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Groups > comp.lang.java.programmer > #11390

Re: cloning

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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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Thread

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

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