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How to define the method clone in your Java class

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Typically, we only clone mutable objects. A mutable object is one that has public methods (such as set methods) that can change its state. Since it is safe to share an immutable object, cloning it is usually unnecessary.

The Object class contains a protected clone method that returns a copy of an object. This copy has shallow copies of any component objects. The method has the following header:

protected Object clone() throws CloneNotSupportedException

Since Object is an ancestor of all other Java classes, your class will inherit this protected clone method. However, a client of your class cannot invoke clone unless you override Object’s version with a public definition of clone.

Classes that define a clone method must implement the standard interface Cloneable, which has the following definition:

public interface Cloneable
{
} // end Cloneable

This interface is empty! It merely signals to the compiler that your class will override its inherited clone method.

NameDemo.java

Name.java

public class NameDemo
{
   public static void main(String[] args)
   {
      // Create a Name object
      Name aName = new Name();
      aName.setName("Jamie", "Java");
		System.out.println("Object A's name is " + aName); // Calls toString
      // Clone the object
      Name theClone = (Name)aName.clone();               // Note the cast
      System.out.println("The clone's name is " + theClone + "\n");
      
      // Change aName's name; see whether it changes the clone
      aName.setName("Phantom", "O'Opera");
      System.out.println("Change A's name to " + aName + "\n");
            
      System.out.println("Did this change affect the clone?");
      System.out.println("The clone's name is " + theClone + "\n");
      // Change clone's name; see whether it changes aName's name
      theClone.setName("Galaxy", "Moonbeam");
      System.out.println("Change theClone's name to " + theClone + "\n");
            
      System.out.println("Did this change affect A's name?");
      System.out.println("A's name is " + aName);
   } // End main
} // End NameDemo

Study the code and notice the following key points about the Name class:

The class header contains the implements Cloneable clause
Name’s public method clone overrides Object’s protected clone method
Both versions of clone return an instance of Object
Name’s clone invokes Object’s clone with the expression super.clone()
super.clone() returns an instance of Object that we cast to Name
Because Object’s clone method throws an exception, Name’s call to it is in a try block. The catch block is empty because we are defining clone for Name.
Name’s clone will not throw an exception, due to its use of try/catch blocks; so, its client can call it without try/catch blocks
Name’s clone finally copies the data fields first and last and then returns the cloned object

We need to say more about the last bullet. We have provided code to make either shallow or deep copies of the fields first and last. You should try each version when you run the program. Because the fields are strings, and strings are immutablecannot be changed, you will see no difference in the way that clone copies them.

PersonDemo.java

Name.java

Person.java

/** Name.java by F. M. Carrano
    A class of names, each containing a first name and a last name.
*/
public class Name implements Cloneable
{
   private String first; // First name
   private String last;  // Last name
	
   /** Creates a default name whose first and last names are empty strings. */
   public Name()
   {
      first = "";
      last = "";
   } // End default constructor	
   
   public Object clone()
   {
      Name theCopy = null;
      try
      {
         theCopy = (Name)super.clone(); // super.clone() returns an instance of Object
      }
      catch (CloneNotSupportedException e)
      {  // This exception will not be thrown, since Name has a clone method.
         // But try/catch blocks are necessary!
         throw new Error(e.getMessage());
      }      
 
      // Shallow copy
      theCopy.first = first;
      theCopy.last = last;
/*
      // Deep copy
      theCopy.first = new String(first);
      theCopy.last = new String(last);
*/
      return theCopy;
   } // End clone
   /** Sets the first and last names of this name object.
       @param firstName  A string giving the name's first name.
       @param lastName  A string giving the name's last name. */
   public void setName(String firstName, String lastName)
   {
      first = firstName;
      last = lastName;
   } // End setName
   /** @return this name's first name as a string. */
   public String getFirstName()
   {
      return first;
   } // End getFirstName
	
   /** @return this name's last name as a string. */
   public String getLastName()
   {
      return last;
   } // End getLastName
   /**  @return this name as a string. */
   public String toString()
   {
      return first + " " + last;
   } // End toString
} // End Name