Shallow and Deep Copy

We generally clone an object using java.lang.Cloneable.clone(). This method makes a shallow copy and not a deep copy.

Difference between the two can be understood by using following example:


public class CloningExample implements Cloneable

{
private LinkedList names = new LinkedList();

public CloningExample()
{

names.add("abc");

names.add("xyz");
}


public Object deepClone() throws CloneNotSupportedException
{
CloningExample copy = (CloningExample)super.clone();

copy.names = (LinkedList)names.clone();

return copy;
}

public boolean equals(Object obj)
{
CloningExample tmp = (CloningExample)obj;

if (this.names == tmp.names)

return true;

else

return false;
}

public static void main(String[] args) throws CloneNotSupportedException
{
CloningExample ce1 = new CloningExample();

System.out.println("\nCloningExample[1] ----------------- " + ce1.names);

CloningExample ce2 = (CloningExample)ce1.clone();

System.out.println("\nCloningExample[2] ----------------- " + ce2.names);

System.out.println("\nCompare Shallow Copy --------------------\n" +

" ce1 == ce2 : " + (ce1 == ce2) + "\n" +

" ce1.equals(ce2) : " + ce1.equals(ce2));

CloningExample ce3 = (CloningExample)ce1.deepClone();

System.out.println("\nCompare Deep Copy --------------------\n" +

" ce1 == ce3 : " + (ce1 == ce3) + "\n" +

" ce1.equals(ce3) : " + ce1.equals(ce3));
}

}


Shallow Copy

CloningExample ce2 = (CloningExample)ce1.clone();

A clone ce2 of ce1 is created by simply calling the clone() method of Object. Our overridden equals() method tells us that the "names" are equivalent, but operator == tells that they aren't equal - that is, that are two distinct objects. This is what referred as a "Shallow Copy". Java simply copied the bits of our variables. This means that the "names" instance variable of both objects (ce1 and ce2) still holds the same information - that is, both objects have a reference to the same LinkedList.


Deep Copy

CloningExample ce3 = (CloningExample)ce1.deepClone();

The code above will perform a deep copy of the ce1 to ce3. The first part of the clone in the deep copy is the same as it was for the shallow copy. deepClone() method now

clones the LinkedList as well. Now, when CloningExample object is cloned, each CloningExample object (ce1 and ce3) will have its own LinkedList.