variable names, by the way.

>      @Override
>      public Object clone() throws CloneNotSupportedException {
>          return (TestImpl)super.clone();
>      }
> }

I suggest that either you eliminate the cast or have a covariant return type.

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg

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#11413

On Mon, 16 Jan 2012 10:27:27 -0800 (PST), albert kao
<albertkao3@gmail.com> wrote, quoted or indirectly quoted someone who
said :

>I want to clone the following class so is the following code ok?
see http://mindprod.com/jgloss/clone.html
-- 
Roedy Green Canadian Mind Products
http://mindprod.com
One of the most useful comments you can put in a program is 
"If you change this, remember to change ?XXX? too".
 

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#11430

On 1/17/2012 12:55 PM, Roedy Green wrote:
> On Mon, 16 Jan 2012 10:27:27 -0800 (PST), albert kao
> <albertkao3@gmail.com>  wrote, quoted or indirectly quoted someone who
> said :
>
>> I want to clone the following class so is the following code ok?
> see http://mindprod.com/jgloss/clone.html

Not doing anything about the mutable fields is not what Java Doc
recommends.

And I not keen on the conversion of CloneNotSupportedException to
a null return either.

Arne

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