Shallow Cloning

[Shivani Yadav]

I am having some confusion in understanding shallow clonning. Just want to clear my understanding.

As per shallow clonning ..."In shallow copy the object is copied without its contained objects.
Shallow clone only copies the top level structure of the object not the lower levels..."

I just tried the below code to see practically.

   

public class TryShallowClone {
	public static void main(String[] args) {
		Shallow origObj = new Shallow();
		System.out.println("origObj str1 : "+origObj.str1);
		System.out.println("origObj str2 : "+origObj.str2);
		System.out.println("origObj myInt : "+origObj.myInt);
		System.out.println("origObj fsh : "+origObj.fsh.getName());
		
		Shallow clonedObj = (Shallow)origObj.clone();
		
		System.out.println("clonedObj str1 : "+clonedObj.str1);
		System.out.println("clonedObj str2 : "+clonedObj.str2);
		System.out.println("clonedObj myInt : "+clonedObj.myInt);
		System.out.println("clonedObj fsh : "+clonedObj.fsh.getName());
		
		clonedObj.fsh.setName("Octopus");
		System.out.println("After modifying cloned's Fish....");
		System.out.println("clonedObj fsh : "+clonedObj.fsh.getName());
		System.out.println("origObj fsh : "+origObj.fsh.getName());
		System.out.println("origObj.fsh == clonedObj.fsh : "+(origObj.fsh == clonedObj.fsh));
		
		clonedObj.str1 = "String33";
		System.out.println("After modifying cloned's str1...");
		System.out.println("clonedObj str1 : "+clonedObj.str1);
		System.out.println("origObj str1 : "+origObj.str1);	
		
	}
}

class Shallow implements Cloneable{
	String str1 = "String1";
	String str2 = str1+ "2";
	Fish fsh = new Fish("Clown");
	int myInt = 2;
	public Object clone(){
		try {
			return super.clone();
		} catch (CloneNotSupportedException e) { 
			e.printStackTrace();
			return null;
		}
	}

}

class Fish {
	String name;
	Fish(String nm){
		name = nm;
		System.out.println("I am in Fish"); 
	}
	public String getName(){
		return name;
	}

	public void setName(String nm){
		name = nm;
	}
}


output :

I am in Fish
origObj str1 : String1
origObj str2 : String12
origObj myInt : 2
origObj fsh : Clown
clonedObj str1 : String1
clonedObj str2 : String12
clonedObj myInt : 2
clonedObj fsh : Clown
After modifying cloned's Fish....
clonedObj fsh : Octopus
origObj fsh : Octopus
origObj.fsh == clonedObj.fsh : true
After modifying cloned's str1...
clonedObj str1 : String33
origObj str1 : String1

 

In the above code i am having a query.
When i am doing " clonedObj.str1 = "String33"; " then why the origObj.str1 value is not changed? i.e. origObj.str1 is "String1" and not the "String33". Here are they not referring to the same String refrence?

This is not happening when i am doing "clonedObj.fsh.setName("Octopus");" i.e origObj.fsh name also gets modified as they are referring to the same Fish object.

Can anybody please explain me the difference between above two scenarios.

Thanks in advance.

[Vijitha Kumara]

In first case, you are changing the "Fish" object's name, but both "Shallow" objects refer to the same "Fish" object. But with "str1" reference though both objects initially refers to the same, changing one's reference doesn't change the other's (that is, Java uses "pass by value" so str1 of the object you changed will refer to the new String object ).

[Shivani Yadav]

Thanks, i got it.

When i am setting the name of the Fish object, i am not changing the object to which clonedObj.fsh and origObj.fsh is referring to. Only the object's state is changed.

But when i am assigning value to clonedObj.str1 the actual object to which it was referring is changed so i will see the difference.

[PrasannaKumar Sathiyanantham]

So while cloning a separate shallow object is created but the same fish object is used by both the shallow objects(the clone one and the parent).Why is that so?

[Vijitha Kumara]

PrasannaKumar Sathiyanantham wrote:So while cloning a separate shallow object is created but the same fish object is used by both the shallow objects(the clone one and the parent).Why is that so?

That's how shallow copy works (clone() in Object class), It only copies object references. If you want deep copy (it's own copies for the cloned object) you should override the clone() in your class.

[Jack Bento]

Is this in the exam?

[Vijitha Kumara]

Fray Bentos wrote:Is this in the exam?

No, nothing about cloning on the SCJP exam. Check the Objectives for SCJP 6.

[Bert Bates]

Agreed, interesting topic, not on the exam.