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clone() method


Joined: Nov 13, 2000
Posts: 9
why do we use the clone() method in java?
Ranch Hand

Joined: Aug 06, 2000
Posts: 60
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:
will be true, this is not an absolute requirement. Copying an object will typically entail creating a new instance of its class, but it also may require copying of internal data structures as well. No constructors are called.
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. The clone method is implemented by the class Object as a convenient, general utility for subclasses that implement the interface Cloneable, possibly also overriding the clone method, in which case the overriding definition can refer to this utility definition by the call:
a clone of this instance.



Joined: Nov 13, 2000
Posts: 9
How can you differentiate shallow copying and deep copying of the objects in the context of clone() method?
Ken Huskerson

Joined: May 16, 2004
Posts: 4
I think it is very thoughtless to answer someone by just posting the javadoc for this method. Seems a little sarcastic to a person asking a very real question.

So again, as I would like to know as well, why do you use this. Maybe someone could offer an example of a situation. This would take a little more thought than a simeple copy-paste.


Michael Herrmann
Ranch Hand

Joined: Dec 06, 2003
Posts: 60
"A shallow copy of an object is a copy in which only the primitive and reference values are copied to the new object. This means that object members like ints, floats, and booleans have the same values in both the new and the existing objects, but object members like arrays, Hashtables, Vectors and so on are shared between the new and existing objects.
A deep copy of an object differs from a shallow copy because all nonprimitive object members are cloned as well. In a deep copy, if an existing object has a Vector object as a member, a clone operation on the existing objects results in a new object being created that points to a new Vector object as well."
Taken from Java Pitfalls: Time-Saving Solutions and Workarounds To Improve Programs. The chapter this excerpt is from from covers the whole clone() issue("Properly Cloning An Object").

Michael Herrmann
I agree. Here's the link:
subject: clone() method
jQuery in Action, 3rd edition