Cloning of Java objects

Parent class of all objects in java is "Object" which contains several important methos. One of them is "clone" which is used to make a copy of object. There are certain things which you should keep in mind while using clone method of Object.
- implement "Cloneable" interface to each class which you want to clone. If a class does not implement "Cloneable" interface, clone method will throw "CloneableNotSupportedException". This interface does not contain any methods. It is just a marker interface. Suppose you have a Class "Book" of which you want to make a clone, then structure of that class will be:

public class Book implements Cloneable
{
//attributes
//methods
@Override protected Object clone() throws CloneableNotSupportedException
{
//Clone logic
}
}

- Obtain cloned object through super.clone
- if your class only contains primitive attributes or wrappers of primitive attributes or immutable object like String, then you don't need to do anything special inside clone method because clone method automatically makes a copy of primitive data types and immutable objects. Suppose above "Book" class contains following two attributes with accessor methdos for attributes:

private String name;
private Float price;
public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public Float getPrice() {
return price;
}

public void setPrice(Float price) {
this.price = price;
}

Now to clone Book class you just need to write following implementation in overriding of clone method:

protected Object clone() CloneNotSupportedException
{
return super.clone();
}

- by default clone method makes shallow copy and does not make a Deep copy of object. Deep copy means if your object contains references of other objects or collections, it will not make a copy of those referenced objects and will use same references as was in origianl objects. Suppose above mentioned "Book" class contains an attribute of class "Publisher". Structure of Publisher class is given below:


public class Publisher{
private String pubName;

public String getPubName() {
return pubName;
}

public void setPubName(String pubName) {
this.pubName = pubName;
}
}

Now try out the following code:

Publisher pub = new Publisher();
pub.setPubName("Java Publisher");
orig.setPub(pub);
Logger.getLogger(CloneTest.class.getName()).log(Level.INFO,orig.getPub().getPubName());
Book clone2 = (Book) orig.clone();
Logger.getLogger(CloneTest.class.getName()).log(Level.INFO,clone2.getPub().getPubName());
orig.getPub().setPubName("java Publisher Changed");
Logger.getLogger(CloneTest.class.getName()).log(Level.INFO,clone2.getPub().getPubName());

Output of above code will be:
15-Jun-2008 1:39:47 AM javasolutionsblog.clonetest.CloneTest main
INFO: Java Publisher
15-Jun-2008 1:39:47 AM javasolutionsblog.clonetest.CloneTest main
INFO: Java Publisher
15-Jun-2008 1:39:47 AM javasolutionsblog.clonetest.CloneTest main
INFO: Java Publisher Changed

Although expected output of third log was "Java Publisher" but since clone didn't make copy of refernce object publisher, it used same object in clone as in origignal object, when i make a change in the publisher name in original object, it also reflected in cloned object.

- In order to do deep copy of Book class, we will aso have to implement "Cloneable" interface in "Publisher" class + override clone method in Publisher class too. If Publisher class contains references of other objects, then you will have to implement Cloneable interface + override clone method in each sub class too.
In our case, structure of Publisher class will be:

public class Publisher implements Cloneable{
private String pubName;

public String getPubName() {
return pubName;
}

public void setPubName(String pubName) {
this.pubName = pubName;
}

@Override
protected Object clone() throws CloneNotSupportedException
{
return super.clone();
}

}

Now change the definition of clone method of Book class as follows:

protected Object clone() throws CloneNotSupportedException {
Book clonedBook = (Book) super.clone();
if (this.getPub() != null) {
clonedBook.setPub((Publisher) this.getPub().clone());
}

return clonedBook;
}