cloning

Discussion in 'Java' started by albert kao, Jan 16, 2012.

  1. albert kao

    albert kao Guest

    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();
    }
    }
    albert kao, Jan 16, 2012
    #1
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  2. albert kao

    Arne Vajhøj Guest

    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()

    <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
    Arne Vajhøj, Jan 16, 2012
    #2
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  3. albert kao

    Lew Guest

    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()
    >
    > <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
    Lew, Jan 16, 2012
    #3
  4. 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
    Arne Vajhøj, Jan 16, 2012
    #4
  5. albert kao

    Lew Guest

    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
    Lew, Jan 16, 2012
    #5
  6. albert kao

    Lew Guest

    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
    Lew, Jan 16, 2012
    #6
  7. 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
    Arne Vajhøj, Jan 16, 2012
    #7
  8. albert kao

    Roedy Green Guest

    On Mon, 16 Jan 2012 10:27:27 -0800 (PST), albert kao
    <> 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".
    Roedy Green, Jan 17, 2012
    #8
  9. albert kao

    Arne Vajhøj Guest

    On 1/17/2012 12:55 PM, Roedy Green wrote:
    > On Mon, 16 Jan 2012 10:27:27 -0800 (PST), albert kao
    > <> 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
    Arne Vajhøj, Jan 17, 2012
    #9
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