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CS242 Advanced Programming Concepts in Java 11/06/07 Serialization Observer Design Pattern Prof. Searleman jets@clarkson.edu Outline Cloning (Cloneable Interface) Serialization (Serializable Interface) Observer Design Pattern: examples HW#5 posted, due: 11/13/07

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cs242 advanced programming concepts in java

CS242Advanced Programming Concepts in Java

11/06/07

Serialization

Observer Design Pattern

Prof. Searleman jets@clarkson.edu

outline
Outline
  • Cloning (Cloneable Interface)
  • Serialization (Serializable Interface)
  • Observer Design Pattern: examples

HW#5 posted, due: 11/13/07

hint: useScanner (orStringTokenizer)

also: do a screen capture of your GUI and turn it in

Exam#2 Tues, Nov 27, 7:00 pm, SC 362

practice problems

cloning it is up to the class designer to determine whether
Cloning – It is up to the class designer to determine whether:
  • the default clone() is good enough:

“Shallow cloning”

  • the default clone() can be patched up by calling clone on each non-primitive, mutable instance variable:

“Deep cloning”

  • give up – don’t use clone
copy constructor
Copy Constructor

A copy constructor for class X is of the form:

public X( X copy )

i.e. the constructor has one argument which is an

object of type X itself

// Example: Copy constructor for class Swimmer

public Swimmer(Swimmer obj) {

this.name = obj.name;

this.swimTime = obj.swimTime;

this.eventDate =

(Date) obj.eventDate.clone(); // Date is cloneable

}

slide5

Swimmer

String

Kristen

name

cu1

swimTime

26.31

eventDate

Date

year

2007

etc.

Suppose you want to save swimmer data on a file, and restore it later. How can this be accomplished?

1. Save each component separately in a file; To restore, read in the data and reconstruct each part

2. Serialize it

object serialization
Object Serialization
  • Any object that implements the serializable interface can be converted into a sequence of bytes
  • Implements “lightweight persistence”;

persistance – object’s lifetime is independent of program execution (lives even when the program terminates)

  • Serializable interface – no methods, just tagging
serializable interface
Serializable Interface

To write a serializable object:

  • Create an OutputStream of some flavor, wrap it inside an ObjectOutputStream
  • Call writeObject() – the object will be serialized and sent to the output stream

To read (restore) a serializable object:

Similar to the above, but call readObject() to “deserialize” the object (remember to cast)

slide8
public class Swimmer

implements Cloneable, Serializable

{

private String name;

private float swimTime;

private Date eventDate;

// methods

}

Consider the swimmer class. When it is serialized, it will also serialize all the object fields as well (String & Date)

slide9
ObjectOutputStream outstrm =

new ObjectOutputStream(

new FileOutputStream(“swimmers.dat”);

// cu1 refers to a swimmer

outstrm.writeObject(cu1);

outstrm.close();

slide10
// later, read in the swimmer data

ObjectInputStream instrm =

new ObjectInputStream(

new FileInputStream(“swimmers.dat”);

Swimmer s1 = (Swimmer) instrm.readObject();

writing serializable objects
Writing serializable objects

To write an object of a given class

  • The class must be public
  • The class must implement Serializable
  • If the base class is not Serializable, then it must have a default constructor
slide12
public class Swimmer

implements Serializable

// ObjectOutputStream objOUT opened

Swimmer[] team = { new Swimmer(…),

new Swimmer(…), …

new Swimmer(…) );

try {

for (int i = 0; i < team.length; i++)

objOUT.writeObject(team[i]);

objOUT.close()

} catch // exceptions

slide13
ObjectInputStream objIN // opened

Swimmer obj = null;

int count = 0;

try {

while (true) { // rewrite to terminate when EOS

obj = (Swimmer) objIN.readObject();

count++;

// add obj to a List

}

objOUT.close();

// convert the list to an array

} catch // exceptions

recap observer design pattern
Recap: Observer Design Pattern
  • key idea: minimize the coupling between the model, views and controllers
    • a controller detects user interaction that changes the data
    • that controller must tell model about the change
    • the model must notify all views of that change
    • all views must repaint() themselves
    • during painting, each view asks the model for the current data to display

model – only knows that views exist and they need to be notified

views – don’t know anything about the controllers (so easy to add more views to a model)

controller – easy to change the controller for a view

slide15
from http://www.javaworld.com

http://www.javaworld.com/javaworld/jw-03-2003/jw-0328-designpatterns.html?

observer method 1
Observer: Method #1

Write your own interfaces for Observer& Subject

public interface Observer {

/** notify Observers that a change has occurred */

public void sendNotify(String s);

}

public interface Subject

{

/** register your interest in the subject */

public void registerInterest(Observer obs);

}

slide17

public class Watch2Windows extends WindowFrame

implements ActionListener, ItemListener, Subject {

private List observers;

private ColorFrame cframe;

private ListFrame lframe;

//------------------------------------------

public Watch2Windows() {

observers = new ArrayList(); //observing frames

//---------create observers---------

cframe = new ColorFrame(this);

lframe = new ListFrame(this);

}

slide18

public void itemStateChanged(ItemEvent e) {

if (e.getStateChange() == ItemEvent.SELECTED)

notifyObservers((JRadioButton)e.getSource());

}

private void notifyObservers(JRadioButton rad) {

String color = rad.getText();

for (int i=0; i< observers.size(); i++) {

((Observer)(observers.get(i))).sendNotify(color);

}

}

public void registerInterest(Observer obs) {

observers.add(obs); //adds observer to list

}

slide19

public class ColorFrame extends JFrame

implements Observer{

public ColorFrame( Subject s) {

super(“Color View”);

s.registerInterest(this);

}

public void sendNotify(String s){

color_name = s;

if (s.equalsIgnoreCase("RED")) color = Color.red;

if (s.equalsIgnoreCase("BLUE")) color = Color.blue;

if (s.equalsIgnoreCase("GREEN")) color = Color.green;

setBackground(color);

} }

slide20

public class ListFrame extends JFrame

implements Observer{

private JList list;

private JListData listData;

public ListFrame( Subject s) {

super(“List View”);

s.registerInterest(this);

listData = new JListData(); // the list model

list = new JList(listData); // the visual list

}

public void sendNotify(String s){

listData.addElement(s);

} }

observer method 2
Observer: Method #2

Use the java.util.Observer interface and the java.util.Observable class

/** any class can implement this interface to be an observer */

public interface Observer {

/** application calls notifyObservers() which automatically

calls this method for each registered observer */

public void update(Observable o, Object arg);

}

observer method 2 cont
Observer: Method #2 (cont.)

/** represents an “observable” object, aka subject or model*/

public class Observable extends Object {

// void addObserver(Observer o)

// void deleteObserver(Observer o)

// int countObservers()

// boolean hasChanged()

// protected void setChanged()

// protected void clearChanged()

/** if data has changed (as indicated by setChanged), then

notify all observers and then clearChanged */

void notifyObservers();

}

slide23

// Account is an Observable class that represents a bank

// account in which funds may be deposited or withdrawn.

package com.deitel.account;

import java.util.Observable;

public class Account extends Observable // subject

{ // set Account balance and notify observers of change

private void setBalance( double accountBalance ) {

balance = accountBalance;

// must call setChanged before notifyObservers

// to indicate model has changed

setChanged();

// notify Observers that model has changed

notifyObservers();

}

slide24

public abstract class AbstractAccountView

extends JPanel

implements Observer { // observer

public AbstractAccountView(Account observableAccount )

throws NullPointerException {

// register as an Observer to receive account updates

observableAccount.addObserver( this );

}

// receive updates from Observable Account

public void update( Observable observable, Object object ){

updateDisplay();

}

}

slide25

// AssetPieChartView is an AbstractAccountView subclass that

// displays multiple asset Account balances as a pie chart.

public class AssetPieChartView extends JPanel

implements Observer{ // observes all accounts

private List accounts = new ArrayList(); // List of observed accts

// add Account to pie chart view

public void addAccount( Account anAccount ) {

if ( anAccount == null ) // do not add null Accounts

throw new NullPointerException();

// add new account to the list of accounts

accounts.add( account );

// register as Observer to receive Account updates

anAccount.addObserver( this );

// update display with new Account information

repaint();

}

slide26

// remove Account from pie chart view

public void removeAccount( Account anAccount ) {

anAccount.deleteObserver( this );// stop receiving updates

accounts.remove(anAccount );// remove Account from accounts list

colors.remove(anAccount ); // remove Account's Color

repaint(); // update display to remove Account information

}

// receive updates from Observable Account

public void update( Observable observable, Object object ) {

repaint();

}

slide27

// AccountController is a controller for Accounts. It provides

// a JTextField for inputting a deposit or withdrawal amount

// and JButtons for depositing or withdrawing funds.

public class AccountController extends JPanel {

private Account account; // Account to control

}

// AccountManager is an application that uses the MVC design

// pattern to manage bank Account information.

public class AccountManager extends JFrame {

public AccountManager() {

super( "Account Manager" );

// create account1 with initial balance

Account account1 = new Account( "Account 1", 1000.00 );

slide28

/* continuation of constructor for AccountManager */

// create GUIs for account1

JPanel account1Panel = createAccountPanel( account1 );

// create account2 with initial balance

Account account2 = new Account( "Account 2", 3000.00 );

// create GUI for account2

JPanel account2Panel = createAccountPanel( account2 );

// create AccountPieChartView to show Account pie chart

AssetPieChartView pieChartView =

new AssetPieChartView();

// add both Accounts to AccountPieChartView

pieChartView.addAccount( account1 );

pieChartView.addAccount( account2 );

} // end AccountManager constructor