Object cloning refers to creation of exact copy of an object. It creates a new instance of the class of current object and initializes all its fields with exactly the contents of the corresponding fields of this object.
Using Assignment Operator to create copy of reference variable
In Java, there is no operator to create copy of an object. Unlike C++, in Java, if we use assignment operator then it will create a copy of reference variable and not the object. This can be explained by taking an example. Following program demonstrates the same.
package com.gs.corejava.collections;
//Java program to demonstrate that assignment
//operator only creates a new reference to same
//object.
//A test class whose objects are cloned
class Test {
int x, y;
Test() {
x = 10;
y = 20;
}
}
// Driver Class
public 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 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.
package com.gs.corejava.collections;
//Java program to demonstrate that assignment
//operator only creates a new reference to same
//object.
//A test class whose objects are cloned
class Test implements Cloneable {
int x, y;
Test() {
x = 10;
y = 20;
}
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
// Driver Class
public 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 = null;
try {
ob2 = (Test) ob1.clone();
} catch (CloneNotSupportedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
// 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
10 20
100 20
Usage of clone() method -Shallow Copy
package com.gs.corejava.collections;
//A Java program to demonstrate shallow 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 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;
// Creating a copy of object t1 and passing it to t2
Test2 t2 = (Test2) t1.clone();
// Change in primitive type of t2 will not be reflected in t1 field
t2.a = 100;
// Change in object type field will be reflected in both t2 and t1(shallow copy)
t2.c.x = 300;
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 clone method after obtaining the copy.
Deep Copy vs Shallow Copy
Shallow copy is 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 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 object.
Shallow copies are cheap and simple to make. In above example, we created a shallow copy of 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 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 below example, we create a deep copy of 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.
package com.gs.corejava.collections;
//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 refernce variable t
Test2 t = (Test2) super.clone();
// Create a new object for the field c
// and assign it to shallow copy obtained,
// to make it a deep copy
t.c = new Test();
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();
// Change in primitive type of t2 will not
// be reflected in t1 field
t3.a = 100;
// Change in object type field of t2 will not
// be reflected in t1(deep copy)
t3.c.x = 300;
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 0
Cloning with inheritance (Shallow) :
Example 1:
package com.gs.corejava.collections;
//: appendixa:HorrorFlick.java
//You can insert Cloneability at any level of inheritance.
//From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
//www.BruceEckel.com. See copyright notice in CopyRight.txt.
class Person {
}
class Hero extends Person {
}
class Scientist extends Person implements Cloneable {
public Object clone() {
try {
return super.clone();
} catch (CloneNotSupportedException e) {
// This should never happen: It's Cloneable already!
throw new RuntimeException(e);
}
}
}
class MadScientist extends Scientist {
}
public class HorrorFlick {
public static void main(String[] args) {
Person p = new Person();
Hero h = new Hero();
Scientist s = new Scientist();
MadScientist m = new MadScientist();
// ! p = (Person)p.clone(); // Compile error
// ! h = (Hero)h.clone(); // Compile error
s = (Scientist) s.clone();
System.out.println(m.clone().getClass());
m = (MadScientist) m.clone();
}
} // /:~
Example 2:
package com.gs.corejava.collections;
class A implements Cloneable {
@Override
protected A clone() throws CloneNotSupportedException // could be public
{
Object clone = super.clone();
System.out.println("Class A: " + clone.getClass()); // will print 'C'
return (A) clone;
}
}
class B extends A {
@Override
protected B clone() throws CloneNotSupportedException {
A clone = super.clone();
System.out.println("Class B: " + clone.getClass()); // will print 'C'
return (B) clone;
}
}
class C extends B {
@Override
protected C clone() throws CloneNotSupportedException {
B clone = super.clone();
System.out.println("Class C: " + clone.getClass()); // will print 'C'
return (C) clone;
}
}
public class TestI {
public static void main(String argv[]) {
C c = new C();
try {
A cloned_c = c.clone();
} catch (CloneNotSupportedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
Output:
Class A: class com.gs.corejava.collections.C
Class B: class com.gs.corejava.collections.C
Class C: class com.gs.corejava.collections.C
