SDSU CS 635 Advanced Object-Oriented Design & Programming
Spring Semester, 2004
Flyweight & Facade
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San Diego State University -- This page last updated 30-Mar-04


Design Patterns: Elements of Reusable Object-Oriented Software , Gamma, Helm, Johnson, Vlissides, Addison-Wesley, 1995, pp. 185-206

The Design Patterns Smalltalk Companion, Alpert, Brown, Woolf, 1998, pp. 179-211


Doc 14, Flyweight & Facade Slide # 2

Sharable & Flyweight

Nation Example

Each country, like India or China, has a lot of information

A Nation object for one country may have a lot of data

A program may only have a few references to an India object

Having all references share the same object

Doc 14, Flyweight & Facade Slide # 3
Symbol & Interned Strings


Only one instance a symbol with a give character sequence in an image

| a b |
a := #cat.
b := (‘ca’ , ‘t’) asSymbol.
a = b    “True”
a == b    “True – a & b point to same location”



Compiler tries to use only one instance of a string with a given character sequence

String>>intern() returns a reference to a unique instance of a string

Doc 14, Flyweight & Facade Slide # 4
Text Example

Use objects to represent individual characters of the alphabet

Using objects allows the program to treat characters like any other part of the document - as an object

A character, like “G”, may require a fair amount of information:

Most of this information is the same for all instances of the same letter


What if there is state information that is different between references?

Doc 14, Flyweight & Facade Slide # 5
Intrinsic State

Extrinsic State

So create one instance of the object and use the same object wherever you need an instance

The one object can store the intrinsic state, but someone else needs to store the extrinsic state for each context of the object

Doc 14, Flyweight & Facade Slide # 6


Doc 14, Flyweight & Facade Slide # 7


The pattern can be used when all the following are true

      MyClass* objectPtrA;
      MyClass* objectPtrB;
      if ( objectPtrA == objectPtrB ) //testing object identity

Doc 14, Flyweight & Facade Slide # 8


Separating state from the flyweight

This is the hard part

Must remove the extrinsic state from the object

Store the extrinsic state elsewhere

Each time you use the flyweight you must give it the proper extrinsic state

Managing Flyweights

Cannot use object identity on flyweights

Need factory to create flyweights, cannot create directly

How do we know when we are done with a flyweight?

Doc 14, Flyweight & Facade Slide # 9


Compiler Example

The VisualWorks Smalltalk compiler system has 75 classes

Programmers only use Compiler, which uses the other classes

Compiler evaluate: '100 factorial'

| method compiler |
method := 'reset
   "Resets the counter to zero"
   count := 0.'.

compiler := Compiler new.
   in: Counter 
   notifying: nil

Doc 14, Flyweight & Facade Slide # 10
Distributed Object Systems

Doc 14, Flyweight & Facade Slide # 11

Subsystems are groups of classes, or groups of classes and other subsystems, that collaborate among themselves to support a set of contracts

There is no conceptual difference between the responsibilities of a class and a subsystem of classes

The difference between a class and subsystem of classes is a matter of scale

A subsystem should be a good abstraction

There should be as little communication between different subsystems as possible

Doc 14, Flyweight & Facade Slide # 12
The Facade Pattern - Basic Idea

Create a class that is the interface to the subsystem

Clients interface with the Facade class to deal with the subsystem

Consequences of Facade Pattern

It hides the implementation of the subsystem from clients

It promotes weak coupling between the subsystems and its clients

It does not prevent clients from using subsystem classes directly, should it?

Facade does not add new functionality to the subsystem

Copyright ©, All rights reserved.
2004 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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