Object Cloning in Java-Deep, Shallow and Copy Constructors

A clone is an exact copy of the original. In java, it essentially means the ability to create an object with similar state as the original object. The java clone() method provides this functionality.

What is clone?

So cloning is about creating the copy of original object. Its dictionary meaning is : “make an identical copy of“.

By default, java cloning is ‘field by field copy’ i.e. as the Object class does not have idea about the structure of class on which clone() method will be invoked.

So, JVM when called for cloning, do following things:

  1. If the class has only primitive data type members then a completely new copy of the object will be created and the reference to the new object copy will be returned.

  2. If the class contains members of any class type then only the object references to those members are copied and hence the member references in both the original object as well as the cloned object refer to the same object.

Apart from above default behavior, you can always override this behavior and specify your own. This is done using overriding clone() method. Let’s see how it is done.

Why use clone() method ?

The clone() method saves the extra processing task for creating the exact copy of an object. If we perform it by using the new keyword, it will take a lot of processing time to be performed that is why we use object cloning.

Advantage of Object cloning

Although Object.clone() has some design issues but it is still a popular and easy way of copying objects. Following is a list of advantages of using clone() method:

  • You don't need to write lengthy and repetitive codes. Just use an abstract class with a 4- or 5-line long clone() method.

  • It is the easiest and most efficient way for copying objects, especially if we are applying it to an already developed or an old project. Just define a parent class, implement Cloneable in it, provide the definition of the clone() method and the task will be done.

  • Clone() is the fastest way to copy array.

Disadvantage of Object cloning

Following is a list of some disadvantages of clone() method:

  • To use the Object.clone() method, we have to change a lot of syntaxes to our code, like implementing a Cloneable interface, defining the clone() method and handling CloneNotSupportedException, and finally, calling Object.clone() etc.

  • We have to implement cloneable interface while it doesn?t have any methods in it. We just have to use it to tell the JVM that we can perform clone() on our object.

  • Object.clone() is protected, so we have to provide our own clone() and indirectly call Object.clone() from it.

  • Object.clone() doesn?t invoke any constructor so we don?t have any control over object construction.

  • If you want to write a clone method in a child class then all of its superclasses should define the clone() method in them or inherit it from another parent class. Otherwise, the super.clone() chain will fail.

  • Object.clone() supports only shallow copying but we will need to override it if we need deep cloning.

Java Cloneable interface and clone() method

Every language which supports cloning of objects has its own rules and so does java. In java, if a class needs to support cloning it has to do following things:

  1. You must implement Cloneable interface.

  2. You must override clone() method from Object class. [Its weird. clone() method should have been in Cloneable interface.]

Read more: Cloneable interface is broken in java

Java docs about clone() method are given below (formatted and extract).

Java clone() method /* 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: 1) x.clone() != x will be true 2) x.clone().getClass() == x.getClass() will be true, but these are not absolute requirements. 3) x.clone().equals(x) will be true, this is not an absolute requirement. */ protected native Object clone() throws CloneNotSupportedException;
  1. First statement guarantees that cloned object will have separate memory address assignment.

  2. Second statement suggest that original and cloned objects should have same class type, but it is not mandatory.

  3. Third statement suggest that original and cloned objects should have be equal using equals() method, but it is not mandatory.

Let’s understand Java clone with example. Our first class is Employee class with 3 attributes – id, name and department.

public class Employee implements Cloneable{ private int empoyeeId; private String employeeName; private Department department; public Employee(int id, String name, Department dept) { this.empoyeeId = id; this.employeeName = name; this.department = dept; } @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); } //Getters and Setters }

Department class has two attributes – id and name.

public class Department { private int id; private String name; public Department(int id, String name) { this.id = id; this.name = name; } //Getters and Setters }

So, if we need to clone the Employee class, then we need to do something like this.

public class TestCloning { public static void main(String[] args) throws CloneNotSupportedException { Department dept = new Department(1, "Human Resource"); Employee original = new Employee(1, "Admin", dept); //Lets create a clone of original object Employee cloned = (Employee) original.clone(); //Let verify using employee id, if cloning actually workded System.out.println(cloned.getEmpoyeeId()); //Verify JDK's rules //Must be true and objects must have different memory addresses System.out.println(original != cloned); //As we are returning same class; so it should be true System.out.println(original.getClass() == cloned.getClass()); //Default equals method checks for references so it should be //false. If we want to make it true, //then we need to override equals method in Employee class. System.out.println(original.equals(cloned)); } }

1 true true false

Great, we successfully cloned the Employee object. But, remember we have two references to the same object and now both will change the state of the object in different parts of the application. Want to see how? Let’s see.

public class TestCloning { public static void main(String[] args) throws CloneNotSupportedException { Department hr = new Department(1, "Human Resource"); Employee original = new Employee(1, "Admin", hr); Employee cloned = (Employee) original.clone(); //Let change the department name in cloned object and we will verify in original object cloned.getDepartment().setName("Finance"); System.out.println(original.getDepartment().getName()); System.out.println(cloned.getDepartment().getName()); } }

Finance Finance

Oops, cloned object changes are visible in original also. This way cloned objects can make havoc in the system if allowed to do so. Anybody can come and clone your application objects and do whatever he likes. Can we prevent this??

Answer is yes, we can. We can prevent this by creating Java deep copy and use copy constructors. We will learn about them later in this post.

Example of clone() method (Object cloning)
class Student18 implements Cloneable{ int rollno; String name; Student18(int rollno,String name){ this.rollno=rollno; this.name=name; } public Object clone()throws CloneNotSupportedException{ return super.clone(); } public static void main(String args[]){ try{ Student18 s1=new Student18(131,"Rama"); Student18 s2=(Student18)s1.clone(); System.out.println(s1.rollno+" "+s1.name); System.out.println(s2.rollno+" "+s2.name); }catch(CloneNotSupportedException c){} } }

131 Rama 131 Rama
Java Shallow Copy

Shallow clone is default implementation in Java. In overridden clone method, if you are not cloning all the object types (not primitives), then you are making a shallow copy.

All above examples are of shallow copy only, because we have not cloned the Department object on Employee class’s clone method. Now, I will move on to next section where we will see the deep cloning.

Deep Copy

It is the desired behavior in most the cases. In the deep copy, we create a clone which is independent of original object and making changes in the cloned object should not affect original object.

Let see how deep copy is created in Java.

//Modified clone() method in Employee class @Override protected Object clone() throws CloneNotSupportedException { Employee cloned = (Employee)super.clone(); cloned.setDepartment((Department)cloned.getDepartment().clone()); return cloned; }

I modified the Employee classes clone() method and added following clone method in Department class.

//Defined clone method in Department class. @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); }

Now testing our cloning code gives desired result and name of the department will not be modified.

public class TestCloning { public static void main(String[] args) throws CloneNotSupportedException { Department hr = new Department(1, "Human Resource"); Employee original = new Employee(1, "Admin", hr); Employee cloned = (Employee) original.clone(); //Let change the department name in cloned object and we will verify in original object cloned.getDepartment().setName("Finance"); System.out.println(original.getDepartment().getName()); System.out.println(cloned.getDepartment().getName()); } }

Human Resource Finance

Here, changing state of the cloned object does not affect the original object.

So deep cloning requires satisfaction of following rules –

  • p style="text-align: justify;text-indent: 0px;">No need to separately copy primitives.

  • p style="text-align: justify;text-indent: 0px;">All the member classes in original class should support cloning and in clone method of original class in context should call super.clone() on all member classes.

  • p style="text-align: justify;text-indent: 0px;">If any member class does not support cloning then in clone method, one must create a new instance of that member class and copy all its attributes one by one to new member class object. This new member class object will be set in cloned object.

Java Copy Constructors

Copy constructors are special constructors in a class which takes argument for its own class type. So, when you pass an instance of class to copy constructor, then constructor will return a new instance of class with values copied from argument instance. It helps you to clone object with Cloneable interface.

public class PointOne { private Integer x; private Integer y; public PointOne(PointOne point){ this.x = point.x; this.y = point.y; } }

This method looks simple and it is until comes inheritance. When you define a class by extending above class, you need to define a similar constructor there also. In child class, you need to copy child specific attributes and pass the argument to the super class’s constructor. Let’s see how?

public class PointTwo extends PointOne { private Integer z; public PointTwo(PointTwo point){ super(point); //Call Super class constructor here this.z = point.z; } }

So, are we fine now? NO. The problem with inheritance is that exact behavior is identified only at runtime. So, in our case if some class passed the instance of PointTwo in constructor of PointOne.

In this case, you will get the instance of PointOne in return where you passed instance of PointTwo as argument. Lets see this in code:

class Test { public static void main(String[] args) { PointOne one = new PointOne(1,2); PointTwo two = new PointTwo(1,2,3); PointOne clone1 = new PointOne(one); PointOne clone2 = new PointOne(two); //Let check for class types System.out.println(clone1.getClass()); System.out.println(clone2.getClass()); } }

class corejava.cloning.PointOne class corejava.cloning.PointOne

Another way of creating a copy constructor is to have static factory methods. They take class type in argument and create a new instance using another constructor of the class. Then these factory methods will copy all the state data to new class instance just created in the previous step, and return this updated instance.

public class PointOne implements Cloneable { private Integer x; private Integer y; public PointOne(Integer x, Integer y) { this.x = x; this.y = y; } public PointOne copyPoint(PointOne point) throws CloneNotSupportedException { if(!(point instanceof Cloneable)) { throw new CloneNotSupportedException("Invalid cloning"); } //Can do multiple other things here return new PointOne(point.x, point.y); } }

Deep copy with serialization

Serialization is another easy way of deep cloning. In this method, you just serialize the object to be cloned and de-serialize it. Obviously, the object which need to be cloned should implement Serializable interface.

Before going any further, I should caution that this technique is not to be used lightly.

  1. First of all, serialization is hugely expensive. It could easily be a hundred times more expensive than the clone() method.

  2. Second, not all objects are Serializable.

  3. Third, making a class Serializable is tricky and not all classes can be relied on to get it right.

Deep copy with serialization
@SuppressWarnings("unchecked") public static T clone(T t) throws Exception { //Check if T is instance of Serializeble other throw CloneNotSupportedException ByteArrayOutputStream bos = new ByteArrayOutputStream(); //Serialize it serializeToOutputStream(t, bos); byte[] bytes = bos.toByteArray(); ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(bytes)); //Deserialize it and return the new instance return (T)ois.readObject(); }

Apache commons – SerializationUtils

In Apache commons, SerializationUtils class also has utility function for deep cloning. If you feel interested the follow their official docs.

org.apache.commons commons-lang3 3.7

SerializationUtils example SomeObject cloned = org.apache.commons.lang.SerializationUtils.clone(someObject);
Java clone best practices
  1. When you don’t know whether you can call the clone() method of a particular class as you are not sure if it is implemented in that class, you can check with checking if the class is instance of “Cloneable” interface as below.

    if(obj1 instanceof Cloneable){ obj2 = obj1.clone(); } //Dont do this. Cloneable dont have any methods obj2 = (Cloneable)obj1.clone();

  2. No constructor is called on the object being cloned. As a result, it is your responsibility, to make sure all the members have been properly set. Also, if you are keeping track of the number of objects in the system by counting the invocation of constructors, you got a new additional place to increment the counter.

I hope that this post has been a refresher for you and help you gain more information about Java 8 clone method and it’s correct usage. It will also help in replying Java clone interview questions.