Java Reference
In-Depth Information
An object of the
ShallowClone
class is composed of an object of the
DoubleHolder
class. The code in the
clone()
method of the
ShallowClone
class is the same as for the
clone()
method of the
DoubleHolder
class. The difference
lies in the type of instance variables that are used for the two classes. The
DoubleHolder
class has an instance variable
of primitive type
double
, whereas the
ShallowClone
class has an instance variable of the reference type
DoubleHolder
.
When the
ShallowClone
class calls the
clone()
method of the
Object
class (using
super.clone()
), it receives a
shallow copy of itself. That is, it shares the
DoubleHolder
object used in its instance variable with its clone.
Listing 7-10 has test cases to test an object of the
ShallowClone
class and its clone. The output shows that after
you make a clone, changing the value through the original object also changes the value in the cloned object. This is
so because the
ShallowClone
object stores the value in another object of the
DoubleHolder
class, which is shared by
both the cloned and the original objects.
Listing 7-10.
A Test Class to Demonstrate the Shallow Copy Mechanism
// ShallowCloneTest.java
package com.jdojo.object;
public class ShallowCloneTest {
public static void main(String[] args) {
ShallowClone sc = new ShallowClone(100.00);
ShallowClone scClone = (ShallowClone)sc.clone();
// Print the value in original and clone
System.out.println("Original:" + sc.getValue());
System.out.println("Clone :" + scClone.getValue());
// Change the value in original and it will change the value
// for clone too because we have done shallow cloning
sc.setValue(200.00);
// Print the value in original and clone
System.out.println("Original:" + sc.getValue());
System.out.println("Clone :" + scClone.getValue());
}
}
Original:100.0
Clone :100.0
Original:200.0
Clone :200.0
In a deep cloning, you need to clone all objects referenced by all reference instance variables of an object. You
must perform a shallow cloning before you can perform a deep cloning. The shallow cloning is performed by calling
the
clone()
method of the
Object
class. Then you will need to write code to clone all reference instance variables.
Listing 7-11 has code for a
DeepClone
class, which performs a deep cloning.
