CS 151 - Lecture 21

Cay S. Horstmann

Lab

• Work with a buddy
• One of you codes, the other types up the answers.
• One of you submits the code in the lab21 subdirectory of your personal repo, the other submits a file report.txt in the lab21 subdirectory of your personal repo.
• Put the name of the coding buddy into the report

Cloning

• Start with this Person class:

  import java.util.*;
  
  public class Person
  {  
     public Person(String name) { this.name = name; }
     
     public void befriend(Person p) { friends.add(p); }
     public Person getFriend(int i)
     {
        return 0 <= i && i < friends.size() ? friends.get(i) : null;
     }
  
     public String toString()
     {
        String result = name + ", friends =";
        for (Person f : friends) result += " " + f.name;      
        return result;
     }
  
     private String name;
     private ArrayList<Person> friends = new ArrayList<>();
  }
  

• Now compile this program:

  public class Prog1
  {
     public static void main(String[] args) throws Exception
     {
        Person fred = new Person("Fred");
        Person barney = new Person("Barney");
        Person cloneOfFred = fred.clone();
        fred.befriend(barney);
        barney.befriend(fred);
        System.out.println(cloneOfFred);
     }
  }
  

  What happens? Why?
• Ok, let's make clone public. Add this to the Person class:

  public Person clone() { return (Person) super.clone(); }
  

  What happens when you compile? Why?
• It's better to have clone declare that it might throw the exception rather than to catch the exception. Do that. Your program should now compile.

A Deep Copy

• Run the program. You should get an exception. Why?
• Fix the exception by making Person implement the appropriate interface. Now your program should compile and run. What friends does cloneOfFred have? Why?
• We don't want that behavior. Make it so that cloneOfFred has its own list of friends, by cloning the original list. How did you do that? Run the program again and verify that the program has been fixed.
• Should we copy even more deeply? Suppose the clone method looks like this:

  public Person clone() throws CloneNotSupportedException
  {
     Person cloned = (Person) super.clone();
     cloned.friends = new ArrayList<>();
     for (Person f : friends) cloned.friends.add(f.clone());
     return cloned;
  }

  Run the program. What happens? Why?

Serialization

• Declare the Person class to be Serializable. Run this program:

  import java.io.*;
  
  public class Prog2
  {
     public static void main(String[] args) throws Exception
     {
        Person fred = new Person("Fred");
        Person barney = new Person("Barney");
        fred.befriend(barney);
        barney.befriend(fred);
        ObjectOutputStream out = new ObjectOutputStream(
           new FileOutputStream("people.ser"));
        out.writeObject(fred);
        out.close();
        fred = null;
        barney = null;
  
        ObjectInputStream in = new ObjectInputStream(
           new FileInputStream("people.ser"));
        Person p = (Person) in.readObject();            
        System.out.println(p);
        System.out.println(p.getFriend(0));
    }
  }

  Which objects are contained in people.ser? Why?

Transient Variables

• Now add an instance variable and, constructor, and a method to the Person class:

  private java.awt.image.BufferedImage image;
  
  public Person(String name, String imageURL) throws java.io.IOException
  {
     this(name);
     image = javax.imageio.ImageIO.read(new java.net.URL(imageURL));
  }
  
  public void show()
  {
     if (image == null) javax.swing.JOptionPane.showMessageDialog(null, name);
     else javax.swing.JOptionPane.showMessageDialog(null, name, "", 1,
        new javax.swing.ImageIcon(image));         
  }
  

  In Prog2, change the declaration of fred to

  Person fred = new Person("Fred", "https://upload.wikimedia.org/wikipedia/en/thumb/a/ad/Fred_Flintstone.png/165px-Fred_Flintstone.png");
  fred.show();
  

  Run the program. What happens? Why?

Make the image transient. The program should now run.

Add p.show() to the end. Why doesn't the image show?

Extra credit: How can you fix it?