Java Clone, Shallow Copy and Deep Copy

Clone (κλών) is a Greek word meaning “branch”, referring to the process whereby a new plant can be created from a twig. In biology it is about copying the DNAs. In real world, if you clone Marilyn Monroe, will you get a copy of her with same beauty and characteristics? No, you will not get! This exactly applies to java also. See how java guys are good at naming technologies.

In java, though clone is ‘intended’ to produce a copy of the same object it is not guaranteed. Clone comes with lots of its and buts. So my first advice is to not depend on clones. If you want to provide a handle / method to deliver a copy of the current instance write a kind of factory method and provide it with a good documentation. When you are in a situation to use a third party component and produce copies of it using the clone method, then investigate that implementation carefully and get to know what is underlying. Because when you ask for a rabbit, it may give monkeys!

Shallow Copy

Generally clone method of an object, creates a new instance of the same class and copies all the fields to the new instance and returns it. This is nothing but shallow copy. Object class provides a clone method and provides support for the shallow copy. It returns ‘Object’ as type and you need to explicitly cast back to your original object.

Since the Object class has the clone method (protected) you cannot use it in all your classes. The class which you want to be cloned should implement clone method and overwrite it. It should provide its own meaning for copy or to the least it should invoke the super.clone(). Also you have to implement Cloneable marker interface or else you will get CloneNotSupportedException. When you invoke the super.clone() then you are dependent on the Object class’s implementation and what you get is a shallow copy.

Deep Copy

When you need a deep copy then you need to implement it yourself. When the copied object contains some other object its references are copied recursively in deep copy. When you implement deep copy be careful as you might fall for cyclic dependencies. If you don’t want to implement deep copy yourselves then you can go for serialization. It does implements deep copy implicitly and gracefully handling cyclic dependencies.

One more disadvantage with this clone system is that, most of the interface / abstract class writers in java forget to put a public clone method. For example you can take List. So when you want to clone their implementations you have to ignore the abstract type and use actual implementations like ArrayList by name. This completely removes the advantage and goodness of abstractness.

When implementing a singleton pattern, if its superclass implements a public clone() method, to prevent your subclass from using this class’s clone() method to obtain a copy overwrite it and throw a CloneNotSupportedException.

Note that clone is not for instantiation and initialization. It should not be synonymously used as creating a new object. Because the constructor of the cloned objects may never get invoked in the process. It is about copying the object in discussion and not creating new. It completely depends on the clone implementation. One more disadvantage (what to do there are so many), clone prevents the use of final fields. We have to find roundabout ways to copy the final fields into the copied object.

Clone is an agreement between you, compiler and implementer. If you are confident that you all three have good knowledge of java, then go ahead and use clone. If you have a slightest of doubt better copy the object manually.

Example source code for java clone and shallow copy

class Employee implements Cloneable {

 private String name;
 private String designation;

 public Employee() {
 this.setDesignation("Programmer");
 }
 public String getDesignation() {
 return designation;
 }

 public void setDesignation(String designation) {
 this.designation = designation;
 }

 public String getName() {
 return name;
 }

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

 public Object clone() throws CloneNotSupportedException {
 /*
 Employee copyObj = new Employee();
 copyObj.setDesignation(this.designation);
 copyObj.setName(this.name);
 return copyObj;
 */
 return super.clone();
 }
}

public class CloneExample {
 public static void main(String arg[]){
 Employee jwz = new Employee();
 jwz.setName("Jamie Zawinski");
 try {
 Employee joel = (Employee) jwz.clone();
 System.out.println(joel.getName());
 System.out.println(joel.getDesignation());
 } catch (CloneNotSupportedException cnse) {
 System.out.println("Cloneable should be implemented. " + cnse );
 }
 }
}

Output of the above program:

Jamie Zawinski
Programmer