	CS 535 Object-Oriented Programming & Design Fall Semester, 2000 Heuristics: Separating Abstractions
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Contents of Doc 17, Heuristics: Separating Abstractions

References

Design Patterns: Elements of Reusable Object-Oriented Software , Gamma, Helm, Johnson, Vlissides, 1995, pp. 293-303 Observer Pattern

Object-Oriented Design Heuristics , Riel, Addison-Wesley, 1996, Heuristics 2.2, 2.8, 2.10, 3.4, 3.5, 4.6, 4.13


Doc 17, Heuristics: Separating Abstractions Slide # 2

Too Many Abstractions

2.8 A class should capture one and only one key abstraction

2.10 Spin off nonrelated information into another class

3.4 Beware of classes that have too much noncommunicating behavior

4.6 Most of the methods defined in a class should be using most of the data members most of the time


Doc 17, Heuristics: Separating Abstractions Slide # 3

Independence from Users

2.2 Users of a class must be dependent on its public interface, but a class should not be dependent on its users

4.13 A class must know what it contains, but it should not know who contains it.

3.5 In applications that consist of an object-oriented model interacting with a user interface, the model should never be dependent on the interface

Example
Build a Timer class (or alarm clock).

We need to be able to display the time remaining and notify an object when the alarm goes off.

How do we satisfy the above heuristics?


Doc 17, Heuristics: Separating Abstractions Slide # 4

Basic Timer Class Here is a class that works, but violates the heuristics. It talks directly to the screen.

import sdsu.test.Assert;
public class Timer extends Thread {
   int secondsRemaining;
   
   public Timer( int minutes, int seconds ) {
      Assert.condition( ( minutes >= 0 ) && ( seconds >= 0 ));
      secondsRemaining = (minutes * 60) + seconds; 
      setPriority( Thread.MAX_PRIORITY );
   }
   
   public String toString()  {
      int minutes = secondsRemaining / 60;
      int seconds = secondsRemaining % 60;
      
      return "" + minutes + ":" + seconds;
   }
   
   public void run() {
      try {
         while (secondsRemaining > 0 ) {
            sleep( 1000 );
            secondsRemaining--;
            System.out.println( this.toString() );
         }
         System.out.println( "Time to wake up" );
      } catch ( InterruptedException stopTimer ){ //stop the timer }
   }
}
Doc 17, Heuristics: Separating Abstractions Slide # 5

Using the Timer class

public class Test  {
   public static void main( String args[] )  throws Exception
      {
      Timer clock = new Timer( 0, 8 );
      clock.start();
      }

How do we decouple the Timer class from the interface and the users?


Doc 17, Heuristics: Separating Abstractions Slide # 6

Observer Pattern

Also know as Observer-Observable, dependants, and publish-subscribe. This basic idea is used in the model-view-controller (MVC). The model is the observable, the view and controller are the observer.

Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically

Applicability

Use the Observer pattern:

When an abstraction has two aspects, one dependent on the other.

When a change to one object requires changing others, and you don't how many objects need to be changed

When an object should be able to notify other objects without making assumptions about who these objects are.

Participants

Observable (Subject, or model)

The model of the application
Maintains a list of dependents
When it changes state, it notifies its dependents

Observer

Object with interest in knowing when observable changes state


Doc 17, Heuristics: Separating Abstractions Slide # 7

Collaborations

Object Diagram

Timing Diagram


Doc 17, Heuristics: Separating Abstractions Slide # 8

Example Code and Java API

Java does include Observable and Observer in the JDK API. They are more complex than the implementation I use here. I did not use the built-in classes to avoid additional complexity. The goal here is to understand the concept not to cover details about Java API. See the JDK on-line documentation (java.util.Observable, java.util.Observer) and http://www.eli.sdsu.edu/courses/fall97/cs535/notes/observer/observer.html for more information about the standard Java classes.

Java’s class Object has a notify() method which is related to threads. To avoid confusion with this method I will use the method name notifyObservers().

The implementation of Observable below does not have all the functionality of java.util.Observable. In particular, my Observable class does not handle the synchronization of threads as best it could. The notifyObservers method needs to be refined. See the source code of java.util.Observable or see notifyModelChanged() method on slide 4 of http://www.eli.sdsu.edu/courses/spring98/cs696/notes/beanEvents/beanEvents.html


Doc 17, Heuristics: Separating Abstractions Slide # 9

Observer

public interface Observer{
   public void update( String message );
}

Observable

import java.util.ArrayList;
import java.util.Iterator;
public class Observable{
   private ArrayList observerList = new ArrayList();
   
   public synchronized void addObserver( Observer myObserver) {
      observerList.add( myObserver);
   }
   public synchronized void removeObserver( Observer myObserver ) {
      observerList.remove( myObserver);
   }
   public synchronized void notifyObservers( String message ) {
      if (message == null )
         message = "";
      Iterator observers = observerList.iterator();
      
      while ( observers.hasNext() ) {
         Observer anObserver =(Observer) observers.next();
         anObserver.update( message );
      }
   }
}
Doc 17, Heuristics: Separating Abstractions Slide # 10

Using the Observer/ObservableTimer

import sdsu.test.Assert;
public class Timer extends Thread {
   public static final String TICK = "tick";
   public static final String ALARM = "alarm";
   Observable observers = new Observable();
   
   int secondsRemaining;
   
   public Timer( int minutes, int seconds ) {
      Assert.condition( ( minutes >= 0 ) && ( seconds >= 0 ));
      secondsRemaining = (minutes * 60) + seconds; 
      setPriority( Thread.MAX_PRIORITY );
   }
   
   public void addObserver( Observer myObserver ){
      observers.addObserver( myObserver );
   }
      
   public String toString() {
      int minutes = secondsRemaining / 60;
      int seconds = secondsRemaining % 60;
      
      return "" + minutes + ":" + seconds;
   }
   
   public void run() {
      try {
         while (secondsRemaining > 0 ) {
            sleep( 1000 );
            secondsRemaining--;
            observers.notifyObservers( TICK );
         }
         observers.notifyObservers( ALARM );
      } catch ( InterruptedException stopTimer ) { //stop the timer
      }
   }
}
Doc 17, Heuristics: Separating Abstractions Slide # 11

ASCIITimerView

public class ASCIITimerView implements Observer {
   Timer myTimer;
   
   public ASCIITimerView( Timer aTimer ) {
      myTimer = aTimer;
      myTimer.addObserver( this );
   }
   
   public void update( String message ) {
      if ( message.equals( Timer.TICK ) )
         System.out.println( "Time left: " + myTimer.toString() );
   }
}
Doc 17, Heuristics: Separating Abstractions Slide # 12

Person

public class Person implements Observer {
   
   public void setAlarm( Timer aTimer ) {
      aTimer.addObserver( this );
   }
   
   public void update( String message ) {
      if ( message.equals( Timer.ALARM ) )
         wakeUp();
   }
   
   public void wakeUp() {
      System.out.println( "Mom, I am awake");
   }
}
Doc 17, Heuristics: Separating Abstractions Slide # 13

Sample Usage

public static void main() {
   Timer clock = new Timer( 0, 8 );
   ASCIITimerView aView = new ASCIITimerView( clock );
   Person teenager = new Person();
   Person child = new Person();
   teenager.setAlarm( clock);
   child.setAlarm( clock);
   clock.start();
}

Output

Time left: 0:7
Time left: 0:6
Time left: 0:5
Time left: 0:4
Time left: 0:3
Time left: 0:2
Time left: 0:1
Time left: 0:0
Mom, I am awake
Mom, I am awake
Doc 17, Heuristics: Separating Abstractions Slide # 14

Variants

Use multiple observer sets for different types of events

The Timer has two different type of events, tick and alarm. The observers must check for the type of event. If the observable contained more than one set of observers, the observers could register interest only in the events they need to know about. Java uses the term “listeners” to refer to this use of the pattern.

Send a reference to the source of the event in the update message

This will allow observers to observer more than one observable and permit them to not keep a reference to the observable.

For more variants see Design Patterns: Elements of Reusable Object-Oriented Software.


Doc 17, Heuristics: Separating Abstractions Slide # 15

Modified ExampleObserver

public interface Observer {
   public void update( Object message );
}

Observable

import java.util.ArrayList;
import java.util.Iterator;
public class Observable {
   private ArrayList observerList = new ArrayList();
   
   public synchronized void addObserver( Observer myObserver ){
      observerList.add( myObserver );
   }
   public synchronized void removeObserver( Observer myObserver ) {
      observerList.remove( myObserver );
   }
   public synchronized void notifyObservers( Object message ) {
      Iterator observers = observerList.iterator();
      
      while ( observers.hasNext() ) {
         Observer anObserver =(Observer) observers.next();
         anObserver.update( message );
      }
   }
}
Doc 17, Heuristics: Separating Abstractions Slide # 16

Timer

import sdsu.test.Assert;
public class Timer extends Thread {
   Observable timeObservers = new Observable();
   Observable alarmObservers = new Observable();
   
   int secondsRemaining;
   
   public Timer( int minutes, int seconds ) {
      Assert.condition( ( minutes >= 0 ) && ( seconds >= 0 ));
      secondsRemaining = (minutes * 60) + seconds; 
      setPriority( Thread.MAX_PRIORITY );
   }
   
   public void addTimeObserver( Observer myObserver ){
      timeObservers.addObserver( myObserver );
   }
   public void addAlarmObserver( Observer myObserver ){
      alarmObservers.addObserver( myObserver );
   }
      
   public String toString() {
      int minutes = secondsRemaining / 60;
      int seconds = secondsRemaining % 60;
      
      return "" + minutes + ":" + seconds;
   }
   
   public void run() {
      try {
         while (secondsRemaining > 0 ) {
            sleep( 1000 );
            secondsRemaining--;
            timeObservers.notifyObservers( this );
         }
         alarmObservers.notifyObservers( this );
      } catch ( InterruptedException stopTimer ) { //stop the timer
      }
   }
}
Doc 17, Heuristics: Separating Abstractions Slide # 17

ASCIITimerView

public class ASCIITimerView implements Observer {   
   public ASCIITimerView( Timer aTimer ) {
      aTimer.addTimeObserver( this );
   }
   
   public void update( Object aTimer ) {
      System.out.println( "Time left: " + aTimer.toString() );
   }
}
Doc 17, Heuristics: Separating Abstractions Slide # 18

Consequences of Observer Pattern

Abstract coupling between Observable and Observer

The observable only knows that it has a lists of observers. It does not know the exact type of the observer. It just knows that the observer has an update method. Hence the coupling is abstract (between the observable and the interface observer) and minimal

Support for broadcast communication

The update message is a broadcast to all observers

Unexpected updates

Simple change one observable can cause numerous updates in the observers of that observable. Those observers may also be observable, so will send update messages to their observers. So one update message can trigger many updates, which can be expensive or distracting.

Updates can take too long

The observable can not perform any work until all observers are done

For more implementation details see: Design Patterns: Elements of Reusable Object-Oriented Software


Doc 17, Heuristics: Separating Abstractions Slide # 19

Spreadsheet Example
How to implement a spreadsheet?

Make SpreadsheetCells both observer and observable. It observers other SpreadsheetCells. When a cell changes it notifies its observers (SpreadsheetCells and SpreadsheetCellsViews) of the change
Make SpreedsheetCellView observers of SpreadsheetCells

How do we make cell C update it self and its view when the user changes the value of cell A?

When the user types a new value in view A, the view changes the value in cell A. Cell A then sends an update message to its observers, which includes cell C. Cell C then recomputes its sum, and sends an update message to its observers, which includes the view on cell C. The view then updates its display value.

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2000 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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