AIM: Copy an object to a new object.

Copying an object

• Normal assignment to copy an object will create two references to the same object.

  public class Employee{
  	private String name;
  	private double salary;
  
  	public Employee(String n, double s){
  		name = n;
  		salary = s;
  	}
  
  	public void setname(String n){
  		name = n;
  	}
  }
  ....
  ....
  Employee e1 = new Employee("Dhruv", 21500.0);
  Employee e2 = e1;
  
  e2.setname("Eknath"); // This will update the name in e1 also.
  

• We want : Two separate but identical objects?

  • e2 should be initialized to a disjoint copy of e1.

• How can we do this?

The clone() method

• Object defines a method clone()

<aside> 💡 e1.clone() returns a bitwise copy of e1.

</aside>

public class Employee{
	private String name;
	private double salary;

	public Employee(String n, double s){
		name = n;
		salary = s;
	}

	public void setname(String n){
		name = n;
	}
}
....
....
Employee e1 = new Employee("Dhruv", 21500.0);
Employee e2 = e1.clone();

e2.setname("Eknath"); // This will NOT update the name in e1.

• Why Bitwise copy?
  • Object does not have access to the private instance variables.
  • So we cannot built up a fresh copy of e1 from scratch.
  • So the way is by copying the object bit by bit.

What could go wrong with a bitwise copy???

Shallow Copy

• What if we add an instance variable Date (object) to Employee ?

  public class Employee{
  	private String name;
  	private double salary;
  	private Date birthday;
  	...
  	public void setname(String n){
  		name = n;
  	}
  	public void setbday(int dd, int mm, int yy){
  			birthday.update(dd,mm,yy);
  	}
  }
  ...
  
  Employee e1 = new Employee("Dhruv" , 21500.0);
  Employee e2 = e1.clone();
  e2.setname("Eknath");  // e1 name will not be updated.
  e2.setbday(16,4,1997); // e1 bday will be updated.
  
  

  • Assume that update() updates the components of the Date object.

• Bitwise copy made by e1.clone() copies reference to the embedded Date.

  • Now : e2.birthday and e1.birthday refers to the same object!
  • e2.setbday also affects e1.birthday

<aside> 💡 Bitwise copy is a shallow copy. i.e. , nested mutable references are copied verbatim.

</aside>

Deep Copy

<aside> 💡 Deep copy recursively clones nested objects.

</aside>