How Cloning Works in java

In java cloning is process of creating exact copy of an object.

Java provides clone() method in java.lang.object package.To create clone of any object we have to implement Cloneable interface.

Method deceleration in Cloneable interface is

    

       protected Object clone() throws CloneNotSupportedException 

If we want to create clone of object then it is necessary that class implement Cloneable Interface if our class not implement Cloneable interface then CloneNotSupportedException type exception will be thrown.

When clone method is called then java create new instance of object, copy each field one by one from one object to another and return instance of object.

java.lang.object provide default implementation of clone method but we can also write our custom implementation.

Below is example of cloning in java 

Suppose we have class Car which implement Cloneable Interface

package com.kodemaker;

public class Car implements Cloneable{

 public String carModel;
 public String carColor;

 public Car(String carModel, String carColor) {
  super();
  this.carModel = carModel;
  this.carColor = carColor;
 }

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

}

Below is main class by which we create clone of object.

package com.kodemaker;

public class CloneMain {
    
 public static void main(String[] args) {
     
  Car audiRedCar = new Car("Audi A3 Sedan","Red");
  Car copyAudiRedCar;
  
  try {
   //create clone of object 
   copyAudiRedCar = audiRedCar.clone();
   
   //original object properties
   System.out.println("---audiRedCar Properties---");
   System.out.println("Model- "+audiRedCar.carModel);
   System.out.println("Color- "+audiRedCar.carColor);
   
   //clone object properties
   System.out.println("---copyAudiRedCar Properties---");
   System.out.println("Model- "+copyAudiRedCar.carModel);
   System.out.println("Color- "+copyAudiRedCar.carColor);
   
   // beacause both object has seperate copy so other object not affect if change one object
   System.out.println("---Both object has seperate copy---");
   copyAudiRedCar.carColor = "Blue";
   System.out.println("Color- "+copyAudiRedCar.carColor);
   System.out.println("Color- "+audiRedCar.carColor);
   
  }catch (CloneNotSupportedException e) {
   e.printStackTrace();
  }
  
 }
}

In above example we can see audiRedCar.clone() != audiRedCar i.e. both object instances are different.
System.out.println("Color- "+copyAudiRedCar.carColor);
System.out.println("Color- "+audiRedCar.carColor);

Output of above programme

---audiRedCar Properties---
Model- Audi A3 Sedan
Color- Red
---copyAudiRedCar Properties---
Model- Audi A3 Sedan
Color- Red
---Both object has seperate copy---
Color- Blue
Color- Red

if Car class not implement Cloneable Interface then

Exception will be thrown

java.lang.CloneNotSupportedException: com.kodemaker.Car
 at java.lang.Object.clone(Native Method)
 at com.kodemaker.Car.clone(Car.java:16)
 at com.kodemaker.CloneMain.main(CloneMain.java:12)

If we have not used cloning then we have to create instance of object and copy fields one by one manually which is very time consuming so cloning is better approach for create clone of object

For more detail of above topic

you can refer http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html

In next article we learn what is deep cloning and shallow cloning.