cloning

[albert kao]

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();
}
}

 

[Arne Vajhøj]

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

 

[Lew]

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:

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

You didn't implement that interface.

 

[Arne Vajhøj]

You didn't implement that interface.

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

Arne

 

[Lew]

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]

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

At runtime.

 

[Arne Vajhøj]

At runtime.

Hopefully runtime during unit tests.

Arne

 

[Roedy Green]

I want to clone the following class so is the following code ok?

see http://mindprod.com/jgloss/clone.html

 

[Arne Vajhøj]

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