Java Reference
In-Depth Information
Cloning Objects
Java does not provide an automatic mechanism to clone (make a copy) an object. Recall that when you assign a
reference variable to another reference variable, only the reference of the object is copied, not the content of the
object. Cloning an object means copying the content of the object bit by bit. If you want objects of your class to be
cloned, you must reimplement the
clone()
method in your class. Once you reimplement the
clone()
method, you
should be able to clone objects of your class by calling the
clone()
method. The declaration of the
clone()
method in
the
Object
class is as follows:
protected Object clone() throws CloneNotSupportedException
You need to observe few things about the declaration of the
clone()
method.
protected
. Therefore, you will not be able to call it from the client code. The
following code is not valid:
•
It is declared
Object obj = new Object();
Object clone = obj.clone(); // Error. Cannot access protected clone() method
This means you need to declare the
clone()
method
public
in your class if you want the client
code to clone objects of your class.
Object
. It means you will need to cast the returned value of the
clone()
method. Suppose
MyClass
is cloneable. Your cloning code will look as
•
Its return type is
MyClass mc = new MyClass();
MyClass clone = (MyClass)mc.clone(); // Need to use a cast
You do not need to know any internal details about an object to clone it. The
clone()
method in the
Object
class
has all the code that is needed to clone an object. All you need is to call it from the
clone()
method of your class. It
will make a bitwise copy of the original object and return the reference of the copy.
The
clone()
method in the
Object
class throws a
CloneNotSupportedException
. It means when you call the
clone()
method of the
Object
class, you need to place the call in a
try-catch
block, or rethrow the exception. You will
learn more about the
try-catch
block in Chapter 9. You have the option not to throw a
CloneNotSupportedException
from the
clone()
method of your class. The following snippet of code is placed inside the
clone()
method of your
class, which calls the
clone()
method of the
Object
class using the
super
keyword:
YourClass obj = null;
try {
// Call clone() method of the Object class using super.clone()
obj = (YourClass)super.clone();
}
catch (CloneNotSupportedException e) {
e. printStackTrace();
}
return obj;
