Object cloning refers to the creation of an exact copy of an object. It creates a new instance of the class of the current object and initializes all its fields with exactly the contents of the corresponding fields of this object.

Using Assignment Operator to create a copy of the reference variable

In Java, there is no operator to create a copy of an object. Unlike C++, in Java, if we use the assignment operator then it will create a copy of the reference variable and not the object. This can be explained by taking an example. The following program demonstrates the same.

// Java program to demonstrate that assignment operator
// only creates a new reference to same object
import java.io.*;

// A test class whose objects are cloned
class Test {
int x, y;
Test()
{
x = 10;
y = 20;
}
}

// Driver Class
class Main {
public static void main(String[] args)
{
Test ob1 = new Test();

System.out.println(ob1.x + " " + ob1.y);

// Creating a new reference variable ob2
// pointing to same address as ob1
Test ob2 = ob1;

// Any change made in ob2 will
// be reflected in ob1
ob2.x = 100;

System.out.println(ob1.x + " " + ob1.y);
System.out.println(ob2.x + " " + ob2.y);
}
}
Output
10 20
100 20
100 20

Creating a copy using the clone() method

The class whose object’s copy is to be made must have a public clone method in it or in one of its parent class.

Every class that implements clone() should call super.clone() to obtain the cloned object reference.
The class must also implement java.lang.Cloneable interface whose object clone we want to create otherwise it will throw CloneNotSupportedException when clone method is called on that class’s object.
Syntax:
protected Object clone() throws CloneNotSupportedException
Usage of clone() method -Shallow Copy

// A Java program to demonstrate
// shallow copy using clone()
import java.util.ArrayList;

// An object reference of this class is
// contained by Test2
class Test {
int x, y;
}

// Contains a reference of Test and
// implements clone with shallow copy.
class Test2 implements Cloneable {
int a;
int b;
Test c = new Test();
public Object clone() throws CloneNotSupportedException
{
return super.clone();
}
}

// Driver class
public class Main {
public static void main(String args[])
throws CloneNotSupportedException
{
Test2 t1 = new Test2();
t1.a = 10;
t1.b = 20;
t1.c.x = 30;
t1.c.y = 40;

Test2 t2 = (Test2)t1.clone();

// Creating a copy of object t1
// and passing it to t2
t2.a = 100;

// Change in primitive type of t2 will
// not be reflected in t1 field
t2.c.x = 300;

// Change in object type field will be
// reflected in both t2 and t1(shallow copy)
System.out.println(t1.a + " " + t1.b + " " + t1.c.x
+ " " + t1.c.y);
System.out.println(t2.a + " " + t2.b + " " + t2.c.x
+ " " + t2.c.y);
}
}
Output
10 20 300 40
100 20 300 40

In the above example, t1.clone returns the shallow copy of the object t1. To obtain a deep copy of the object certain modifications have to be made in the clone method after obtaining the copy.

Deep Copy vs Shallow Copy

Shallow copy is the method of copying an object and is followed by default in cloning. In this method, the fields of an old object X are copied to the new object Y. While copying the object type field the reference is copied to Y i.e object Y will point to the same location as pointed out by X. If the field value is a primitive type it copies the value of the primitive type.
Therefore, any changes made in referenced objects in object X or Y will be reflected in other objects.
Shallow copies are cheap and simple to make. In the above example, we created a shallow copy of the object.

Usage of clone() method – Deep Copy

If we want to create a deep copy of object X and place it in a new object Y then a new copy of any referenced objects fields are created and these references are placed in object Y. This means any changes made in referenced object fields in object X or Y will be reflected only in that object and not in the other. In the below example, we create a deep copy of the object.
A deep copy copies all fields and makes copies of dynamically allocated memory pointed to by the fields. A deep copy occurs when an object is copied along with the objects to which it refers.

// A Java program to demonstrate
// deep copy using clone()

// An object reference of this
// class is contained by Test2
class Test {
int x, y;
}

// Contains a reference of Test and
// implements clone with deep copy.
class Test2 implements Cloneable {
int a, b;

Test c = new Test();

public Object clone() throws CloneNotSupportedException
{
// Assign the shallow copy to
// new reference variable t
Test2 t = (Test2)super.clone();

// Creating a deep copy for c
t.c = new Test();
t.c.x = c.x;
t.c.y = c.y;

// Create a new object for the field c
// and assign it to shallow copy obtained,
// to make it a deep copy
return t;
}
}

public class Main {
public static void main(String args[])
throws CloneNotSupportedException
{
Test2 t1 = new Test2();
t1.a = 10;
t1.b = 20;
t1.c.x = 30;
t1.c.y = 40;

Test2 t3 = (Test2)t1.clone();
t3.a = 100;

// Change in primitive type of t2 will
// not be reflected in t1 field
t3.c.x = 300;

// Change in object type field of t2 will
// not be reflected in t1(deep copy)
System.out.println(t1.a + " " + t1.b + " " + t1.c.x
+ " " + t1.c.y);
System.out.println(t3.a + " " + t3.b + " " + t3.c.x
+ " " + t3.c.y);
}
}
Output
10 20 30 40
100 20 300 40
In the above example, we can see that a new object for the Test class has been assigned to copy an object that will be returned to the clone method. Due to this t3 will obtain a deep copy of the object t1. So any changes made in ‘c’ object fields by t3, will not be reflected in t1.

Advantages of clone method:

If we use the assignment operator to assign an object reference to another reference variable then it will point to the same address location of the old object and no new copy of the object will be created. Due to this any changes in the reference variable will be reflected in the original object.
If we use a copy constructor, then we have to copy all the data over explicitly i.e. we have to reassign all the fields of the class in the constructor explicitly. But in the clone method, this work of creating a new copy is done by the method itself. So to avoid extra processing we use object cloning.